Investigation on the transparency of resonant absorption in the process of resonance-enhanced multiphoton ionization

The variation of absorption feature in the process of resonance enhanced multiphoton ionization (REMPI) as a function of resonance detuning, Rabi frequency and ionization rate is simulated theoretically. It is shown that at the beginning of laser radiation, absorption presents the character of a two-level system. The absorption intensity increases with the decrease of resonance detuning and gets to a maximum peak when the detuning equals zero. An interesting phenomenon will be observed after a period of time for laser radiation. Resonant absorption show itself as transparency. The transparent depth increases with the decrease of ionization rate. This is attributed to the evacuation of the population in excited resonant state decreases with the ionization rate. It is also found that with the increase of Rabi frequency, the resonant absorption increases at first and then decreases after it gets to a maximum, absorption transparency will appear at last. The transparent width increases with Rabi frequency. We interpreted these to the splitting of the energy level (ac Stark effect) induced by laser field. When resonant absorption changes to the splitting levels, transparency appears at the position of original resonant point. The larger Rabi frequency is, the wider of the interval of the splitting levels will be. So the transparent width increases with Rabi frequency.