Influence of Doppler effect on the absorption in the process of resonance multiphoton ionization

The theory of density matrix equation is used to describe the interaction between laser field and atoms or molecules (referred as particles). With this theory, this paper investigated the influence of Doppler effect on the resonance absorption in the process of resonance enhanced multiphoton ionization (REMPI) of particles. The results show that at the beginning of laser radiation, Doppler effect will broaden the absorption line. This is generally known as Doppler broadening. After laser radiates for a period of time, light transparency will appear at the position of near zero resonance detuning. This is caused by AC Stark splitting. Thus this phenomenon appears only when Rabi frequency is large enough. Doppler effect can make light transparency disappear, because the gap between the two peaks beside of transparent window is smaller than the line-width of Doppler broadening. It will also enlarge the Rabi frequency for transparency.