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Forum Code of Conduct__________________________________________________ _________________________________________
Below is a list of rules that explains what behavior is expected of you and other users on the forums. Please note that the following guidelines and policies are not exhaustive and may not address all manner of offensive behavior. Therefore, the forum moderators have full discretion of addressing individuals who have engaged in behavior that is perceived as offensive. Your access to these forums is a "privilege," and not a "right." Elsword reserves the right to suspend your access to these forums at any time for reasons that include, but are not necessarily limited to, your failure to abide by these guidelines.
We assess each player's violations on a case by case basis. This means that previous violations will be taken into account when determining the length of suspension. Repeat violations, despite severity of each individually will result in a permanent ban on the Elsword forums.
KOG Games and Moderators have elasticity when enforcing the rules. In other words the "it wasn't written exactly in the rules" CoC loophole argument is not going to fly. If you do anything that is disruptive to the community you will be warned or banned.
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Forum Rules:
SPAMMING AND TROLLING: This is broad and is not limited to the items mentioned below. Please review these before posting on the forums.
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Final Notes:
The bottom line is that we want ElswordOnline.com to be a fun and safe environment for all players. In posting on these forums, you will encounter thousands of other players who share different experiences and come from vastly different backgrounds. While certain language or dialects and images may not be offensive to you, consider the fact that that same language and images may have a completely different effect on someone else. We've done everything we can to give you great games but now it's up to you, the players, to breathe life into these worlds!!


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LONDON (Reuters) - Helen Mirren was crowned best actress at Britain's top theatre awards on Sunday for reprising her Oscar-winning portrayal of the queen, while a stage version of a novel about a boy with autism was on track to be the top winner of the Olivier awards.
Mirren, 67, has won stellar reviews for starring in "The Audience", Peter Morgan's play about the private weekly meetings between Queen Elizabeth and the 12 British prime ministers during the six decades of her reign.
Mirren is no stranger to royalty having won an Academy award for the same role in the 2006 film "The Queen".
The actress said the queen certainly deserved an Olivier award after receiving one of Britain's most prestigious entertainment honors, a BAFTA, earlier this month for her support for the film and television industry.
"I think she deserves one for the most committed and consistent performance of the 20th century and probably the 21st century," said Mirren on accepting the award from Harry Potter actor Daniel Radcliffe.
Her co-star, Richard McCabe, won the award for best supporting actor for playing the prime minister Howard Wilson.
But the biggest winner of the 37th Olivier awards looked set to be "The Curious Incident Of The Dog In The Night-Time" that opened at the National Theatre in London in 2012 and transferred to London's West End theatre district this year.
The play, based on Mark Haddon's 2003 award-winning novel, was nominated for eight awards and picked up several in the first half of the glitzy awards ceremony at London's Royal Opera House.
Luke Treadaway won the best actor award for playing 15-year-old Christopher, a maths prodigy with autism who sets out to solve the mystery of who killed his neighbor's dog.
"The book created such an amazing central character who people seem to relate to even though he has behavioral problems and his way of viewing of the world," Treadaway, 28, told Reuters on the red carpet on a chilly London evening.
Reporting by Belinda Goldsmith; Editing by Jon Hemming

subset: commoncrawl


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Federal Jury Convicts Man of String of Armed Robberies of Las Vegas Convenience Stores During 2013
LAS VEGAS, NV—Following an 11-day federal jury trial, a man accused of robbing 13 convenience stores and a small casino in the Las Vegas area during 2013 was convicted today on all counts charged, announced Daniel G. Bogden, United States Attorney for the District of Nevada.
Abdul Howard, 48, of Las Vegas, was convicted of one count of felon in possession of a firearm, 14 counts of interference with commerce by robbery, and 12 counts of possession of a firearm during, in relation to, and in furtherance of a crime of violence. Howard is scheduled to be sentenced by U.S. District Judge Gloria M. Navarro on November 6, 2014, at 9:00 a.m. and faces mandatory life in prison.
"Violent criminals such as Abdul Howard are a threat and danger to the entire community," said U.S. Attorney Bogden. "We are pleased to be able to assist local law enforcement and use federal laws in these types of cases, which carry stiff mandatory minimum penalties and ensure that career criminals such as Mr. Howard are incarcerated for very long periods of time."
According to the court records and evidence introduced at trial, Howard robbed 13 convenience stores and one casino in the Las Vegas area between January 15, 2013 and April 16, 2013. Howard robbed most of the businesses late at night using a semi-automatic handgun that he used to threatened store employees and some customers. In some of the robberies, Howard pointed the handgun at an employee or stuck a gun into the employee's body or head. In one instance, Howard fired a handgun at an employee, and in another, Howard shot an employee in the neck. Investigators dubbed Howard the Cinched Hoodie Robber, because he typically entered the businesses wearing a hooded sweatshirt with the hood "cinched" up around his face in an effort to conceal his identity.
Howard has at least five prior felony convictions in New York, Florida, and Nevada related to robbery, burglary, cocaine distribution, and sexually motivated coercion.
This case was investigated by the FBI, Las Vegas Metropolitan Police Department Robbery Section, and North Las Vegas Police Department, as part of the Safe Streets Task Force and Project Safe Neighborhoods program and prosecuted by Assistant U.S. Attorneys Phillip N. Smith, Jr. and Cristina D. Silva.


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Issued on: Modified:
Indian police in New Delhi on Saturday clashed with demonstrators protesting the gang rape of a young woman aboard a bus earlier in December. Police employed batons, tear gas and water cannons, injuring several protestors in the scuffle.
Police in India's capital used tear gas and water cannons Saturday to push back thousands of people who tried to march to the presidential mansion to protest the gang rape and brutal beating of a 23-year-old student on a moving bus.
Some protesters suffered injuries when they tried to break through steel barricades in a high-security zone in New Delhi. Police fired tear gas and chased the protesters with sticks, and some of the protesters attacked police with stones.
The protesters were demanding the death penalty for the five suspects who have been arrested by police following the Dec. 16 attack in New Delhi. Police are still looking for the sixth and final assailant.
The government said Friday that it is seeking life sentences for the assailants.
The attack sparked days of protests across the country from women demanding authorities take tougher action to protect them against the daily threat of harassment and violence. On Friday, Indian officials announced a broad campaign to protect women in New Delhi.
Some of the protesters Saturday carried placards reading "Save women. Save India" and "Hang the rapists."
V.K. Singh, a retired Indian army chief, joined the protesters and blamed "political and bureaucratic apathy for crimes against women." He demanded immediate police reforms to train and arm security forces.
C.P.N. Singh, a junior home minister, appealed to the demonstrators, who were mostly students, to protest peacefully and avoid vandalizing government property.
"The government is hearing you and taking steps necessary to ensure the safety of women," Singh told reporters.
Ravi Shankar Prasad, a spokesman for the main opposition Bharatiya Janata Party, however, condemned the police action against the protesters and asked government leaders to talk to them.
Police reinforcements rushed to the area as the crowds of protesters swelled Saturday morning on the lawns near Parliament House. The area is a high-security zone, with the presidential mansion, the prime minister's office and various ministries located there.
The victim and a companion were attacked after getting a ride on a chartered bus. Police said men on the bus gang-raped the woman and beat her and her companion with iron rods as the bus drove through the city for hours, even passing through police checkpoints. The assailants eventually stripped the pair and dumped them on the side of a road.
The victim is recovering from injuries in a New Delhi government hospital.


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Walking into Laduree in Paris is a bit like walking into Tiffany or Cartier: it is one of those supremely luxurious places that has the ability to make you feel fancy by simply walking through the door.
Of course, while both are luxury brands, buying a few of the delights spun from sugar at Laduree is far more reasonable to the typical shopper than shelling out cash for something silver (or gold, or platinum, or diamond-studded) from Tiffany.
Not only is it a delightful place to visit, but it's an important landmark in the world of pastry: founded in 1862, the cafe pioneered the concept of the salon de thé. Per the Laduree site:
Under the Second Empire, cafes developed and became more and more luxurious. They attracted Parisian high society. Along with the chic restaurants around the Madeleine, they became the showcases of the capital.
The beginning of this century found Paris wrapped up in a frenzy of distraction and going out in public. Parisians flocked to the Universal Exposition. Women were also changing. They wanted to make new acquaintances. Literary salons and literature circles were outmoded.
Ernest Ladurée's wife, Jeanne Souchard, daughter of a well-known hotelier in Rouen, had the idea of mixing styles: the Parisian café and pastry shop gave birth to one of the first tea salons in town. The "salon de thé" had a definite advantage over the cafés: they permitted ladies to gather in freedom. Jeanne Souchard succeeded in combining the turn-of-the-century trend to modernism with knowledge of the merits of a craft transmitted by her family.
So you can probably see why visiting Laduree is one of those pivotal pastry experiences that every sweet tooth should experience at least once (even if the company which now owns it, Holder, is responsible for putting macarons in French McDonalds too).
While they are perhaps best known for their macarons, on this visit, I had my eye not on the little sweetburgers but on their iconic and infinitely lovely religieuse.
A religieuse is a pastry which is said to take its name from its resemblance to a nun's habit--but being composed of choux pastry filled with thick custard and topped with delicate and pretty icing with buttercream piping on the sides, some harcore pastry lovers might argue that the name stems from its taste, which approaches an absolutely religieuse experience.
And at Laduree, they have a few different flavors; we chose the intriguing Blackcurrant-Violet, which is described as "Choux pastry, blackcurrant & violet flavoured confectioner's custard."
As a general rule, I am not a huge fan of lavender or rose-infused pastries, which I feel often can err toward tasting a bit perfumey. However, if there is one that could turn me around, this would probably be it: while assertively flavored, the violet flavor is beautifully done: buttery and floral and full. But like I said, it's powerful--I don't think I could polish one of these off in the same way that I might attack, say, a chocolate variety, but it sure was a delight to share and savor with others (we shared it among a group of four).
But as always, it was a delight to visit Laduree. Next on my list to try there, though? The Marie-Antoinette, an exquisitely appointed little cake...or maybe the mont blanc?
Laduree has various locations in Paris and beyond; for locations and more information, visit laduree.fr. And as a P.S., if you want to try making your own religieuse pastries, why not check out this excellent post on Not Quite Nigella?


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Essential tremor is a type of involuntary shaking movement in which no cause can be identified. Involuntary means you shake without trying to do so.
See also:
Essential tremor is the most common type of tremor. Everyone has some tremor present, but the movements are often so small that they can't be seen. Essential tremors are most common in people older than 65.
The exact cause for essential tremor is unknown. Tremors occur when there is a problem with the nerves that supply certain muscles. Some research suggests that the part of the brain that controls muscles movements does not work correctly in patients with essential tremor.
Essential tremor can also occur with other brain and nervous system problems, such as dystonia, parkinsonism, and certain nerve conditions passed down through families.
If an essential tremor occurs in more than one member of a family, it is called a familial tremor. This type of essential tremor is passed down through families (inherited), which suggests that genes play a role in its cause.
Familial tremor is usually a dominant trait, which means that you only need to get the gene from one parent to develop the tremor. It often starts in early middle age, but may be seen in people who are older or younger.
The tremor is more likely to be noticed in the hands, but may affect the arms, head, eyelids, or other muscles. The tremor rarely affects the legs or feet. People with essential tremor may have trouble holding or using small objects such as silverware or a pen.
The shaking most often involves small, rapid movements -- more than 5 times a second.
Specific symptoms may include:
The tremors may:
Your doctor can make the diagnosis by performing a physical exam and asking questions about your medical and personal history.
A physical exam will show shaking with movement, usually small movements that are faster than 5 times per second. There are usually no problems with coordination or mental function.
Further tests may be needed to rule out other reasons for the tremors. Other causes of tremors may include:
Blood tests and imaging studies (such as a CT scan of the head, brain MRI, and x-rays) are usually normal.
Treatment may not be needed unless the tremors interfere with your daily activities or cause embarrassment.
For tremors made worse by stress, try techniques that help you relax. For tremors of any cause, avoid caffeine and get enough sleep.
For tremors caused or made worse by a medication, talk to your doctor about stopping the drug, reducing the dosage, or switching. Do NOT change or stop medications on your own.
Severe tremors may make it harder to do daily activities. You may need help with these activities. Devices may help with everyday activities, such as:
Buying clothes with Velcro fasteners, using button hooks
Cooking or eating with utensils that have a larger handle
Using straws to drink
Wearing slip-on shoes and using shoehorns
Medicines may help relieve symptoms. The most commonly used drugs include:
The drugs can have side effects.
Other medications that may reduce tremors include:
Botox injections, given in the hand, have been used to reduce tremors by weakening local muscles.
In severe cases, surgery may be tried. This may include:
An essential tremor is not a dangerous problem, but some patients find the tremors annoying and embarrassing. In some cases, it may be dramatic enough to interfere with work, writing, eating, or drinking.
Sometimes the tremors affect the voice box, which occasionally leads to speech problems.
Call for your health care provider if
Alcoholic beverages in small quantities may decrease tremors but can lead to alcohol dependence and alcohol abuse, especially if you have a family history of such problems. How alcohol helps relieve tremors is unknown.
Tremor - essential; Familial tremor; Tremor - familial
Deuschl G, Raethjen J, Hellriegel H, Elble R. Treatment of patients with essential tremor. Lancet Neurol. 2011 Feb;10(2):148-61.
Reviewed by: Kevin Sheth, MD, Department of Neurology, University of Maryland School of Medicine, Baltimore, MD. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine;David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
Read More
Notice: The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. Copyright 1997-2012, A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.


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Undergraduate Degrees, Majors and Concentrations
COMM3140 - Health Communication Campaign
Summer 2019
This course is designed to review various levels of the health communication campaign process and the basic types of health communication programs that address health awarenenss and behavior change. This course will introduce students to advanced theories and concepts of health behavior change, and methods of health communication strategies, message development and execution. Students will learn how to create and test effective health messages, identify what media works best for health messages distribution and target audiences, and how to evaluate the effectiveness of health messages.
Prerequisite: COSK2230 or COSK2245
3 Credits
Schedule Book for All Active and Available Future Terms, Course starting with COMM3140
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Pleasant Valley senior Dakota Everett didn't get a chance to play for the conference title two years ago. Now the Bears' linebacker is a big reason Pleasant Valley has a shot to win the Mountain Valley Conference title again.
BUSHKILL — Pleasant Valley senior Dakota Everett didn't get a chance to play for the conference title two years ago.
Now the Bears' linebacker is a big reason Pleasant Valley has a shot to win the Mountain Valley Conference title again.
Everett spearheaded the PV defensive attack, limiting East Stroudsburg North to just 45 yards of offense in the second half as Pleasant Valley rallied from two touchdowns behind to beat North 30-21 on Friday night.
Pleasant Valley (7-2, 7-0 MVC) will take on Stroudsburg (8-1, 7-0) in a winner-take-all game next Friday night. The Mounties defeated Dieruff, 51-13, in Allentown on Friday night for its eighth straight win.
"They're a great set of athletes and so are we," said Everett who missed the title game two years ago with an injury. "So we're going to get on the field and we're going to see who comes out on top."
It looked as though PV's game with Stroudsburg next week wasn't going to mean very much. North started the game by running all over the Bears' defense.
Lorenzo Melchiorre scored twice, while Abdul Murphy got into the end zone once in the first half to stake Eastburg North to a 21-6 halftime lead.
Pleasant Valley was stunned, and its chances of a MVC title were about to slip away. PV coach Jim Terwilliger said he didn't need to say much at halftime, the players were saying plenty, when the team rallied to play its best half of football all year.
"That was the most inspiring second half of football that I've seen in all of my coaching," Terwilliger said. "We didn't have time, our backs were against the wall. With North being the offense that they were, we had to play inspired in all areas. It was outstanding defense."
After watching North treat the game like a track meet in the first half, the Timberwolves were shut down in the second half. Murphy had the best night, running for 117 yards, but 72 came in the first half.
On the other side, PV's offense came to life. Freshman running back Austyn Borre finished with 80 yards after having just 27 in the first half.
But it was a special teams play that swung the momentum for good. Trailing 21-13 after a Brandon Leap to Andrew Romeo TD pass, PV forced North to punt.
That's when Tony Bell returned the kick 62 yards and a two point conversion tied the game at 21.
"We needed something big and Tony's punt return down the sideline I thought was a game-changing play," Terwilliger said.
PV took a 24-21 lead on a Jordan Caffrey 21-yard field goal with 11:50 left in the fourth.
North (6-3, 5-2) answered. The Timberwolves marched to the PV 6, but on fourth and 6 at the 9, Everett knocked down a Melchiorre pass to end the threat.
PV drove 90 yards, eating up clock as they marched and Josh Thornton put the game out of reach with a 4-yard run for the final points.


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South African Journal of Science
versão On-line ISSN 1996-7489
versão impressa ISSN 0038-2353
S. Afr. j. sci. vol.105 no.9-10 Pretoria Set./Out. 2009
Femtosecond pump probe spectroscopy for the study of energy transfer of light-harvesting complexes from extractions of spinach leaves
S. Ombinda-LemboumbaI, II; A. du PlessisI, II, *; R.W. SparrowIII; P. MolukaneleI, III; L.R. BothaI, II; E.G. RohwerII; C.M. SteenkampII; L. van RensburgIV
ICSIR National Laser Centre, P.O. Box 395, Pretoria 0001, South Africa
IILaser Research Institute, Department of Physics, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
IIICSIR Biosciences, P.O. Box 395, Pretoria 0001, South Africa
IVEnvironmental Science and Management, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa
Measurements of ultrafast transient processes, of temporal durations in the picosecond and femtosecond regime, are made possible by femtosecond pump probe transient absorption spectroscopy. Such an ultrafast pump probe transient absorption setup has been implemented at the CSIR National Laser Centre and has been applied to investigate energy transfer processes in different parts of photosynthetic systems. In this paper we report on our first results obtained with Malachite green as a benchmark. Malachite green was chosen because the lifetime of its excited state is well known. We also present experimental results of the ultrafast energy transfer of light-harvesting complexes in samples prepared from spinach leaves. Various pump wavelengths in the range 600680 nm were used; the probe was a white light continuum spanning 420700 nm. The experimental setup is described in detail in this paper. Results obtained with these samples are consistent with those expected and achieved by other researchers in this field.
Key words: pump probe spectroscopy, ultrafast spectroscopy, transient absorption, femtosecond laser, light-harvesting complex II, Malachite green
Photosynthesis is the process by which light energy is converted into chemical energy and then, through a complex series of biochemical and biophysical processes, to any one of a number of bio-machines.1 There is a large diversity of photosynthetic organismshigher plants, cyanobacteria and photosynthetic bacteria2 for which some fundamental processes are common. These are light absorption, excitation energy transfer, charge separation and vectoral charge transfer.1 In the photosynthetic process, chlorophylls are light-absorbing pigments which bind to proteins and are also known as pigment proteins. These pigment proteins have two main functions. They form light-harvesting antenna complexes which are organised into an antenna system to absorb light and transfer the excitation energy to the second functional entity called a reaction centre. It is at the reaction centre that charge separation and transfer occurs.1,3
In the natural state there is a high degree of disorder within the system. This is due to natural light from the sun irradiating the photosynthetic pigments with a range of different wavelengths of light which the pigments can absorb. This means that the energy of the photons, the intensity of the light and the time between photon excitations occurs in a random manner. The environmental conditions in which these systems grow also vary.4 In order to investigate the processes of light absorption and energy transfer, these variables need to be reduced, and a number of strategies are used. Over recent years, several photosynthetic components have been isolated and detailed structural information obtained using crystallographic data57 and atomic force microscopy.810 A significant amount of the work on elucidating the structures of energy transfer processes within light-harvesting complexes (LHCs) and reaction centres has been conducted on purple photosynthetic bacteria.2 In photosynthetic organisms, such as the bacteria Rhodobacter sphaeroides, there are two types of light-harvesting antenna complexestypes 1 and 2. These are termed LH1 (B875) and LH2 (B800/850). In the more general case of non-bacterial organisms, these light-harvesting complexes are termed LHC I and LHC II. LHC II acts as a light-absorbing and transferring complex. LHC I is closely associated with a reaction centre.11,12 In all species of purple bacteria, the LHC I surrounds the reaction centre.2 However, LHC I and LHC II are constructed on the same modular basis.13,14 This organisation of reaction centres surrounded by aggregates of chlorophylls and associated carotenoids, seems to be universal to photosynthetic bacteria and higher plants and is called the photosynthetic unit.15 When photons are absorbed by one of the chromophores in the LHC II antenna complex, non-radiative photophysical processes occur in a series that channels the excitation energy with high efficiency from the site of absorption. There are two types of ultrafast photophysical processes that occur. One type is energy migration (exciton process), which is the reversible series of energy transfer processes that occurs between identical coupled chromophores in this case between LHC II antenna complexes of the photosynthetic membrane. The other type is energy transfer, which refers to the down-hill energy transfer process which will then terminate the exciton process16 within the photosynthetic unit. Energy transfer takes place in the order LHC II → LHC I → reaction centre. An irreversible electron transfer reaction occurs at a special pair of chromophores in the reaction centre, preventing any possible back transfer of energy.16 Both of these occur in the singlet state of an excited pigment.16 The energy transfer is a sequential process, is multi-exponential and occurs in a total time of less than 100 ps, at about 95% efficiency.15 Within a bacteriochlorophyll ring, it is estimated that energy transfer around the ring takes place in a few hundred femtoseconds.17,18 The LH2 → LH1 transfer time is 35 ps19 and energy transfer from LH1 → reaction centre is 3050 ps.20 This longer time is due to the greater LH1reaction-centre distance. Energy migration times are strongly dependent on the relative orientation of the transition moments within the macrocycle array.
One line of study is to isolate these functional components and study the energy transfer characteristics separately. The energy transfer processes of these photosynthetic systems can be investigated using ultrafast pump probe transient absorption spectroscopic techniques.2123 The use of ultrafast pulsed lasers enables the pigments to be excited in unison with a quasi-monochromatic light source. This pump pulse excites the pigments at a pre-set temporal origin, after which the energy transfer can be probed by a second pulse with an adjustable delay.4 In our experiment, the sample used was isolated LHC II from spinach leaves.
The LHC II is the most abundant antenna in the photosystem II. The function of the LHC II is to transfer excitation energy towards a reaction centre of either photosystem I (PS I) or photosystem II (PS II). Normally, there are four LHC II trimers per PS II reaction centre.24 In addition to LHC II, the PS II contains other complexes such as CP29, CP26, CP47, CP43, D1 and D2. The CP43, CP47, D1 and D2 form the PS II core complex.2527 In its native form, the LHC II is a trimeric protein. The structure of this complex was first described in 1994 by Kühlbrant et al.29 Each monomer contains 78 chlorophyll a molecules, 45 chlorophyll b molecules and 3 carotenoids, namely lutein, neoxanthin and violaxanthin.28,29 These chlorophylls have been assigned specific site energies. The site energies of chlorophyll a are 602604 and 610614, those for chlorophyll b are 601 and 605609.5 These chlorophylls are arranged in two separate layers: one close to the stromal surface and the other by the luminal surface with a distance ranging from 13.918 Å between them. Close to the stromal surface of the membrane are three clusters of chlorophylls. There is a chlorophyll a trimer (a610a611a612) and a chlorophyll a dimer (a602a603). These are closely associated with three chlorophyll b molecules (b601, b608 and b609). On the luminal side of the membrane there is a chlorophyll a cluster (a613 and a614), a group of chlorophyll b (b606, b607 and b605) and a single chlorophyll a (a604).30 When assembled, the LHC II trimer contains between 36 and 42 chlorophyll molecules as well as 10 to 12 xanthophyll (lutein, neoxanthin and violaxanthin) molecules. In vivo and in vitro, there is a further level of structural organisation that LHC II can adopt. This is either a regular lamellar sheet, usually 2D in shape, or, alternatively, in a 3D or disordered 3D structure.31 Thus, LHC II has a large degree of flexibility in its structural organisation, with accompanying changes in optical properties.
The interpretation of pump probe transient absorption data is highly complex and the results obtained are influenced by many factors, such as the degree of aggregation of the LHC II25 or whether the complexes are in the monomeric or trimeric state.25,32 However, in this study, the main focus of the research is not on elucidating the photophysical processes of photosynthetic energy transfer. We propose to develop light-harvesting and energy-transfer systems with architectures to enable specific, site-directed energy transfer. Our aim is to characterise these systems with respect to the direction, efficiency and rate of energy transfer from the site of photon absorption to the terminal energy transfer process. We report these preliminary results as a proof of principle of our experimental setup, using Malachite green dye, and to indicate the presence of the above-mentioned energy transfer processes in LHC II extractions from spinach leaves.
Many biological and chemical processes occur on relatively long time scales (from milliseconds to seconds). These long processes are the result of combinations of several very fast elementary processes ranging from femtoseconds to picoseconds.33 This generally is referred to as the ultrafast or ultrashort pulse regime. In order to observe ultrafast processes, such as the translation or rotation of part of a molecule, energy transfer, charge transport or the formation or breaking of a chemical bond, a measurement technique with a temporal resolution in the range of femtoseconds to picoseconds is required. However, most detectors are not fast enough to resolve such time scales.
Femtosecond transient absorption (pump probe) spectroscopy has become a widely-used and useful technique for studying ultrafast processes.34
In the pump probe spectroscopy technique, a beam consisting of a train of ultrafast laser pulses is split into pump and probe beams. The pump and probe beams are sent along different optical paths, and then focused into a sample with a time delay between them.35 The pump is used to trigger a photoinduced process in the sample and the probe beam is used to probe the process, for example molecules or atoms in their excited states. The evolution of this process is then followed by monitoring the delayed probe pulse, for example by absorption, and varying this delay time. The resulting spectra are therefore measured at a range of delay times to provide transient absorption at a range of wavelengths.
The pump-induced variation of the probe absorbance (ΔA)asa function of delay time can be measured and analysed. This is done by observing pumped as well as non-pumped probe spectra. Using the Beer-Lambert law for linear absorption in a liquid sample, the ΔA can be written as:
where II is the incident intensity and Isignal the transmitted intensity. Considering that Iref is proportional to II in the setup depicted in Fig. 1, the absorbance change is given by:
This equation indicates that the absorbance change (ΔA)isa function of the ratio of the reference and probe signals at a given wavelength, measured after the sample excitation ('on'), and of the ratio of the same signals measured in an unexcited sample ('off').
In order to benchmark our experimental setup, Malachite green dye was used as a sample. Several spectroscopic studies have been conducted on Malachite green. Previously measured ultrafast pump probe signals have shown that Malachite green has an ultrashort electronic excited state lifetime that depends on the solvent viscosity but is in the region of 3 ps.36 In another study, transient absorption signals of Malachite green in ethanol solution, pumped at 580 nm and probed at 620 nm, showed a fast kinetic process with a time constant of approximately 2.1 ps.37
There has been considerable research directed towards developing artificial photosynthetic systems and a number of groups have synthesised artificial light-harvesting, energy migration and charge transfer components.3841 In this collaborative research effort between the Femtosecond Science Group and the Synthetic Biology Group of the Council for Scientific and Industrial Research (CSIR), the aim was to use ultrafast pump probe spectroscopy to determine energy transfer times experimentally for both natural and artificial light-harvesting systems. It is envisaged that we will explore a range of pigments from higher plants as well as photosynthetic bacteria arrayed in a variety of organisational configurations. One of the goals is to establish the distance over which energy can be transferred. It is also envisaged that polymeric structures will be used as an architectural scaffold onto which the pigments will be attached. With this arrangement it should be possible to develop multi-pigment systems for directed energy transfer.
Sample preparation and characterisation: LHC II
Isolation and characterisation of loosely stacked lamellar aggregates of LHC II with long-range chiral order
LHC II samples were prepared from spinach leaves (with good turgor and dark green colour) as described before42 with a few modifications to suit the experimental conditions. The leaves were grown at a local greenhouse and the entire experiment was performed under normal laboratory light and on ice. spinach leaves were washed with cold water and chilled distilled water, and then homogenised in a blender with ice-cold buffer A (50 mM Hepes, 0.4 M sorbitol, pH 7.8 with NaOH). The suspension was filtered through four layers of muslin and then centrifuged at 5 000 g for 5 min at 4ºC. The pellet was resuspended in cold buffer B (20 mM Hepes, 5 mM EDTA, 50 mM sorbitol, pH 7.8 with NaOH). A medium-sized brush was used for crude homogenisation of the pellet to ensure all material was finely homogenised without large visible particles. The suspension was again centrifuged at 10 000 g for 10 min at 4ºC. The pellet was resuspended in cold buffer C (20 mM Hepes, pH 7.8 with NaOH) and again homogenised as above. Triton x-100 was added to the suspension to obtain the final detergent concentration of 0.70.9%, and the suspension was then stirred continuously on ice and in the dark for at least 45 min, followed by centrifugation at 30 000 g for 40 min at 4ºC. The supernatant predominantly contained LHC II and hence was kept at this stage and the pellet was discarded. All the centrifugation procedures were carried out using a Beckman Coulter, Avanti J-30I centrifuge. The preparations can be stored in the dark at 87ºC for up to a month without the loss of their lamellar structure. Thawing and refreezing can lead to complete destruction of these properties, therefore a sample can be used only once. The prepared sample was diluted in Buffer C to obtain an absorbance of ~1 in a 1-cm optical pathlength cuvette at 430 nm for all the characterisation methods as well as the pump probe experiments. The chlorophyll content of the sample was calculated using a method based on Arnon's equations: Add 50 µl sample, 10 cm3 of 80% v:v acetone, filter and read absorbance at 665 nm and 649 nm. Arnon's equations are given by:
Chlorophyll a = [(A665 × 11.63) (A649 × 2.39)] × 0.2 mg cm3
Total chlorophyll = [(A665 × 6.4) + (A649 × 17.72)] × 0.2 mg cm3
Chlorophyll b = Total chlorophyll chlorophyll a,
where A665 and A649 are the absorbance of the sample at 665 nm and 649 nm. By using these equations we found the following values: Chlorophyll a = 1.68 mg cm3, chlorophyll b = 0.69 mg cm3, total chlorophyll content = 2.38 mg cm3 and chlorophyll a/b ratio = 2.4:1
Characterisation methods
Circular dichroism measurements
Circular dichroism (CD) was measured at room temperature using the Chirascan Circular Dichroism Spectrometer (Applied Photophysics) and the results can be presented in units of absorbance or millidegrees (mdeg). LHC II, when isolated with mild detergent treatments, readily assembles into loosely stacked lamellar aggregates, which exhibits a significant degree of long-range chiral order of their chromophores. Long-range chirality is in turn essential for long distance energy migration. This characteristic of LHC II is essential for the purposes of this study: to prepare a LHC II sample that can undergo energy migration. This is evident when the CD spectrum of LHC II is observed in Fig. 2. The asymmetric bands (psi-type CD band) at ~430505 nm and ~675 nm indicate the formation of large macro-aggregates with long-range chiral order within the sample, which allows energy transfer.
Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE)
A SDS-PAGE gel of 12% acrylamide was cast (using a Bio-Rad mini-protean tetra cell and PowerPac universal). This technique was performed to identify proteins in the sample mixture. Figure 3 illustrates the results obtained from performing the SDS-PAGE using the extracted LHC II sample at room temperature. Lane 1 contained protein molecular makers, while Lanes 2 to 7 contained LHC II sample buffered with a 5× sample buffer in different concentrations to obtain optimal resolution. Lanes 2 to 4 contained a sample that was more concentrated and gave a better resolution, while Lanes 5 to 7 contained sample that was strongly diluted (with little protein) and hence very difficult to see using Coomassie blue staining solution. Results obtained from this technique showed that the prepared sample contained mostly LHC II complexes and this can be seen from the broken-down complexes of PS II, as shown in Fig. 3. CP47 (45k50kDa) and CP43 (35 kDa), which are complexes from the peripheral core complex of PS II, together with D1 and D2 proteins closely associated to the reaction centre, were present; pure LHC II (56 kDa) was also present. All these pigment protein complexes could be detected using Coomassie blue staining solution. The gel also indicated the presence of small subunits (2025 kDa) from LHC I and CPI. This implies that the sample was not purely LHC II; it contained some components from LHC I as well as some from PS II reaction centre.
Experimental setup
A generic femtosecond pump probe experimental setup was used and is described below. A commercial Ti:sapphire femtosecond oscillator (Coherent Mira 900-F), operating at a repetition rate of 76 MHz and pumped by the 5 W output of a continuous diode-pumped Nd:YVO4 laser (Coherent Verdi V5), produced pulses of approximately 130 fs duration, as measured by a home-built autocorrelator. These pulses were stretched and then amplified by a regenerative Ti:sapphire amplifier (Coherent Legend-F) pumped by a diode-pumped, Q-switched Nd:YLF laser (Coherent Evolution) at a repetition rate of 1 kHz, and finally compressed to produce 130-fs pulses of up to 1 mJ per pulse. This pulse duration was also measured by a home-built autocorrelator. These pulses have a bandwidth of about 8-nm full-width-at-halfmaximum (FWHM) and a central wavelength of 795 nm.
As shown in Fig. 4, the amplified beam is split by a beam splitter (BS1) into two beams (90% transmitted and 10% reflected)the pump and probe beams, respectively. The probe beam is sent to a variable optical delay line, which comprises a retro-reflector mirror on a precision translation stage controlled by a computer. The probe beam is then focused on a 2.15-mm-thick sapphire plate to generate a white light continuum. A short pass filter is placed after the plate in the probe path in order to suppress the strong residual peak, at 795 nm, from the Ti:sapphire laser. The probe beam is split into two beams reference and signal. The signal is focused on the sample in such a way that it overlaps spatially with the pump beam in the liquid sample, while the reference beam is also sent through the sample, as indicated in Fig. 1. The pump pulse is sent through an optical parametric amplifier (TOPASCOPA)in order to obtain a wide tuning range of the pump beam (53020000 nm). After the optical parametric amplifier, a chopper is inserted in the pump beam path in order to record spectra that are classified as pumped and not-pumped, thereby reducing background effects. The probe reference and signal beams are focused on optical fibres which guide the beams into the entrance slit of an imaging spectrometer. The spectrometer (MS 2004 I) uses a Czerny-Turner optical configuration. The light, after passing through the entrance slit, is directed by a collimating spherical mirror onto a diffraction grating. The grating used in this study is 600 lines per mm diffraction grating, blazed at 500-nm for 330-nm to 1 000-nm operation. The grating disperses the entrance parallel beams (signal and reference beams), and these are again collimated, using a second collimating mirror, onto two vertically-separated photodiode arrays. This spectrometer disperses both beams to a maximum of 206 nm across the active area of the photodiode arrays. Each photodiode array has 1024 pixels and a spectral response range from 200 nm to 1000 nm. All pump probe measurements were performed at room temperature and in dark conditions. The sample was held in a rotating cuvette cell of 1-mm thickness. Unless otherwise specified, averaging was done over 100 pulses per measurement at each delay time. The entire pump probe setup described above is a commercial system from CDP systems, the ExciPro-D.
Experimental results and discussion
Malachite green
A white light continuum was generated using a sapphire plate in order to use this as a probe pulse in the pump probe experiment. Figure 5 shows spectra of the white light continuum, recorded using an Ocean Optics USB 2000 spectrometer. Two spectra are shown, with and without a short-pass filter, in order to illustrate the strong residual signal from the 795-nm laser output and hence the need for the short-pass filter. This white light was then split, using a 50:50 beam splitter, into two: the probe and reference beams. Malachite green in ethanol was used as a sample for this proof of principle experiment, to demonstrate the working of the system. A typical result of the pump probe transient absorption signal, at one particular wavelength, is shown in Fig. 6. In this experiment, the pump pulse energy was 1.5 µJ with a focused beam size of about 300 µm. The 610-nm absorption change as a function of delay time is presented here. Since the optical parametric amplifier was set also to 610 nm (pump wavelength), this is effectively a one-colour pump probe measurement, although the probe is in fact a white light continuum and therefore more information is available from a single experiment. The temporal delay time at which the strongest absorption change (0.045 OD) is observed is relative to the physical position of the retro-reflector in the optical delay line, and has not been calibrated for zero position. We observe a fast decay process after this point, which can be attributed to the excited state decay as expected. A single exponential fit of the decay data, which seems to fit reasonably well in this case, indicates a time constant of 2.9 ± 0.1 ps.
Figure 7 shows a contour graph of the measured transient absorption of the Malachite green for the entire wavelength range 500720 nm. It can be seen in this figure that the strongest absorption change occurs at about 620 nm and no other absorption changes of interest occur in this range, as expected.36,37 The maximum absorbance changes for our experimental measurements were between 0.055 OD and 0.060 OD, and the noise level was 0.002 OD. The temporal resolution depends on the pulse duration of the pump pulse (130 fs), the degree of chirp of the white light continuum and the step size of the delay line. The temporal resolution was better than 200 fs in all experiments reported here, and was obtained experimentally by inference from the data recorded for Malachite green, as indicated in Fig. 8; this resolution is a compromise between error margins obtained and the total time required for the completion of a single experiment.
White light continuum chirp measurement and correction
The white light continuum is obtained by focusing a femtosecond pulse onto an optical medium (a sapphire plate in our case). The propagation of the femtosecond pulse through this medium leads to spectral broadening (which is useful in this experiment), but also temporal lengthening of the pulse due to chirp. This chirp is due to the spectral components with low-frequency (red components) propagating faster than those with higher frequency (blue components). Therefore, different wavelength components in the probe pulse arrive at different times at the sample. It is important to correct for the chirp introduced by the white light generation in the analysis of the pump probe results.
The effect of this chirp is that the relative delay between the pump pulses and the different wavelength components of the white light spectrum will be different, and dependent on the amount of chirp induced. To correct this, one must compensate these delay differences, which requires that the kinetics of a known specimen, which exhibits a localised temporal response in the spectral range of interest, must be recorded. In our experiment, we used dyes as specimens with localised temporal response in the spectral range from 400 nm to 700 nm (Oxazine 170 and sulphorhodamine B dyes). Pump probe measurements were done on Oxazine 170 and sulphorhodamine B, and the absorbance change spectra and kinetics were recorded. These experiments were carried out with a pump energy measured at the laser output of 35.51 µJ, which resulted in roughly 1.2 µJ before the sample, by the appropriate use of filters. The goal was to plot relative time delay versus the wavelength of the dye. For every wavelength of the Oxazine 170 recorded spectra (Fig. 9, inner graph), a corresponding kinetics graph (Fig. 9, outer graph) was determined and a corresponding delay time, which corresponded best to the overlap between pump and that particular probe wavelength, was extracted. When a kinetics spectrum is obtained, 90% and 10% of the maximum value of the absorbance change is determined. In order to find the corresponding time delay of a specific wavelength, first a value of absorbance change at half of the difference between these two values (90% and 10% of maximum absorbance change) is found. Then a corresponding time is determined, as indicated in Fig. 9. A set of relative delay time values are then obtained for the 400700-nm spectral range. It is therefore possible to plot the relative time delay against the wavelength. Figure 9 shows the plot of relative time delay against the wavelength of two different dyes: Oxazine 170 and sulphorhodamine B. As expected, it was observed that, under the same conditions, the two dyes exhibited the same behaviour (Fig. 10). In Fig. 10, it can be seen that, as the wavelength increases (from blue toward red) the relative time delay increases and the white light spectrum in our experiment, which ranges from 410 nm to 710 nm, has a total temporal span of about 1.6 ps. To obtain the chirp correction equation, data from the plot of relative time delay against wavelength, using Oxazine 170, was polynomial fitted. Figure 10 shows the third-order polynomial fit, as well as the polynomial equation (in red). This equation was used to correct for the chirp introduced by the white light in further analyses of other samples.
Light-harvesting complex II
The absorption spectrum of our samples containing LHC II, extracted from spinach leaves, was recorded using a simple Ocean Optics lamp and spectrometer (USB2000) at room temperature, and is shown in Fig. 11. This spectrum shows similar shapes and peak positions to those reported in the literature.43 The absorption spectrum shows one main peak in the red region at about 675 nm, which is due mainly to chlorophyll a,44 and a shoulder at 650 nm, which is related mostly to chlorophyll b. In addition, the absorption spectrum exhibits a broadband from 400 nm to 520 nm, with the main peak positioned at 440 nm and shoulders on both sides. The peak at 440 nm represents chlorophyll a, while the peak at 467 nm represents chlorophyll bboth in the blue absorption region of LHC II.29
The preparation of the LHC II from PS II also indicated the presence of CP47 and CP43, which are complexes from the peripheral core complex of PS II, and D1 and D2 proteins closely associated to the reaction centre, as shown in Fig. 3. We also noticed the presence of LHC I residual in the sample, as indicated in Fig. 3. By using a set of adjustable filters, the pump pulse energy was set to 2 µJ and the beam size at the focus was about 300 µm. The sample was inserted into a rotating glass cell to ensure fresh sample for every pulse, in order to avoid sample degradation and to prevent the accumulation of long-lived states. Figure 12 shows the transient absorption spectra obtained in the region from 350 nm to 800 nm, although the white light continuum only had significant intensity from 420 nm to 700 nm. In this case, in order to excite both chlorophyll a and chlororphyll b components, the LHC II sample was pumped by a wavelength of 610 nm. Although 610 nm does not correspond to the peak absorption wavelength, our system provided high pulse energies and good pulse to pulse stability at this wavelength. Figure 12 shows five broadband absorbance changes of interest. The negative absorbance changes with peaks at 436 nm and 678 nm correspond to ground-state bleaching of chlorophyll a. The pump excites many molecules into the higher excited state, which results in a reduction of the number of molecules in the ground state. Therefore, when the probe pulse arrives, there are fewer molecules in the ground state, causing negative absorbance changes. The broadband positive absorbance change (about 500640 nm), with a peak at about 542 nm, is due to excited state absorption. This signal is caused by the excited state population, which is then excited further to a higher energy level by the probe pulse. This causes an increase of the absorption of the probe beam. In addition, shoulder bands observed at 472 nm (Fig. 12, outer graph) and 650 nm (Fig. 12, inner graph) can be ascribed to ground-state bleaching of chlorophyll b. However, the large contribution of the broadband excited state absorption (positive absorbance change from 500 nm to 640 nm) resulted in a less pronounced chlorophyll b shoulder at 650 nm than expected. The peak observed at 610 nm is as a result of laser scatter from the sample, due to particles in the sample, indicating imperfect sample preparation. At 610 nm, chlorophyll a and b are excited and energy transfers are expected to occur between the chlorophyll molecules. Several studies on the energy transfer between the chlorophylls have shown that the transfer usually takes place from chlorophyll b to chlorophyll a.3,45 In addition to the transfer from chlorophyll b to chlorophyll a, there is also energy equilibration that takes place from chlorophyll a to chlorophyll a and from chlorophyll b to chlorophyll b. The LHC II monomer is composed of 78 chlorophyll a molecules, 45 chlorophyll b molecules and 3 carotenoids, at a chlorophyll a:b ratio of 1.6:2. The energy transfer from chlorophyll b to chlorophyll a, within this monomer, exhibits three components: a very fast component that occurs at about 100150 fs and two other components that have lifetimes of 600800 fs and 57 ps.45
In order to obtain complete information on the five bands observed in Fig. 12, a contour graph can be plotted (Fig. 13). Figure 13 shows the transient absorption of LHC II as a function of wavelength and relative delay time between pump and probe. It is possible to observe the pump at 610 nm in this case, which remains constant over all experimental time delays and is due to laser scatter. In addition, the variation of the ground-state bleaching at 678 nm and 436 nm, and the excited state absorption with a peak at about 542 nm, can be clearly observed.
To extract further information regarding the time constants of the energy transfer of the LHC II from the experiment, the spectra are actually measured at a large number of delay times, such that specific fast absorption changes can be temporally quantified and their time constants extracted. Figure 14 shows the evolution of the absorbance changes of 678 nm (the strongest absorption in Fig. 11), 542 nm, 472 nm and 436 nm as a function of delay time. In the 678-nm, 472-nm and 436-nm bands, we observed a fast drop in the absorbance change to quickly reach a minimum, followed by a slow recovery. In the 542-nm transient we observed a fast increase followed by a slow downturn. To obtain the time constants, we applied a second-order exponential fit procedure to the data set obtained at each peak (436 nm, 472 nm, 548 nm and 678 nm). This was done because we expected the sample to exhibit at least two separate energy transfer mechanisms with different time constants, and because a single exponential did not fit the data well. The experimental values of the energy transfer time constants are tabulated in Table 1. The results of the second-order exponential fit gave us two time constants, 1.3 ± 0.5 ps and 25 ± 4 ps, for the 678-nm peak. The 1.3-ps time constant can be ascribed to singlet-singlet annihilation within the LHC II monomer and the 24.5-ps component to the annihilation in the trimer. Comparable time constants, 1.9 ps and 20 ps, for monomer and trimer annihilation, respectively, at 679 nm have been reported.46 This process could limit the energy transfer.46
In the blue absorption region, we were able to detect the time constants of energy transfer between chlorophylls. The 436-nm peak relates to chlorophyll a and gives us two time constants (860 ± 200 fs and 45 ± 4 ps), whereas the 472-nm peak relates to chlorophyll b and gives two time constants (750 ± 100 fs and 29 ± 2 ps). The 29-ps and 45-ps time constants are not accurate extracted values since the measurement was done only over a total range of 15 ps. However, the time constant of 750 fs is related to the energy transfer from chlorophyll b to chlorophyll a and is reflected in the disappearance of the chlorophyll b bleaching (472-nm peak) after about 800 fs, as indicated in Fig. 12. The time constant of the major component of energy transfer from chlorophyll b to chlorophyll a was reported in the literature to be about 600 fs.3,4548
Time constants obtained for 542 nm are 4.76 ± 1 ps and 43 ± 6 ps, again the latter value is less accurate due to the limited range of the experimental measurement. This broadband feature, from 500 nm to 600 nm (with peak at 542 nm), is related to the relaxation of the vibrational energy of the excited state of the chlorophyll molecules. Upon excitation at 610 nm, the molecules are excited to a high vibrational energy level of the electronic excited state. The molecules will then undergo vibrational relaxation. Therefore the probe gives us a broadband spectrum which is the overlap of absorption spectra of molecules in different vibrational energy levels.
The time constants obtained for LHC II for the absorbance changes at 678 nm correspond well with those expected, while the others are less accurate, probably due to a long-lived state being populated and not resolved temporally here due to its long lifetime. The observation of a long-lived state could be due to the presence of LHC I, D1 and D2 residue in our sample. These complexes are expected to have long lifetimes, in the order of hundreds of picoseconds up to the nanosecond regime.
The transient absorption experiment was also carried out at different pump wavelengths in order to compare results obtained for the energy transfer times from different degrees of pump excitation. Figure 15 indicates the absorbance changes of the same sample, containing LHC II, pumped at different wavelengths: 610 nm, 630 nm, 650 nm and 680 nm at zero time delay (or strongest absorption change). As expected, we observed the strongest absorbance changes, about 0.052 OD, when the pump was set at 680 nm. The pump of 650 nm, 630 nm and 610 nm produced the strongest absorption changes of 0.027 OD, 0.020 OD and 0.013 OD, respectively. By changing the pump wavelengths, we excited different pigments (chlorophyll a, chlorophyll b or chlorophyll a/b), which resulted in different energy transfer processes taking place; in other words, different combinations of chlorophyll b to chlorophyll a energy transfers and chlorophyll a to chlorophyll a energy transfers occurred. Different time constants are therefore expected.
Time constants extracted from each of these experiments for the 678-nm peak are given in Table 2. By setting the pump at 650 nm, which is the strongest chlorophyll b absorption wavelength, we were able to observe two time constants, 0.81 ± 0.3 ps and 9.95 ± 1 ps, related to the energy transfer from chlorophyll b to chlorophyll a. These values compare well with the values of 600 fs and 57 ps reported in the literature.3,45,47,48 The excitation of the sample at 680 nm gave us time constants of about 0.35 ± 0.1 ps and 6.90 ± 0.3 ps. These values are correlated to the chlorophyll a to chlorophyll a energy equilibration and are in agreement with the values reported in the literature,48 which are 300 fs and 6 ps. By changing the pump wavelength to 630 nm, we obtained 2.8 ± 1.1 ps and 47 ± 17 ps; these long times could be due to internal conversion.
In the present study, the ultrafast energy transfer components which occurred in less than 200 fs were not resolved due to the temporal limitation of our setup. Some values did not correspond well with those reported in the literaturean indication that a longer time range measurement is required for more accurate exponential fit procedures. The process may also be more complicated than consisting of only two time constants. These are challenges for future work in improvements in the extracted time constants and interpretation thereof.
Conclusions and future work
We have presented the development of a technique of femtosecond pump probe spectroscopy for ultrafast time-resolved spectroscopy, and the preliminary results achieved using this technique. Our experimental setup was standardised using Malachite green dye and the measured lifetime of 2.9 ps corresponds well to those reported in the literature for this dye. We presented our method of correcting for the chirp induced in the white light continuum generation process, as this is required for accurate time resolution of the measurements. We have also presented preliminary measurements of the ultrafast energy transfer in light-harvesting complexes extracted from spinach leaves. Upon excitation at 610 nm, we were able to measure the time constants of the energy transfer between the chlorophylls, as well as annihilation processes. Future work involves the detailed study of the energy transfer lifetimes as a function of pump laser intensity and wavelength, optimisation and confirmation of the ultrafast dynamics of the extracted light-harvesting complexes, as well as further work in interpretation of energy transfer information from the pump probe measurements.
The ultimate goal is to use this technique to characterise the light-harvesting and energy-transfer characteristics of artificial systems developed at the CSIR, and to compare these with natural systems. The work proposed by this group focuses on the development of artificial light-harvesting and energy-transfer systems which match the rates and efficiencies of the natural photosynthetic systems. Two approaches are being pursued. One approach uses porphyrins and produces nanorod structures via ionic self-assembly. These porphyrin nanorods are then embedded in a polymeric scaffold to enhance the structural properties of the nanorods. The second approach uses a polymeric scaffold and attach porphyrins to specific sites on the polymer backbone. The polymer acts to produce specific architectures for site-directed energy transfer. The aim will be to produce structures for which the direction of energy transfer is defined. The next step is to produce technical specifications for the energy-transfer system in terms of the direction, rate and efficiency of the energy-transfer process and robustness of the system.
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Received 10 March. Accepted 3 September 2009.
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