Competition between spontaneous radiation and ionization in the process of resonance enhanced multi-photon ionization

摘要

Photo-ionization probability is an important factor in the practical use of resonance enhanced multiphoton ionization (REMPI). To a certain experimental condition, it depends on the competition between spontaneous radiation and ionization of the excited particles. In this work, we investigate the influence of laser resonance detuning, Rabi frequency and ionization rate on spontaneous radiation and ionization in the process of REMPI with the theory of density matrix equation. A model of three energy level system is adopted. It is found that the spontaneous radiation and ionization probability increase with the decrease of laser resonance detuning. They get to the maximum when resonance detuning equals zero. The line width of spontaneous emission will decrease with the increase of ionization rate due to the competition between spontaneous radiation and ionization. In addition, the spontaneous radiation and ionization probability increase with Rabi frequency until gets to saturation. Laser resonance detuning has no influence on the saturation value. It only influences the Rabi frequency for saturation. If Rabi frequency increases further after saturation, the spontaneous radiation will decrease because of the phenomena of energy level splitting in strong laser field. Now that resonant absorption and large laser intensity can increase the ionization probability greatly, so we must select suitable laser frequency and large laser intensity in the practical use of REMPI, in order to get optimum detection result.