Anomalies in above-threshold ionization observed inH2and its excited fragments

Abstract

The photoelectron energy spectra resulting from a multiphoton process in ${\mathrm{H}}_2$ with high laser intensities is, in general, determined by the character of the intermediate resonance state. Above-threshold ionization (ATI) is observed in the ionization of the molecule ${\mathrm{H}}_2$ as well as of its excited atomic fragments, and even of atomic hydrogen in the ground state. ATI is observed with five, three, and two extra photons for three different wavelengths λ=532 nm, λ=355 nm, and λ=266 nm, respectively. In the λ=532 nm spectra radiative coupling between dressed states is observed. A shift to higher values of the center of the vibrational distribution of the ion by seven vibrational quanta is observed between ATI peaks which differ by four photon energies. A calculation, taking into account a coupling of the ${\mathit{B}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ state to the dressed ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$(2p${\mathrm{\sigma }}_{\mathit{u}}$ ${)}^2$ state, and of the X ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$ state by the dressed 2p${\mathrm{\sigma }}_{\mathit{u}}$ state, shows the trend of the observed ionization spectrum. The calculation is performed to describe the effect on the average of the vibrational distribution and not on the distribution itself. The structure of the ATI spectrum in the λ=355 nm and λ=266 nm is nearly a reproduction of the normal ionization spectrum, repeated several times at higher electron energies. In the λ=266 nm spectra both ATI of the molecule, as well as ATI of the atomic fragments, are observed.