Few-photon absorption and emission in the laser-assisted single photoionization

We investigate the few-photon absorption and emission in the laser-assisted single photoionization of hydrogen (H) atom. The results show that the electron wave ionized by the ultraviolet (UV) field can absorb or emit more than one infrared (IR) field photon. With the IR photon increases, the intensity of corresponding photoionization peak is getting weak and the distribution of them in time gets narrower. Furthermore, the central peak of photoelectron spectrum without absorbing or emitting IR photon shows a valley, which minimum decreases with the increase of IR intensity and wavelength. The sideband peaks of photoelectron spectrum absorbing or emitting one or two IR photon show a peak, which maximum increases with the increase of IR intensity and wavelength. Through analysis, we present that the changes in the photoionization peak are decided by the delay-dependent Bessel function.