Quantum mechanical study of electronic and nuclear dynamics of molecules in intense laser fields

Abstract

Abstract The dynamical behaviour of H 2 + in near-infrared, intense laser fields ( I >10 13 W/cm 2 and λ >700 nm) is examined with accurate evaluation of the electronic and nuclear wave packet by the dual transformation method we developed to deal with the dynamics in Coulombic systems. Using “field-following” time-dependent adiabatic states defined as eigenfunctions of the “instantaneous” electronic Hamiltonian, we have clarified the dynamics of the bound electron, ionization, Coulomb explosion, and molecular vibration of H 2 + . The present analysis of H 2 + indicates that the multi-electron dynamics and nuclear dynamics of polyatomic molecules in intense fields can be described by using the potential surfaces of time-dependent adiabatic states and the nonadaiabatic coupling elements between those states. To obtain time-dependent adiabatic states of a molecule, the electronic Hamiltonian including the interaction with the instantaneous laser electric field is diagonalized by ab initio molecular orbital (MO) methods. While the time-dependent adiabatic potentials calculated by MO methods are used to evaluate the multichannel nuclear dynamics until the next ionization process, the charge distributions on individual atomic sites can be used to judge whether ionization occurs or not at a given light intensity. We have applied the time-dependent adiabatic state approach to reveal characteristic features of the dynamics of structural deformations of CO 2 and its cations in a near-infrared intense laser field (∼10 15 W cm −2 ). In the CO 2 and CO 2 + stages, ionization occurs before the field intensity becomes high enough to deform the molecule. In the CO 2 2+ stage, simultaneous symmetric two-bond stretching occurs as well as one-bond stretching. Two-bond stretching is induced by an intense field in the lowest time-dependent adiabatic state |1〉 of CO 2 2+ , and this two-bond stretching is followed by the occurrence of a large-amplitude bending motion mainly in the second-lowest adiabatic state |2〉 nonadiabatically created from |1〉 at large internuclear distances by the field. It is concluded that the experimentally observed stretched and bent structure of CO 2 3+ just before Coulomb explosions originates from the structural deformation of CO 2 2+ . We also applied the present approach to investigate dissociative ionization of C 2 H 5 OH in near-infrared, intense laser fields. The calculated ratio of the probability of C–O bond cleavage to that of C–C bond cleavage becomes smaller with decreases in the pulse length, as was observed in the experiment by Itakura et al. [J. Chem. Phys. 119 (2003) 4179]. In the case where the C–C axis is parallel to the polarization direction of an applied laser field, upper C–C bond dissociative adiabatic states are created owing to field-induced nonadiabatic transitions even if the pulse length is as short as 30 fs. This example clearly shows that field-induced nonadiabatic transition plays a decisive role in the reaction dynamics of molecules in intense laser field.