Pushing multiphoton resonant ionization of the argon atom to the low-intensity regime

Abstract

Resonance-enhanced multiphoton ionization process of the argon atom by an 800-nm 30-fs linearly polarized laser field is investigated at intensities range from 1.1 to $4.55\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. At $4.55\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ intensity the experimental photoelectron energy spectrum is in a good agreement with the time-dependent Schr\"odinger equation (TDSE) calculation where the double structure originating from dressed $4p\ensuremath{-}4d$ coupled transition is clearly identified. At lower intensity of $1.1\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$, the resonant ionization process via the $4f$ state is observed, however, the expected peak (jet) at ${90}^{\ensuremath{\circ}}$ in the photoelectron angular distribution from the zeroth order above threshold ionization, has vanished completely. Such behavior is attributed to the destructive interference phenomenon in the coherent contributions of different partial waves of the photoelectron, namely, $\ensuremath{\epsilon}d$ and $\ensuremath{\epsilon}g$ states, and has been confirmed in our TDSE calculations.