Deformations of CO 2 , CO 2 + , and CO 2 2+ in intense laser fields (>10 14 W/cm 2 ) are investigated by using potential energy surfaces of field-following adiabatic states at various instantaneous field strengths. The adiabatic states are obtained by ab initio molecular orbital calculations. To predict tunnel ionization of multi-electron molecules, we propose a new approach based on the idea that electron transfer induced by an intense laser field charges each atom in a molecule and that ionization proceeds via the most negatively (or least positively) charged atomic site. We conclude that bond stretching in CO 2 2+ accompanied by large amplitude bending motion is responsible for the experimentally determined geometrical structure of Coulomb explosion species CO 2 3+ , namely, that the C-O bond length is stretched to about 1.6 Å and the mean amplitude of bending is relatively large (∼40°). † Part of the special issue "Edward W. Schlag Festschrift".

To elucidate the nature of a pseudo-Jahn-Teller (JT) distortion from planarity, ab initio MCSCF calculations with 6-31G(d) basis sets are carried out on the ground state of the titled molecules. Cycloheptatriene undergoes the pseudo-JT distortion from a planar C 2V to a boat C s conformation. In heptalene, the pseudo-JT distortion takes place in two stages, the initial step being from a symmetric planar D 2h to a skew planar C 2h structure and the subsequent step from the skew C 2h to a twisted C 2 one. In heptafulvalene, the symmetric planar D 2h structure exhibits a third-order saddle point and, hence, the pseudo-JT distortions take place from the D 2h to D 2 , C 2V , and C 2h ones through the respective out-of-plane nuclear deformations. Besides, the twisted D 2 structure exhibits a second-order saddle point with respect to the syn-and anti-folding motions of the sevenmembered rings and the structural changes take place from the D 2 to the C 2V and C 2h structures. A comparison of the total energy reveals that the ground state of heptafulvalene takes the anti-folded C 2h conformation. These structural characteristics are in good agreement with the available experimental facts. Energy component analyses reveal that in these molecules the pseudo-JT distortion from planarity should lead markedly to a lowering of the electron-nuclear attraction energy and concomitantly to raising of both the interelectronic and the internuclear repulsion energy. These energetic behaviors are accounted for in terms of the electrostatic interactions and the proximity between the nuclei and electron clouds involved in the CC and CH bonds, owing to the folding of the carbon skeleton.

To clarify the origin of nonplanarity in eight π-electron heterocyclic conjugated systems, an energy component analysis was carried out for the ground states by using ab initio MCSCF method with 6-31++G(d,p) basis set. Inspection of the energy components comprised in the total energy reveals that the type of pseudo-JT stabilization is classified into two groups, one in which the stability of nonplanar structure arises from a lowering of the interelectronic and internuclear repulsion energies and the other in which the stability results from a lowering of the electron-nuclear attraction energy. This sharp distinction in energy changes is accounted for in terms of an expansion or contraction of the molecular skeleton and a proximity among the nuclei and the electron clouds owing to a folding of the six-membered ring. Besides, it is shown that the theoretical structural characteristics for 1,2-dihydro-and 1,4-dihydro-1,2,4,5-tetrazines are in good agreement with available experimental facts.