Multiplateau structure in photoemission spectra of strong-field ionization of dense media

Strong-field ionization of dense molecular gases in a short infrared laser pulse is studied by means of photoelectron spectroscopy combined with a liquid microjet technique. By increasing the gas density, we observe how the laser-assisted electron scattering on neighboring particles becomes a dominant mechanism of hot electron emission. The angle-resolved energy distributions of rescattered electrons are obtained by analyzing the density dependency of emission spectra. A semiclassical consideration of electron trajectories is shown to provide a good description of experimental spectra. The model predicts the existence of four energy plateaus. Two cutoffs at higher energies are evident in the spectra.