Rotational Structure of a Super-Excited State of the NO Molecule Revealed by OODR-Multiphoton Laser Spectroscopy
The optical\xe2\x80\x93optical double resonance time of flight (OODR-TOF) spectroscopy technique was employed to examine the 65 000\xe2\x80\x9366 500 cm\xe2\x80\x931 region of the nitric oxide spectrum. In this region, we detected the following three electronic states: E 2\xce\xa3+ (\xce\xbd = 2) (Rydberg state), B 2\xce\xa0 (\xce\xbd = 23) (valence state), and L 2\xce\xa0 (\xce\xbd = 4) (valence state). The rotational structure analysis of an unexpected band in the red part of the spectra revealed the presence of a new super-excited 2\xce\xa3+ Rydberg state at ~13.3 eV, which was populated through a three-photon transition from the intermediate A 2\xce\xa3+ (\xce\xbd = 0) state. This super-excited state converges to the NO (a3\xce\xa3+) ionic state with electronic configuration (1\xcf\x83)2(2\xcf\x83)2(3\xcf\x83)2(4\xcf\x83)2(5\xcf\x83)2(1\xcf\x80)3(2\xcf\x80)1(3s\xcf\x83)1.
1. Introduction<!>2. Experimental Section<!>3. Results and Discussion<!>3.1. States Below the First Ionization Continuum<!>3.2. States Above the First Ionization Continuum<!>3.3. Electronic Configuration of the New State
<p>The implementation of optical–optical double resonance schemes has led to an enormous simplification of molecular spectra and may facilitate one to reach Franck–Condon regions not accessible from the ground state and to drive multiphoton transitions.1–9 The implementation of mass detection opened up the prospects of studying excited states that lie in the vacuum ultraviolet region, thereby overcoming the problem of light absorption by air. In this work, we report a new electronic state of NO that lies at 13.3 eV, taking advantage of the features mentioned. This state is located in the region of the so-called "super-excited states" that have been studied using high-order laser harmonics or synchrotron radiation.10–13 Computational studies have also been done in this region to understand the photoionization processes.14,15 The new state was reached by first pumping individual rotational levels of the intermediate A 2Σ+ (ν = 0) state, and then scanning a dye laser in the visible region to drive a three-photon transition. In the same OODR-TOF spectra also appear the following states: E 2Σ+ (ν = 2) (Rydberg state), B 2Π (ν = 23) (valence states), and L 2Π (ν = 4) (valence states). These states lie at ~6.3 eV and are reached through one-photon absorption from the intermediate state. The excitation scheme is similar to that of Ishiwata et al., who recorded OODR spectra of Br2 using a two-photon transition from the intermediate state to reach a new ion-pair state.16 We deduced that this is a Rydberg 2Σ+ state through a rotational analysis, which has been considered to be the most reliable tool to assign the term symbol of an electronic state. Nevertheless, because the information about rotational structures has been missing in studies that involve the one-photon excitation process with the synchrotron radiation as the excitation source, the super-excited Rydberg states at energies above 13 eV were usually assigned by using the quantum defect analysis.11,12</p><!><p>The instrumental setup employed in this work has been described elsewhere.17–19 Commercial nitric oxide (99.5%, AGA Products) mixed with argon was expanded into the source chamber using a pulsed-nozzle valve. The NO/Ar gas pulse was skimmed before it entered the detection chamber. The molecular beam direction was perpendicular to the propagation direction of the two laser beams. The pressure in the detection chamber was below 1 × 10–4 Torr when the pulsed nozzle valve was operating. Individual rotational states of the A electronic state of NO were excited using an excimer-pumped dye laser, equipped with a BBOI doubling crystal. The beam was focused onto an area of ~1 mm2 using a cylindrical lens (focal length 100 cm). A tunable dye laser (Scanmate, Lambda Physik) pumped by a Nd:YAG laser (LPY 150, Lambda Physik) was used to carry out the second resonance as well as to ionize the highly excited molecules. The delay time between the two laser pulses was adjusted to 50 ns using a digital pulse generator. The NO+ ion signal was detected by a homemade Wiley–McLaren linear time-of-flight spectrometer (TOF) system with a flight distance of a half meter.17,19</p><!><p>In the present Article, we report the observation of a new excited state at 13.3 eV of nitric oxide, which lies at 4 eV above the onset of the first ionization continuum. The detection of this so-called super-excited state was achieved using two tunable dye lasers. The first laser pulse (UV light, ~4 eV) prepared individual rotational levels of the intermediate A 2Σ+ (ν = 0) state of NO, denoted from now on as the intermediate state. The second laser (visible light, ~2.6 eV) excited two types of transitions: (i) a one-photon transition to reach electronic states of NO below the first ionization continuum, and (ii) a three-photon transition to reach the excited state above the onset of the first ionization continuum. Although quite different energies are involved, both types of transitions appear in the same spectral region. We first present an analysis of the states observed through the one-photon process because the information generated facilitated the analysis of the "super-excited" states.</p><!><p>In Figure 2 are observed the following three electronic states: E 2Σ+ (ν = 2) (Rydberg state), B 2Π (ν = 23) (valence state), and L 2Π (ν = 4) (valence state). These states are reached through a one-photon transition driven by the second laser, using the A state as a stepping stone. The relative position of these states as a function of rotational quantum number is presented in Figure 1, which was constructed using the molecular constants listed in Table 1. For each rotational level of the NO intermediate state, the spectra display the following eight lines for the B–A (23, 0) band: OPf12, Pe11, Qf11, Re11, Pf22, Pe21, Qf21, and SRe21 (from left to right). While the L–A (4, 0) band should also present eight lines, some are missing, and the same observation has been made by other authors.20 Above J = 13.5, the B–A (23, 0) transitions become extremely weak and are no longer observed. An energy against the N(N + 1) plot for the E–A (2,0) band exhibits a linear behavior from N = 0 to N = 20, as shown in Figure 3. This means that the E2 state is not perturbed. Because both the E and the A states are Rydberg states and belong to Hund's case (b), the E–A (2, 0) band consists of only two lines. The line positions of the E–A (2, 0) and B–A (23, 0) bands have been compiled in Tables 2 and 3, respectively.</p><!><p>Besides the three bands previously discussed, the spectra exhibit a hitherto unassigned band that consists of two lines located in the left part of the spectra of Figure 2. The observation of only two transitions in each spectrum is characteristic of a 2Σ+–2Σ+ transition. The midpoint between the P and R lines maintains a constant separation from the E–A (2, 0) band, as expected for a Rydberg 2Σ+ state. However, what it is peculiar of this band is that the separation between the P and R lines is one-third of the corresponding separation on the E–A (2, 0) band. This difference may be explained postulating a three-photon absorption process from the A state, as illustrated in Figure 4. The following arguments support this assignment. First, the selection rule for one-photon and three-photon transitions is essentially the same, which means that the pattern of the new band is similar to that of the E–A band.16,21,22 In Table 4 are listed the energies of the rotational states reached. Second, we can discard the absorption of three photons from the ground state to reach a state around 7.5 eV because otherwise we should be detecting numerous lines corresponding to the rotational states thermally populated in the ground state. Third, the ground state has 2Π symmetry, and the band observed clearly corresponds to a 2Σ+–2Σ+ transition. Forth, from the E versus N(N + 1) curve in Figure 5, we derived the rotational constant to be 2.00 cm–1, which agrees with the value expected for a Rydberg state. The term value of this state is 106 735 (~13.3 eV), which is ~4eV above the threshold of the first ionization continuum (~9.26 eV). This term value belongs to one of those super-excited states that have recently been investigated exciting NO (with the photon energies of 9–35 eV) using high-order harmonic laser or synchrotron lights.11,12,23–27 The nearby super-excited Rydberg 2Σ+ state with the extremely sharp intense line, located at the energy of 13.818 eV, has been observed with the angle-resolved constant-ionic-state (CIS) technique, using one-photon excitation from the ground state.12,27 Salzmann et al. reported that the autoionization via this resonant Rydberg excitation leads to a 10-fold increase of the ionization cross section.27 This enhanced autoionization mechanism may explain why we can observe ion signal from this new Rydberg 2Σ+ state. While the assignment of the symmetry of the super-excited states of NO has usually been done using the quantum defect theory, an assignment based on a rotational structures analysis, as has been presented in Figure 5, is more definitive.11</p><!><p>The electronic configuration for this new super-excited state may be assigned with the aid of existing theoretical and experimental results.12–15,27 The electronic configuration for the NO ground state (X2Π) is usually represented as (1σ)2(2σ)2(3σ)2(4σ)2(5σ)2-(1π)4(2π)1. The removal of one electron from the (2π), (1π), and (5σ) orbitals gives rise the lowest three ionic states: X1Σ+, a3Σ+, and b3Π states, with ionization energies of 9.26, 15.67, and 16.56 eV, respectively. The potential energy curves for the NO neutral ground state and the ionic states may be found in the literature.27,28 The nomenclature of the term symbols used to describe the NO super-excited states is different from that used for the excited states. A series of neutral super-excited states will be called b3Π or a3Σ+ series if they converge to the NO+ (b3Π) or NO+ (a3Σ+) ionic state. The states are described by providing information of the converging ionic state and information of the highest occupied electron: NO (converging ionic state, highest occupied orbital). As far as the neutral states of overall 2Σ+ symmetry above 13 eV are concerned, we need to note that the (2π)–1 ionization via the 2π–kσ transition only gives rise to a continuum shape resonance centered at 14 eV. Although both NO (b3Π, npπ) series and NO (a3Σ+, nsσ) series include 2Σ+ states, our newfound 2Σ+ state should belong to the a3Σ+ Rydberg series for energetic reasons. That is to say, the lowest member of the NO (b3Π, 3p) series of Rydberg states has an energy of 13.818 eV (or possibly as low as 13.654 eV), whereas the state we observed lies at 13.3 eV.12,14,15,27 In addition, theory predicts that the NO (a3Σ+, nsσ) series should have members with 2Σ+ symmetry in the region between 12.61 and 15.06 eV,15 and experimentally the NO (a3Σ+, 4sσ)–NO (X2Π) transition bands have been observed between 14.1 and 15.2 eV.12 It follows that the new state can only be a resonant state of NO (a3Σ+, 3sσ), which has the following electronic configuration: (1σ)2(2σ)2(3σ)2(4σ)2(5σ)2(1π)3(2π)1(3sσ)1.</p>