The analytic formula of the ionization efficiency in the process of double resonance enhanced multi-photon ionization (DREMPI) is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the ionization efficiency and the laser power index versus laser intensity in the DREMPI process of NO molecule, via A2\sum and S2\sum intermediate resonant states, is numerically simulated. It is shown that the ionization efficiency of NO molecule increases with the laser intensity until getting saturation, while the laser power index decreases with the enhancement of the laser intensity and changes to zero at last. The variation of the laser power index with the laser intensity indicates that the ionization efficiency reaches saturation in the one, two, and three excitation steps respectively. It is also found that the narrower the laser pulse duration is, the higher becomes the laser intensity for saturation.

在激光强度接近实际的时间和空间分布下，计算了三能级系统(1+1)多光子过程电离效率和光强指数随激光能量的变化关系。并将各种分布情况下的计算结果与采用方波近似得到的结果进行了比较,得到了激光强度的时间和空间分布对多光子过程的影响。实验也证实了模拟计算结果的正确性。