Two-color photoionization in xuv free-electron and visible laser fields

Two-photon ionization of atomic helium has been measured by combining femtosecond extreme-ultraviolet pulses from the free-electron laser in Hamburg (FLASH at DESY) with intense light pulses from a synchronized neodymium-doped yttrium lithium fluoride laser. Sidebands appear in the photoelectron spectra when the two laser pulses overlap in both space and time. Their intensity exhibits a characteristic dependence on the relative time delay between the ionizing and the dressing pulses and provides an inherent time marker for time-resolved pump-probe experiments. The measurements of the sidebands are in good agreement with theoretical predictions and allow for a direct analysis of two-photon ionization, free from processes related to interference between multiple quantum paths.

Figure 1

(Color online) Experimental setup for the pump-probe experiments using the combination of the FEL with an optical laser.

Figure 2

Photoelectron spectrum for the one-photon ionization of He atoms by the xuv radiation from the FEL. The energy positions where sidebands created by the additional dressing field would be observed are indicated by the dotted lines. The inset shows the high-kinetic-energy part of the spectrum.

Figure 3

(Color online) Part of the photoelectron spectrum in the region of the He $1s^{-1}$ main line and the high-energy sideband. Different spectra are given as a function of the time delay between the pulses of the FEL and the optical ($12\mathrm{ps}$, $523\mathrm{nm}$) laser.

Figure 4

(Color online) Theoretical photoelectron spectrum of He obtained from TDSE calculations for the present characteristics of the FEL and optical laser. The interaction with the FEL radiation leads to one-photon and two-photon ionization of the He $1s$ shell (thin black line); the optical laser field creates the additional sideband structures (bold red line). The inset shows the He $1s^{-1}$ photoline and the first sidebands on a linear intensity scale.

Figure 5

Cross-correlation signal between the pulses of the FEL ($50\mathrm{fs}$, $32.2\mathrm{nm}$) and of the optical ($12\mathrm{ps}$, $523\mathrm{nm}$) laser.