I. Paidarová

Atoms-in-molecules calculations on diatomic fragments of polyatomic systems. HeH and HeH+

Abstract Two different modifications of the atoms-in-molecules method are applied to various states of the HeH and HeH + species. The calculations are carried out in a wide range of internuclear separations in order to obtain a description of the diatomics adequate to serve as input for a subsequent DIM calculation of potential energy surfaces of (HeH 2 ) + . The results are compared with other presently available calculations.Abstract Two different modifications of the atoms-in-molecules method are applied to various states of the HeH and HeH + species. The calculations are carried out in a wide range of internuclear separations in order to obtain a description of the diatomics adequate to serve as input for a subsequent DIM calculation of potential energy surfaces of (HeH 2 ) + . The results are compared with other presently available calculations.

DIM model calculations for (O2H)+ interaction potentials

Abstract Several low-lying potential energy surfaces for the (O 2 H) + system have been estimated using the diatomics-in-molecules (DIM) approach in a model calculation calibrated with known ab initio information. Main features of ground and lower excited states are qualitatively well reproduced in spite of the fact that the DIM basis was kept rather small. The error from the truncation of the basis set had to be compensated for by empirical adjustments of some of the input diatomic potential energy curves of the excited states. From the topography and crossing behaviour of the resulting potential energy surfaces some predictions concerning the mechanism of inelastic and charge transfer collisions are made.

Ab initio calculation of nuclear quadrupole coupling constants of rovibrational levels in the X 1Σ+ state of all isotopic variants of BH

Abstract The 11 B, 10 B, and 2 H quadrupole coupling constants of rovibrational levels of 11 B 2 H, 10 B 2 H, 11 B 1 H, and 10 B 1 H in their ground electronic state have been calculated from molecular wavefunctions which explicitly describe nuclear displacement. The 11 B and 10 B coupling is predicted to be relatively very strong for all isotopic variants. The vibrational dependence of the B coupling is found to be rather insignificant and its rotational dependence is predicted to be quite unimportant. The deuteron coupling in 10 B 2 H and 11 B 2 H is found to be weak, its strength is predicted to decrease with vibrational excitation. The change with rotational excitation is unimportant. The quadrupole hyperfine patterns of 10 B 2 H and 11 B 2 H in the X 1 Σ + state are dominated by relatively very strong B coupling and their basic features are predicted to be independent of the vibrational and rotational state considered.

Atoms-in-molecules calculations on diatomic fragments of polyatomic systems: HeH, HeH+ and HeH− for a dim model of penning ionization of H2 by He*

Abstract Potential energy curves and mixing coefficients for various states of HeH, HeH + and HeH − are calculated using the orthogonalized Moffitt method. Special attention is paid to characterization of those states which are relevant to the Penning ionization of the atomic hydrogen by helium metastables, and important for the construction of DIM potential energy hypersurfaces describing the Penning ionization of H 2 by He*.

Break-down of the born-oppenheimer approximation in the He(23S)-H2 and He(21S)-H2 autoionizing systems?

Abstract The possibility of non-adiabatic behaviour of the He(21S)-H2 and He(23S)-H2 collision systems is reexamined by means of extended diatomics-in-molecules calculations. In the triplet case, the calculations support the idea that at collision energies above ≈ 1 eV the system could behave non-adiabatically. In the singlet He*-H2 system, a break-down of the Born-Oppenheimer approximation is predicted to take place at higher energies.

Modified version of the trajectory surface leaking method for dynamical calculations on autoionizing atom-diatom collision systems

Abstract A modified version of the trajectory surface leaking method for dynamical calculations on autoionizing atom-diatom collision systems is elaborated. The modification is designed so as to account for the most prominent quantum features of the ionization event. Test calculations on He(2 3 S)-H 2 are presented. The approach is found to yield an improved picture of the He(2 3 S)-H 2 →[He…H 2 + ]+e − ionization event, which is consistent with Penning electron spectra measurements. Some implications for future theoretical studies of atom-diatom autoionizing collision systems are outlined.

Non-adiabatic coupling in the autoionizing He(2 3S)—H2 system

Abstract Non-adiabatic interaction between the He(2 3S)—H2 and He(2 3P)—H2 potential energy surfaces is studied. The potentials and non-adiabatic coupling vector are calculated by use of the hermitean version of the DIM scheme for description of polyatomic autoionizing systems. The shape and location of the non-adiabatic region are determined and used to estimate translational states of the He*—H2 collision system which lead to non-adiabatic transitions. The effect of the behaviour of the system in the non-adiabatic region on the autoionization feature of a He(2 3S)—H2 collision is outlined.

Nuclear quadrupole coupling constants of rovibrational levels of all isotopic species of BH+ in their X 2Σ+ and B′ 2Σ+ states

Abstract The 11 B, 10 B, and 2 H quadrupole coupling constants of rovibrational levels in the X 2 Σ + and B′ 2 Σ + states of 11 B 2 H + , 10 B 2 H + , 11 B 1 H + , and 10 B 1 H + are calculated from molecular wavefunctions which explicitly describe nuclear motion. Except for the boron coupling in the B′ 2 Σ + state, the vibrational dependence of the nuclear quadrupole coupling constants is found to be significant for all the isotopic species studied. The rotational dependence of the 2 H nuclear coupling constants is predicted to be unimportant. The changes with rotational excitation of the boron coupling constants are significant only for higher vibrational levels in the ground electronic state. The 11B and 10B coupling is found to be relatively very strong for all isotopic variants, the 2 H coupling in 10 B 2 H + and 11 B 2 H + is, especially in the B′ 2 Σ + state, rather weak. The quadrupole hyperfine structure of 10 B 2 H + and 11 B 2 H + is essentially determined by relatively very strong boron coupling, the energy levels being perturbed by the deuteron coupling. In the B′ 2 Σ + state the perturbing effect is found to be negligible, in the low-lying rovibrational levels of the ground electronic state the corresponding splitting is found to be quite sizeable.

MRSD-CI calculations of deuteron quadrupole coupling constants for low-lying rovibrational levels of HD and D2 in their X1Σg+ and B1Σg+ states

Abstract The possibility of using the approximate MRSD-CI potential in ab initio calculations of deuteron quadrupole coupling constants of rovibrational levels of HD and D 2 in their X Σ g + and B Σ g + states is explored. It is shown that the ensuing approximate treatment of vibrational displacement of the molecules does not prevent the method from describing correctly the main features of the deuteron quadrupole coupling in the low-lying rovibrational levels of these systems. Some aspects for the future use of approximate potentials in the calculation of quadrupole coupling constants are outlined.

Theoretical model and ab initio calculation of potential energy surfaces for the reaction NH+·(2Π)+H2(1Σ+g)

Abstract This paper examines the doublet potential energy surfaces (PESs) governing the NH + (H 2 )→NH + 2 (H) reaction by combining ab initio calculations of the PES with a model Hamiltonian. It appears that in C 2v collisions, the deep ground-state minimum is separated from the reactants by a barrier of about 1.2 eV. In the region around the barrier, several PESs exhibit crossings which become avoided for lower symmetry. Thus, in this region low energy paths between reactants and products can arise which do not necessarily sample the deep minimum of the ground state. These paths, together with possible non-adiabatic transitions, can therefore account for the experimentally observed behaviour of the reaction.