M. P. de Lara-Castells

Vibrational quenching of CO2(010) by collisions with O(3P) at thermal energies: A quantum-mechanical study

The CO(2)(010)-O((3)P) vibrational energy transfer (VET) efficiency is a key input to aeronomical models of the energy budget of the upper atmospheres of Earth, Venus, and Mars. This work addresses the physical mechanisms responsible for the high efficiency of the VET process at the thermal energies existing in the terrestrial upper atmosphere (150 K</=T</=550 K). We present a quantum-mechanical study of the process within a reduced-dimensionality approach. In this model, all the particles remain along a plane and the O((3)P) atom collides along the C(2v) symmetry axis of CO(2), which can present bending oscillations around the linear arrangement, while the stretching C-O coordinates are kept fixed at their equilibrium values. Two kinds of scattering calculations are performed on high-quality ab initio potential energy surfaces (PESs). In the first approach, the calculations are carried out separately for each one of the three PESs correlating to O((3)P). In the second approach, nonadiabatic effects induced by spin-orbit couplings (SOC) are also accounted for. The results presented here provide an explanation to some of the questions raised by the experiments and aeronomical observations. At thermal energies, nonadiabatic transitions induced by SOC play a key role in causing large VET efficiencies, the process being highly sensitive to the initial fine-structure level of oxygen. At higher energies, the two above-mentioned approaches tend to coincide towards an impulsive Landau-Teller mechanism of the vibrational to translational (V-T) energy transfer.

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.

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.

Mode excitation dynamics in the fragmentation of Ar3+: An helicity decoupling study

A full quantum study of the fragmentation dynamics for argon trimer ions is carried out using a previously computed potential energy surface (PES). The initial and final internal states which are being considered are the bending–stretching states of the trimer and the rotovibrational states of the residual dimer ion. The treatment employs the helicity decoupling approximation over a broad range of total angular momentum values (J) and analyses the final distribution of rotational–vibrational states as a function of the metastable states of the J values and of the “tumbling” (helicity) quantum number |Ω|. It is found that at least two different effects, one mainly dynamically related to the centrifugal barrier and another to the orientational nature of the coupling potential field, can be discerned to explain the fragmentation results.