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 $(22-68) \mathrm{TW}/{\mathrm{cm}}^2$. The observed doubly charged ion momentum distribution exhibits a distinct transition from a flat-top structure near zero longitudinal momentum at $22\mathrm{TW}/{\mathrm{cm}}^2$ to the one with two maxima at nonzero longitudinal momentum at $37\mathrm{TW}/{\mathrm{cm}}^2,52\mathrm{TW}/{\mathrm{cm}}^2$, and $68\mathrm{TW}/{\mathrm{cm}}^2$, 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.

• Received 8 September 2016

DOI:https://doi.org/10.1103/PhysRevA.94.053412