Full-dimensional multi configuration time dependent Hartree calculations of the ground and vibrationally excited states of He2,3Br2 clusters

摘要

The first experiments on van der Waals (vdW) clusters consisting of a dihalogen molecule and rare-gas atoms have been performed by Levy et al.,1,2 followed by a large number of studies on systems such as HenI2 (n=1–3),3,4 NenI2 (n=1–6),2,5,6 NenBr2,7 Rg1, 2X2 (Rg=He, Ne, Ar and X=Cl, Br),8,9,10,11 NenICl (n=1–5),12,13 HenICl ((n=1–3).14,15 More recently the interest in studying such species has been intensified due to additional experiments, rotational, and infra-red spectroscopy on doped helium nanodroplets,16,17,18 and on small/intermediate-sized doped helium clusters.19nDespite the fact that experimental data are available over the last 30 years, the theoretical treatment of such systems is still a significant challenge. For the triatomic complexes exact full-dimensional quantum calculations of the dynamics are feasible,20,21,22,23 with limitations imposed by the accuracy of the assumed potential surface. Recently high quality ab initio potential energy surfaces (PESs) have been calculated for such triatomic species,20,24,25,26,27,28,29,30 and attempts have also been made using pairwise three-body interaction potentials to represent the PESs of larger complexes.31,32,33,34,35 Thus the first issue in this context, from the theoretical side, is how to properly describe the multidimensional potential energy surface or, in other words, at what extent the usual assumption of additive forces holds. Reported ab initio results have been shown that for He2Br2 system, the pairwise atom-atom interactions are not able to describe the PES of the complex, while a sum of three-body HeBr2 terms,29 plus the He–He interaction, can accurately represent the interaction energies for this cluster.34 This latter has been also supported by recent experimental data on He2,3Br2, He2,3ICl, and Ne2Br2 complexes.14,36 In particular, laser-induced fluorescence and two-laser action spectra have been recorded, and different isomers for these vdW clusters have been stabilized and identified.14 The assignment of the spectroscopic features to multiple conformers associates them to configurations of the system with the He atoms localized to linear and/or T-shape wells of the corresponding PES. These findings have been found to be in accord with predictions made by previous theoretical investigations for the tetra-atomic clusters.31,34nThe work presented here expands our previous efforts to evaluate the multidimensional surfaces of such vdW complexes obtained from first principles ab initio computations, by direct comparison with the experiment. In this way we can study the role of uncertainties in the PES on the spectroscopic and dynamic properties of these systems. This involves full-dimensional exact quantum dynamics simulations, which as the number of intermolecular degrees of freedom increases are getting computationally difficult, or impossible for the larger complexes. Thus, such calculations for small clusters (two or three He atoms), where experimental data are available, can serve as benchmark for approximate treatments37,38 in order to deal with larger species, and provide with required information, such as input data, for such approaches.39