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

Abstract

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 ${P}_{\mathrm{ion}}$ has shown characteristic dependences on the scaled coupling parameter ${U}_{gi}$ between two levels of the ground ($g$) and intermediate ($i$) resonance states, namely, ${P}_{\mathrm{ion}}\ensuremath{\propto}{({U}_{gi})}^{n}$, with $n$ being equal to \ensuremath{\sim}2, less than 1, and \ensuremath{\sim}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., $n$ \ensuremath{\sim} 1. Thus, the observed linear laser-power dependence has been rationalized as being caused by the strong Rabi oscillations between the (2$p$)--(1$s$) states.