Suppression of resonant two-photon ionization rate by Rabi oscillation (Vol. 42, No. 3)

In the form of resonantly enhanced multiphoton ionization (REMPI), few-photon spectroscopy has become a tool in a number of applications. This is because multiphoton ionization (n+1) rate increases in proportion to the (n+1)-th power of laser intensity, where n is the number of photons for the resonance excitation and the resonance enhances excitation probability, compared to non-resonance channel. Unlike REMPI in optical laser frequency regime, the intermediate resonance is reached by absorbing one photon in VUV laser frequency regime. In such case, the resonance state emits a photon with the same phase and frequency as the laser light. This cycling process of absorbing and emitting is called Rabi oscillations, which might affect the multiphoton ionization rate.

In fact, we have revealed the strong suppression of the resonant two-photon ionization (1+1) rate of Ar+ ion that increases according to the second power of laser intensity within the framework of perturbation theory. Our interpretation of the new results, the Rabi oscillation on the course of the duration of a single light pulse of the VUV free-electron laser plays the crucial role to determine the resonant two-photon ionization rate, bridges a gap between multiphoton physics and quantum optics.