Suppression of ionization probability due to Rabi oscillations in the resonance two-photon ionization of He by EUV free-electron lasers

摘要

The two-photon resonance ionization probability of atoms in strong extreme-ultraviolet free-electron laser (EUV FEL) pulses has been investigated by the model of time-dependent wave packet propagation of a light-coupled multilevel atom. Under the simulation within the model assuming single-mode FEL pulses, the ionization probability P_(ion) has shown characteristic dependences on the scaled coupling parameter U_(gi), between two levels of the ground (g) and intermediate (i) resonance states, namely, P_(ion) ∝ (U_(gi))~n , with n being equal to ～2, less than 1, and ～1 for the small, medium, and large U_(gi) regimes, respectively. This power dependence of the ionization probability has been interpreted due to Rabi oscillations between g and i states. To compare with recent experimental results on the same condition, the multimode nature of self-amplitude spontaneous emission (SASE) FEL pulses has been managed in the simulation. Then, the recent experimental laser-power dependence of the two-photon resonance ionization of He [Sato et al., J. Phys. B 44, 161001 (2011)] has been well described by that for the large U_(gi) regime of the simulation, i.e., n～1. Thus, the observed linear laser-power dependence has been rationalized as being caused by the strong Rabi oscillations between the (2p)-(1s) states.