TUNNEL IONIZATION OF H-2 IN A LOW-FREQUENCY LASER FIELD - A WAVE-PACKET APPROACH

The dynamics of multielectron dissociative ionization (MEDI) of H-2 in an intense IR laser pulse are investigated using a wave-packet propag ation scheme. The electron tunneling processes corresponding to the su ccessive ionizations of H-2 are expressed in terms of field-free Born- Oppenheimer (BO) potential energy surfaces (PES) by transforming the t unnel shape resonance picture into a Feshbach resonance problem. This transformation is achieved by defining anew, time-dependent electronic basis in which the bound electrons are still described by field-free BO electronic states while the ionized ones are described by Airy func tions. In the adiabatic, quasistatic approximation, these functions de scribe free electrons under the influence of the instantaneous electri c field of the laser and such an ionized electron can have a negative total energy. As a consequence, when dressed by the continuous ejected electron energy, the BO PES of an ionic channel can be brought into r esonance with states of the parent species, This construction gives a picture in which wave packets are to be propagated on a continuum of c oupled electronic manifolds. A reduction of the wave-packet propagatio n scheme to an effective five-channel problem has been obtained for th e description of the first dissociative ionization process in H-2 by u sing Fano's formalism [U. Fano, Phys. Rev. 124, 1866 (1961)] to analyt ically diagonalize the infinite, continuous interaction potential matr ix and by using the properties of Fano's solutions. With this algorith m, the effect that continuous ionization of H-2 has on the dissociatio n dynamics of the H-2(+) ion has been investigated. In comparison with results that would be obtained if the first ionization of H-2 was imp ulsive, the wave-packet dynamics of the H-2(+) ion prepared continuous ly by tunnel ionization are markedly nonadiabatic. The continuous ioni zation appears to give rise to a population in the dissociative contin uum that is localized at small internuclear distances throughout the a ction of the laser pulse, and is released only when the laser pulse is over, yielding a complex fragment kinetic energy spectrum. Comparison with available experimental data is made.