Tunnel ionization of H_{2} in a low-frequency laser field: A wave-packet approach

The dynamics of multielectron dissociative ionization (MEDI) of ${\mathrm{H}}_2$ in an intense IR laser pulse are investigated using a wave-packet propagation scheme. The electron tunneling processes corresponding to the successive ionizations of ${\mathrm{H}}_2$ are expressed in terms of field-free Born-Oppenheimer (BO) potential energy surfaces (PES) by transforming the tunnel shape resonance picture into a Feshbach resonance problem. This transformation is achieved by defining a new, 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 functions. In the adiabatic, quasistatic approximation, these functions describe free electrons under the influence of the instantaneous electric 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 resonance with states of the parent species. This construction gives a picture in which wave packets are to be propagated on a continuum of coupled electronic manifolds. A reduction of the wave-packet propagation scheme to an effective five-channel problem has been obtained for the description of the first dissociative ionization process in ${\mathrm{H}}_2$ by using Fano’s formalism [U. Fano, Phys. Rev. 124, 1866 (1961)] to analytically diagonalize the infinite, continuous interaction potential matrix and by using the properties of Fano’s solutions. With this algorithm, the effect that continuous ionization of ${\mathrm{H}}_2$ has on the dissociation dynamics of the ${\mathrm{H}}_2^{+}$ ion has been investigated. In comparison with results that would be obtained if the first ionization of ${\mathrm{H}}_2$ was impulsive, the wave-packet dynamics of the ${\mathrm{H}}_2^{+}$ ion prepared continuously by tunnel ionization are markedly nonadiabatic. The continuous ionization appears to give rise to a population in the dissociative continuum that is localized at small internuclear distances throughout the action 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.