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The Journal of Chemical Physics

The structure of a weakly bound ionic trimer: Calculations for the He24H− complex

Franco Antonio Gianturco, Francesco Paesani, I. Baccarelli, Gerardo Delgado-Barrio, Tomás González‐Lezana, Salvador Miret-Artés, Pablo Villarreal, Gianluigi Bendazzoli, Stefano Evangelisti

Published Apr 1, 2001• 19 citations
Publisher

Abstract

The weakly bound diatomic systems He24 and HeH−4 have been found able to support only one bound state for J=0, although the latter also supports an additional bound state for J=1. In the present paper we, therefore, study the structure of the bound states which might exist for the weakly bound triatomic He24H−, in its J=0 state, by describing the full potential as a simple addition of two-body (2B) interactions. We carry out bound state calculations using both Jacobi coordinates within a discrete variable representation (DVR) and pair coordinates with a distributed Gaussian function (DGF) expansion. The system is shown to possess two bound states with respect to its lower dissociation threshold and two further “ghost” states before the complete break-up threshold. The spatial structures of such states and of the floppy complex are analyzed in detail, as is the possibility of detecting Efimov-type states in such a weakly bound aggregate. Finally, the inclusion of three-body (3B) forces in the description of the full interaction and its effect on the number of possible bound states is also discussed.

DOI
10.1063/1.1352034

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The weakly bound diatomic systems He24 and HeH−4 have been found able to support only one bound state for J=0, although the latter also supports an additional bound state for J=1. In the present paper we, therefore, study the structure of the bound states which might exist for the weakly bound triatomic He24H−, in its J=0 state, by describing the full potential as a simple addition of two-body (2B) interactions. We carry out bound state calculations using both Jacobi coordinates within a discrete variable representation (DVR) and pair coordinates with a distributed Gaussian function (DGF) expansion. The system is shown to possess two bound states with respect to its lower dissociation threshold and two further “ghost” states before the complete break-up threshold. The spatial structures of such states and of the floppy complex are analyzed in detail, as is the possibility of detecting Efimov-type states in such a weakly bound aggregate. Finally, the inclusion of three-body (3B) forces in the description of the full interaction and its effect on the number of possible bound states is also discussed.
Helium trimer bound states are calculated with a variational method described in terms of pair atom coordinates and distributed Gaussian functions by assuming zero total angular momentum. With the two-body interaction potential used, a weakly bound state for the dimer and only two bound states for the trimer are found. Although strictly speaking the first excited trimer state is not an Efimov state, we show that this state presents several characteristics of the Efimov behavior which are extensively discussed in the present work. Special emphasis is placed on the main geometrical configurations contributing to the two bound trimer states, and suggestions are made on the possible ways of experimentally detecting the Efimov-type state.
In the present study, we carry out a full search of the bound states and resonances of the He(2)Be triatomic system, with its isotopic variants (3)He(2)(9)Be, (3)He(4)He(9)Be, and (4)He(2)(9)Be using the hyperspherical method. Three-body long-range effects are also included in the computation by adding to the additive potential the Axilrod-Teller triple-dipole term. In addition, the possibility of the occurrence of Efimov-type states in these systems is discussed. We have found a bound state for each of the (3)He(2)(9)Be and (3)He(4)He(9)Be trimers, while one weakly bound excited state is also found to exist for the (4)He(2)(9)Be system.
The 4He3 weakly interacting system is analysed by constructing the full interaction as a sum of two-body (2B) potentials chosen among the most recent proposals from the literature. The spatial density distributions of the three bound atoms are obtained using a diffusion Monte Carlo (DMC) algorithm and a stochastic analysis under specific geometric constraints is carried out with the resulting densities in order to recover a more conventional structural picture for such floppy system. The total binding energies were obtained with the chosen potentials analysed in the present work, using the DMC algorithm, and are compared with previous published results. The ensuing spatial distributions are analysed in some detail to select the dominant structures from a conventional triangular description of this very floppy molecule.
We confirm the existence of and study the location and spatial shape for the ground bound state of the triatomic system that contains one lithium (6Li or 7Li) atom and two boson helium partners. The computational method employed involves a variationally determined multi-center expansion over Gaussian functions of variable width, as discussed in our previous recent work. The overall behavior of this very diffuse, bound wave-function with zero angular momentum is analyzed in detail to extract specific indicators on its spatial features. The strength of the available three-particle interaction is arbitrarily scaled in order to investigate the possible appearance of either halo or Efimov states in the present system.
We study the structure of triatomic molecules containing two helium atoms and one alkali-metal atom X (X = Li, Na). The three-body Schr\"odinger equation is solved in hyperspherical coordinates using the He-He and He-X pair van der Waals potentials available in the literature. We calculate the structural properties of the He2Li{\mathrm{He}}_{2}\mathrm{Li}He2​Li and He2Na{\mathrm{He}}_{2}\mathrm{Na}He2​Na bound states, and analyze one-dimensional pair and angle distribution functions as well as two-dimensional pair-pair and angle-angle distribution functions. These bound states are characterized by so peculiar weakly bound structures that classifying them into particular sizes and geometrical shapes appears to be elusive. Particularly, the ${J}^{\mathrm{\ensuremath{\Pi}}}={0}^{+}$ excited states of 4He26Li^{4}\mathrm{He}_{2}^{6}\mathrm{Li}4He26​Li and 4He27Li^{4}\mathrm{He}_{2}^{7}\mathrm{Li}4He27​Li are found to constitute giant triatomic molecules with their size amounting to several hundred bohrs.
The rotationless, vibrational states of two weakly bound triatomic species are calculated using a variational expansion over multicentred Gaussian functions (the DGF method described in the main text) and corrections are introduced by using the three-body potentials vis à vis the sum-of-potentials approximation. The effects are seen to be very small and isotopic substitution is further introduced to evaluate changes on the vibrational structures of both systems, aiming at possibly detecting Efimov behaviour in such special trimers: no Efimov states are found with the available potentials and when the two-body potential strengths are varied. An additional study on the effects of isotopic substitutions for H atoms is also presented.