Mixed quantum/classical theory for rotationally inelastic scattering of identical collision partners revised.

Mixed quantum/classical theory (MQCT) for the treatment of rotationally inelastic transitions during collisions of two identical molecules, described either as indistinguishable or distinguishable partners, is reviewed. The treatment of two molecules as indistinguishable includes symmetrization of rotational wavefunctions, introduces exchange parity, and leads to state-to-state transition matrix elements different from those in the straightforward treatment of molecules as distinguishable. Moreover, the treatment of collision partners as indistinguishable is eight times faster. Numerical results presented herein for H2 + H2, CO + CO and H2O + H2O systems indicate good agreement of MQCT calculations with full-quantum calculations from the literature and show that an a posteriori correction, applied after treatment of the collision partners as distinguishable, generally produces good results that agree well with the rigorous treatment of collision partners as indistinguishable. This correction for the cross section includes either multiplication by 2 or a summation over physically indistinguishable processes, depending on the transition type. After this correction, the results of the two treatments agree within 5% for most but may reach 10-20% for some transitions. At low collision energies dominated by scattering resonances, these differences can be larger, but they tend to decrease as collision energy is increased. It is also shown that if the system is artificially forced to follow the same collision path in the indistinguishable and distinguishable treatments, then all differences between the results of the two treatments disappear. This interesting finding gives new insight into the collision process and indicates that the indistinguishability of identical collision partners comes into play through the collision path itself, rather than through matrix elements of inelastic transitions.

Potential energy and dipole moment surfaces of HCO- for the search of H- in the interstellar medium.

Potential energy and permanent dipole moment surfaces of the electronic ground state of formyl negative ion HCO(-) are determined for a large number of geometries using the coupled-cluster theory with single and double and perturbative treatment of triple excitations ab initio method with a large basis set. The obtained data are used to construct interpolated surfaces, which are extended analytically to the region of large separations between CO and H(-) with the multipole expansion approach. We have calculated the energy of the lowest rovibrational levels of HCO(-) that should guide the spectroscopic characterization of HCO(-) in laboratory experiments. The study can also help to detect HCO(-) in the cold and dense regions of the interstellar medium where the anion could be formed through the association of abundant CO with still unobserved H(-).

On the electron-induced isotope fractionation in low temperature (32)O(2)/(36)O(2) ices--ozone as a case study.

The formation of six ozone isotopomers and isotopologues, (16)O(16)O(16)O, (18)O(18)O(18)O, (16)O(16)O(18)O, (18)O(18)O(16)O, (16)O(18)O(16)O, and (18)O(16)O(18)O, has been studied in electron-irradiated solid oxygen (16)O(2) and (18)O(2) (1 ∶ 1) ices at 11 K. Significant isotope effects were found to exist which involved enrichment of (18)O-bearing ozone molecules. The heavy (18)O(18)O(18)O species is formed with a factor of about six higher than the corresponding (16)O(16)O(16)O isotopologue. Likewise, the heavy (18)O(18)O(16)O species is formed with abundances of a factor of three higher than the lighter (16)O(16)O(18)O counterpart. No isotope effect was observed in the production of (16)O(18)O(16)O versus(18)O(16)O(18)O. Such studies on the formation of distinct ozone isotopomers and isotopologues involving non-thermal, non-equilibrium chemistry by irradiation of oxygen ices with high energy electrons, as present in the magnetosphere of the giant planets Jupiter and Saturn, may suggest that similar mechanisms may contribute to the (18)O enrichment on the icy satellites of Jupiter and Saturn such as Ganymede, Rhea, and Dione. In such a Solar System environment, energetic particles from the magnetospheres of the giant planets may induce non-equilibrium reactions of suprathermal and/or electronically excited atoms under conditions, which are quite distinct from isotopic enrichments found in classical, thermal gas phase reactions.