D. López-Durán

Raman spectra of (He)N-Br2(X) clusters: The role of boson/fermion statistics in a quantum solvent

The aim of this paper is to elucidate the role played by the bosonic/fermionic character of N He atoms solvating a Br2(X) molecule. To this end, an adiabatic model in the molecular stretching coordinate is assumed and the ground energy levels of the complexes are searched by means of Hartree (or Hartree-Fock) Quantum Chemistry calculations for 4He (or 3He) solvent atoms. Simulations of vib-rotational Raman spectra point at the spin multiplicity as the main feature responsible for the drastic difference in the rotational structures of molecules embedded in boson or fermion helium drops as already observed by the experiments of Grebenev et al. [S. Grebenev, J. P. Toennies, and A. F. Vilesov, Science 279 (1998) 2083].

Quantum features of a barely bound molecular dopant: Cs2(3Σu) in bosonic helium droplets of variable size.

We present in this work the study of small (4)He(N)-Cs(2)((3)Σ(u)) aggregates (2 ≤ N ≤ 30) through combined variational, diffusion Monte Carlo (DMC), and path integral Monte Carlo (PIMC) calculations. The full surface is modeled as an addition of He-Cs(2) interactions and He-He potentials. Given the negligible strength and large range of the He-Cs(2) interaction as compared with the one for He-He, a propensity of the helium atoms to pack themselves together, leaving outside the molecular dopant is to be expected. DMC calculations determine the onset of helium gathering at N = 3. To analyze energetic and structural properties as a function of N, PIMC calculations with no bosonic exchange, i.e., Boltzmann statistics, at low temperatures are carried out. At T = 0.1 K, although acceptable one-particle He-Cs(2) distributions are obtained, two-particle He-He distributions are not well described, indicating that the proper symmetry should be taken into account. PIMC distributions at T = 1 K already compare well with DMC ones and show minor exchange effects, although binding energies are still far from having converged in terms of the number of quantum beads. As N increases, the He-He PIMC pair correlation function shows a clear tendency to coincide with the experimental boson-liquid helium one at that temperature. It supports the picture of a helium droplet which carries the molecular impurity on its surface, as found earlier for other triplet dimers.

Microsolvation of Cationic Dimers in 4He Droplets : Geometries of A+2(He)N (A = Li, Na, K) from Optimized Energies

Ab initio computed interaction forces are employed to describe the microsolvation of the A+2(2Sigma) (A=Li, Na, K) molecular ion in 4He clusters of small variable size. The minimum energy structures are obtained by performing energy minimization based on a genetic algorithm approach. The symmetry features of the collocation of solvent adatoms around the dimeric cation are analyzed in detail, showing that the selective growth of small clusters around the two sides of the ion during the solvation process is a feature common to all three dopants.

Spectral simulations of polar diatomic molecules immersed in He clusters: application to the ICl (X) molecule

A recently developed quantum-chemistry-like methodology to study molecules solvated in atomic clusters is applied to the ICl (iodine chloride) polar diatomic molecule immersed in clusters of He atoms. The atoms of the solvent clusters are treated as the ‘electrons’ and the solvated molecule as a structured ‘nucleus’ of the combined solvent-solute system. The helium‐helium and helium-dopant interactions are represented by parametrized two-body and ab initio three-body potentials, respectively. The ground-state wavefunctions are used to compute the infrared (IR) spectra of the solvated molecule. In agreement with the experimental observations, the computed spectra exhibit considerable differences depending on whether the solvent cluster is comprised of bosonic ( 4 He) or fermionic ( 3 He) atoms. The source of these differences is attributed to the different spin-statistics of the solvent clusters. The bosonic versus fermionic nature of the solvent is reflected in the IR absorption selection rules. Only P and R branches with single state transitions appear in the spectrum when the molecule is solvated in a bosonic cluster. On the other hand, when the solvent represents a fermionic environment, quasi-degenerate multiplets of spin states contribute to each branch and, in addition, the Q-branch becomes also allowed. Combined, these two factors explain the more congested nature of the spectrum in the fermionic case.

Weakly bound finite systems: (4He)N–Rb2(3Σu), clustering structures from a quantum Monte Carlo approach *

We report here ((4)He)(N)-Rb(2)((3)Σ(u)) complexes, 2 ≤ N ≤ 20, analysed through a quantum diffusion Monte Carlo stochastic approach. The calculations show that the spin stretched dimer molecule is bound outside the pure He sub-complex, due to the stronger He-He potential as compared with the He-Rb(2) interaction, while the rare gas atom moiety presents, in turn, a shell-like structure with ten He adatoms completing the first shell. Our results agree with previous findings on this and similarly weakly interacting systems.

Diffusion Monte Carlo description of Cs2(3Σu)–(4He)N clusters: an example of weak dopant–helium interaction

In this paper, we study the energy and geometric properties of Cs2(3Σu)–(4He)N clusters, 2≤N≤20, N even, through a diffusion Monte Carlo methodology. Considering the results for doped clusters in which the He–impurity interaction dominates over the He–He one, our aim is to investigate the case when this assumption is not fulfilled anymore and the helium–helium potential becomes dominant. We find a scenario where a pure helium subcomplex is formed, leaving out the alkaline dimer, with the largest species gathering the helium atoms in a two-shell-like structure, the first of which is filled with ten particles. Our results are in agreement with previous theoretical and experimental findings.

Dynamics and Potential Energy Surfaces for small to medium size Hen‐dihalogen clusters

The intermolecular forces between atoms and molecules are of great importance in studies of solids, liquids and clusters. Our current studies serve to bridge the gap between small cluster and large cluster limit.

Structure and dynamics of van der Waals complexes: From triatomic to medium size clusters

Proceedings of the XXIV International Conference. Held 20-26 July 2005 in Rosario, Argentina.