Search

The potential energy surface (PES) for the interaction between Li-2((1)Sigma(g)(+)) and He-4 has been computed using an accurate, post-Hartree-Fock quantum calculation for its ground electronic state. The orientational anisotropy of the forces and the interplay between repulsive and attractive effects within the PES are analyzed to extract information on the possible existence of bound states in the triatomic system. The structures of a few of the Li-2(He)(n) small clusters are examined by comparing a classical approach with a full quantum one to generate bound configurations and to extract information on the possible spatial arrangements of the smaller clusters via a vis the location of the Li-2 dopant. Some significant consequences on the Li-2 behavior in larger clusters and droplets are drawn from the above findings., University of Rome ‘‘La Sapienza’’ Scientific Committee; CASPUR Supercomputing Center; INFM Institute

A new quantum time-dependent treatment has been employed to model the photoexcitation of LiH2+ by looking at the process both in a constrained configuration and in the full three-dimensional space. The study has been carried out by monitoring at the end of the process the fragmentation probabilities, the final vibrational distributions of the molecular fragments, and the angular evolution of the wave functions of the complex on the excited electronic surface. The comparison between different initial conditions is able to shed light on the microscopic mechanism of the energy redistribution, with particular reference to the role of the angular coordinate that turns out to provide efficient energy channeling during the evolution. The possibility of extending the method to larger systems is briefly discussed.

A new spin coupled multireference balence bond scheme for the treatment of electronic excited states is presented. The method involves the specific optimization of excited spin coupled wave functions which act as reference functions and are used to introduce correlation corrections directly in the excited state of interest. The variational optimization of such excited wave functions is achieved by using simple orthogonality constraints on the spin coupled orbitals and it preserves all the appealing features of the valence bond method. The applications to the lower lying states of [LiH2]+ and BeH show the advantages and some of the limitations of this new approach.

Recent calculations on the rigid rotor surface for the LiH–He interaction [B. K. Taylor and R. J. Hinde, J. Chem. Phys. 111, 973 (1999)] surmised that the well depth and anisotropy of the new surface differ substantially from the latest valence bond calculations for the same system [F. A. Gianturco, S. Kumar, S. K. Pathak, M. Raimondi, M. Sironi, J. Gerratt, and D. L. Cooper, Chem. Phys. 215, 227 (1997)]. We examine in detail the performance of both these potential energy surfaces once employed to generate quantum observables which can be compared with experiments. Our computations clearly show that the two surfaces exhibit small differences in their short-range repulsive anisotropy and larger differences in well depths. The latter however cannot be assessed as yet from existing experiments. The different behavior of their wells in supporting van der Waals bound states is also examined and spectroscopic experiments are suggested for detecting possible transitions between such states.

We present an analysis of the liquid and of a small isolated cluster of n-ethyl ammonium nitrate based on “first principles” molecular dynamics. We discover that the peculiar properties of ionic liquids make such compounds ideal candidates for such an analysis. We have been able to characterize some important features of the liquid structure and we have validated our simulations by comparing our findings with experimental vibrational spectra of the liquid phase. Theoretical spectra, which present a remarkable agreement with the measurements, besides the assignment of the main spectra features, allow an interpretation of the spectra at high frequencies where the vibrational motions involve the hydrogen-bonded atoms, thus providing a picture of the hydrogen bonding network that exists in such compounds.

Ab initio calculations at the multiconfiguration self-consistent field level followed by a multireference configuration interaction were carried out along the two possible collinear approaches of the [LiHHe]+ system, while a three-dimensional calculation of the structures of that complex with LiH+ kept at its equilibrium geometry was also completed at the same level of accuracy. The interaction forces of the lowest two electronic states indicate possible reactive behavior, with the first excited potential-energy surface clearly showing a barrierless path to HeH+ product formation. The details of the reactive pathways and their possible bearing on reaction processes, which could occur at the low temperature of a He nanodroplet holding LiH+ as an impurity, are analyzed and discussed.

Diffusion Monte Carlo calculations are carried out for clusters of OH- (1Sigma+) with N 4He atoms, N varying up to 15, while classical configurations from a genetic algorithm optimization are obtained up to N=20. The overall interaction potential is assembled from ab initio data for the partners using the sum-of-potentials scheme. In contrast with the cationic dopants' behavior, the results indicate a very marked spatial delocalization and quantum features of the solvent adatoms surrounding the anionic impurity, thus making classical calculations of solvent's spatial locations of only limited use. In spite of the generally known repulsive interaction of negative charges in He droplets, the calculations show that this polar molecular anion is solvated by a liquidlike solvent layer, reminiscent of what happens in pure helium droplets.

Quantum coupled-channel scattering calculations have been carried out at ultralow energies (down to 10(-5) K) for rotational quenching of ionic and spin-stretched states of the lithium dimer in collision with He atoms. Marked cross section differences, to be related to changes in their interactions with He, have been observed with respect to the singlet case while little changes in the collisional behavior are seen to occur upon ionization when the spin-stretched target is considered. Both effects originate from an interplay of structural and dynamical features of the colliding partners which are analyzed in detail.

Calculations have been carried out for the vibrational quenching of excited H(2) molecules which collide with Li(+) ions at ultralow energies. The dynamics has been treated exactly using the well-known quantum coupled-channel expansions over different initial vibrational levels. The overall interaction potential has been obtained from the calculations carried out earlier by our group using highly correlated ab initio methods. The results indicate that specific features of the scattering observables, e.g., the appearance of Ramsauer-Townsend minima in elastic channel cross sections and the marked increase of the cooling rates from specific initial states, can be linked to potential properties at vanishing energies (sign and size of scattering lengths) and to the presence of either virtual states or bound states. The suggestion is made such that by selecting the initial state preparation of the molecular partners, the ionic interactions would be amenable to controlling quenching efficiency at ultralow energies.

Full quantum dynamics calculations have been carried out for the ionic reaction 4He2+ + 3He and state-to-state reactive probabilities have been obtained using both a time-dependent (TD) and a time-independent (TI) approach. An accurate ab-initio potential energy surface has been employed for the present quantum dynamics and the two sets of results are shown to be in agreement with each other. The results for zero total angular momentum suggest a marked presence of atom exchange (isotopic replacement) reaction with probabilities as high as 60%. The reaction probabilities are only weakly dependent on the initial vibrational state of the reactants while they are slightly more sensitive to the degree of rotational excitation. A brief discussion of the results for selected higher total angular momentum values is also presented, while the l-shifting approximation [1] has been used to provide estimates of the total reaction rates for the title process. Such rates are found to be large enough to possibly become experimentally accessible.