Pulse-delay effects in the angular distribution of near-threshold EUV + IR two-photon ionization of Ne

Photoelectron angular distributions (PADs) from two-photon near-threshold ionization of Ne atoms by the combined action of femtosecond pulses from an extreme ultraviolet (EUV) free-electron laser and infrared (IR) laser have been studied experimentally and theoretically. Solutions of the time-dependent Schrödinger equation indicate that the PADs strongly depend on the time delay between EUV and IR pulses. The experimental results obtained for two extreme cases of completely overlapping and nonoverlapping pulses fully confirm the prediction, illustrating that the measurements of the time-delay dependence of the PAD provide a tool for investigating the fundamental problem of the relative importance of the resonant and nonresonant pathways in the two-color two-photon processes.

Figure 1

(a) Spectrum of photoelectrons measured at the EUV photon energy of 24.3 eV. The central peak at 2.7 eV is due to ionization of the Ne $2p$ subshell by the EUV pulse. Two smaller peaks at the energies of 1.15 and 4.25 eV are sidebands which arise due to interaction with the IR field. Right panels show the measured (dashed lines) and calculated (solid lines) angular distributions of photoelectrons at the LSB (b) and the HSB (c). The bars at 15 and 45${}^{\circ }$ show typical error bars of the measured angular distributions.

Figure 2

(a) Spectrum of photoelectrons measured at photon energy 21.3 eV. The peak at 1.25 eV corresponds to the ionization of the Rydberg wave packet by the IR field. (b) The angular distribution of photoelectrons of the 1.25-eV peak for the case when EUV and IR pulses are not overlapped. Dashed line, measured in experiment 1 (10-ps delay); dash-dotted line, measured in experiment 2 (0-ps delay) by selecting the nonoverlapping events; solid line, calculations. (c) The angular distribution of photoelectrons of the 1.25-eV peak for the case in which EUV and IR pulses are overlapped. Dash-dotted, measured in experiment 2 (0-ps delay) with selection of the overlapping events; solid line, calculations. The bars at 15 and 45${}^{\circ }$ show typical error bars of the measured angular distributions.

Figure 3

The calculated dependence of asymmetry parameters ${\beta }_2$ (closed circle) and ${\beta }_4$ (open circle) on delay between EUV and IR pulses for the case of EUV photon energy 21.3 eV. The parameters are shown for the angular distributions at the maximum of the peak (1.25 eV).