Recoil-ion momentum distribution for nonsequential double ionization of Xe in intense midinfrared laser fields

Abstract

We experimentally investigate the recoil-ion momentum distribution along the laser polarization direction for nonsequential double ionization of Xe by 50 fs, 2400 nm laser pulses at intensities of $(22\ensuremath{-}68) \mathrm{TW}/{\mathrm{cm}}^{2}$. The observed doubly charged ion momentum distribution exhibits a distinct transition from a flat-top structure near zero longitudinal momentum at $22\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/{\mathrm{cm}}^{2}$ to the one with two maxima at nonzero longitudinal momentum at $37\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/{\mathrm{cm}}^{2},\phantom{\rule{4pt}{0ex}}52\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/{\mathrm{cm}}^{2}$, and $68\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/{\mathrm{cm}}^{2}$, which is remarkably different from the case of 800 nm. Simulation based on a semiclassical model is used to obtain the ratios of contributions from the recollision-impact ionization (RII) and the recollision-induced excitation with subsequent field ionization (RESI) in nonsequential double ionization. Our calculation reveals that the increasing contribution of the RII channel is responsible for the more prominent double-hump structure at longer wavelength or higher laser intensity. Moreover, a simple fitting based on the calculated ratios allows one to reproduce the experimental ion momentum distributions well and obtain contributions from these two channels.