Molecular above-threshold-ionization spectra: The effect of moving nuclei

Abstract

Exact non-Born-Oppenheimer simulations of a one-dimensional model of one-electron ${\mathrm{H}}_{2}^{+}$ and linear ${\mathrm{H}}_{3}^{2+}$ in an intense short laser pulse are used to investigate the nonlinear multiphoton electron emission spectra, called above threshold ionization (ATI). Due to the rapid proton motion on near-femtosecond time scale, the ATI spectra are found to be produced at the critical internuclear distance ${R}_{c}\ensuremath{\sim}7--8\mathrm{a}.\mathrm{u}.,$ leading to charge-resonance-enhanced ionization (CREI). As a consequence, maxima in the ATI spectra are displaced with respect to the similar H-atom spectra by a laser-induced Stark energy ${E}_{M}{R}_{c}/2,$ where ${E}_{M}$ is the maximum amplitude of the laser field. Highly oscillating ATI spectra occur, which are enhanced by the nuclear motion. These are interpreted as due to coherent excitations of the lowest unoccupied molecular orbitals and highest occupied molecular orbitals, which are also responsible for CREI. Electron rescattering effects in the energy regions of ${10U}_{p}$ and ${8U}_{p},$ where ${U}_{p}$ is the ponderomotive energy, are shown to be substantially reduced due to the rapid molecular dissociation and Coulomb explosion. Nevertheless, the ATI spectra in these energy regions reflect the different electron energies in the rescattering process. Fine structures of the ATI spectra are found to be enhanced by moving nuclei, reflecting the enhancement of resonant transitions by varying Franck-Condon factors during the nuclear motion.