Dissociative ionization ofH2+in an intense laser field: Charge-resonance-enhanced ionization, Coulomb explosion, and harmonic generation at 600 nm

Abstract

The time-dependent Schr\"odinger equation for ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$ in a 600-nm, intense (I\ensuremath{\geqslant}${10}^{14}$ W/${\mathrm{cm}}^{2}$) laser field is solved numerically for a model which uses the exact three-body Hamiltonian with one-dimensional nuclear motion restricted to the direction of the laser electric field and three-dimensional electronic motion. High ionization rates of ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$ are found, exceeding those of neutral atomic hydrogen. This confirms, by the rigorous, full dynamical calculation, the recently discovered charge-resonance-enhanced ionization (CREI)---all previous demonstrations of CREI were based on the ``frozen nuclei'' model. The numerical kinetic-energy spectra of dissociating fragments are compared with recent experimental results. They can be interpreted by a simple bond softening mechanism (or laser-induced avoided crossing in a dressed-state representation), in which the binding forces are completely suppressed by the strong electric field and thus the dissociating fragments move as free particles with a kinetic energy close to their initial vibrational energy until they reach a critical distance R=${\mathit{R}}_{\mathit{c}}$\ensuremath{\cong}8 bohr, where they are rapidly ionized, due to CREI. The harmonic generation spectra calculated from our non-Born-Oppenheimer simulations show that the high harmonics are also generated when the nuclei cross this critical distance R=${\mathit{R}}_{\mathit{c}}$. \textcopyright{} 1996 The American Physical Society.