Isolating resonant excitation from above-threshold ionization

Abstract

We measure photoelectron energy distributions from single ionization of xenon atoms by a linearly polarized laser pulse $(800\phantom{\rule{0.16em}{0ex}}\mathrm{nm},\phantom{\rule{0.28em}{0ex}}25\phantom{\rule{0.16em}{0ex}}\mathrm{fs})$ with successively varying the laser intensity within the region of $1.4\ensuremath{-}7.0\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$. By measuring the photoelectron energy shifted with the ponderomotive potential, we have calibrated the laser peak intensity precisely with an uncertainty less than $5%$. Employing the quantum trajectory Monte Carlo theory, we simulate the photoelectron energy spectra with respect to the laser intensity. By comparison between the measurement and the simulation, we are able to identify resonant structures from photoelectron energy spectra in the low-energy region, which do not shift with the laser intensity. Inspecting the momentum distribution along the laser propagation axis, we have further shown that the quantum interference effect has a significant effect on the width of the momentum distribution perpendicular to the laser polarization direction beside the Coulomb focusing effect.