On the Nature of Optical Excitations in Hydrogenated Aluminium Cluster Al 4 H 6 : A Theoretical Study Sridhar Sahu and Alok Shukla Department of Physics, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, INDIA ∗ In this paper, we present a theoretical investigation of photoabsorption spectrum of the newly synthesized hydrogenated cluster of aluminium, Al 4 H 6 . The calculations are performed within the wave-function-based semi-empirical method employing the complete neglect of differential overlap (CNDO) model, employing a large-scale configuration interaction (CI) methodology, and our re- sults are found to be in very good agreement with the earlier ones obtained from the ab initio time-dependent density functional theory (TDDFT). We carefully analyze the many-particle wave functions of various excited states up to 8 eV, and find that they are dominated by single-particle band to band excitations. This is in sharp contrast to bare aluminium clusters, in general, and Al 4 , in particular, whose optical excitations are plasmonic in nature. We attribute this difference to be a consequence of hydrogenation. PACS numbers: 36.40.Vz, 31.10.+z, 31.15.bu, 31.15.vq I. INTRODUCTION Because of the future importance of hydrogen as a fuel, now-a-days a considerable amount of research ac- tivity is taking place in the field of hydrogen storage materials[1, 2]. This is because hydrogen is an extremely combustible gas, therefore, the preferred way to store it is in physically or chemically adsorbed forms. It is from this point of view that a large amount of theoretical and experimental research is taking place in the field of metal- lic hydrides. Among all possibilities, hydrides of group III elements, boron and aluminium, which are called bo- ranes and alanes, respectively, are considered to be strong candidates for the purpose[3–5]. It is with this aim in mind, that Li et al .[6] recently synthesized and studied the properties of a new alane, Al 4 H 6 . The same group fol- lowed up this work, by performing an experimental and theoretical study of the electronic structure and prop- erties of closo-alanes, Al 4 H 4 , Al n H n +2 , 4 ≤ n ≤ 8 [7]. Besides their possible applications, alanes are of inter- est from a fundamental point of view as well. Boron and aluminium are in the same group of periodic table with three valence electrons each, yet their properties are very different. In the bulk form boron is a semiconduc- tor, while aluminium is a metal. As far as the hydride chemistry is concerned, boron exhibits a huge variety of boranes with n-vertex polyhedral structures[8, 9], while the number and types of alanes is much smaller. The pio- neering works of Lipscomb[10] which provide the explana- tions of the cage structures of boranes using the concepts of three-centered two-electron(3c-2e) bonding, ultimately resulted in an ingenious molecular orbital theory known as polyhedral skeletal electron pair theory (PSEPT) or simply Wade-Mingos rule[11–13]. On the other hand only a small number of hydrides of aluminium have been investigated theoretically and experimentally such as molecules AlH[14–18], AlH 3 [19], Al 2 H 4 [20] Al 2 H 6 [19– 24], of the general type Al n H n +2 [25, 26], and cage-like clusters such as Al 13 H $^{−}$m [27]. Fu et al. [26] in a recent comparative study investigated theoretically Al n H n +2 clusters, and their borane analogues. Grubisic et al .[7] have tried to arrive at a Wade-Mingos type of rule set for aluminium hydrides, so that their structures could be predicted in a way similar to boranes, though Martinez and Alonso[25] have recently raised doubts on this sim- ilarity. With this background in mind, the recent study of Al 4 H 6 ,[6] a material whose structure is consistent with Wade-Mingos rule, has again revived the analogy of alu- minium hydrides with boranes. In this work we present an extensive study of the electronic structure and op- tical properties of Al 4 H 6 , with the aim of predicting its photoabsorption spectrum, and to understand the nature of its optical excitations, which can be used for optical characterization of this substance. For the purpose, we use our recently developed CNDO/INDO Hamiltonian- based semiempirical multi-reference singles-doubles con- figuration interaction (MRSDCI) methodology described elsewhere[28–30]. To benchmark our approach, we also apply this approach to AlH molecule and demonstrate good agreement with the experimental results. Martinez and Alonso[25] calculated the photoabsorption spectra of several alanes of the type Al n H n +2 including Al 4 H 6 , us- ing the ab initio time-dependent density functional the- ory (TDDFT), and our results are found to be in very good agreement with their results[25]. Upon analyzing the many-particle wave functions of the excited states corresponding to important peaks in the spectrum of Al 4 H 6 , we conclude that all the way up to 8 eV the states correspond to inter-band excitations, and not to plasmonic excitations common in bare Al clusters. The remainder of this paper is organized as follows. In section II we briefly describe the theoretical methodology employed for the present calculations. This is followed by the presentation and discussion of our results in section III. Finally, in section IV we present our conclusions. ∗ Electronic address: sridhar@phy.iitb.ac.in, shukla@phy.iitb.ac.in arXiv:1001.0635v2 [physics.atm-clus] 10 Jan 2010