High-order above-threshold ionization in stretched molecules

Abstract

By numerical solution of the time-dependent Schr\"odinger equation, highly accurate electron spectra are calculated for strong-field ionization of a one-dimensional $\mathrm{H}_{2}{}^{+}$ molecular ion at large internuclear distances. Compared to the atomic spectra, we find additional cutoffs at high electron energies. The classical model for ionization (simple-man's model) adapted to the molecular geometry predicts cutoffs up to $50{U}_{\mathrm{p}}$, where ${U}_{\mathrm{p}}$ is the ponderomotive potential. The cutoffs correspond to various scattering scenarios of the tunneled electron from the molecular sites. For certain internuclear distances the agreement between the classical predictions and the numerical spectra is unsatisfactory. We propose a modified simple-man's model based on complex electron trajectories, allowing for electrons appearing in the continuum with nonzero initial velocity from the tunneling ionization process. Agreement of the resulting cutoffs with the numerical results is recovered.