(1+2+1)双共振多光子电离概率的理论研究

Theoretical Study of 1+2+1 Double-Resonance Multiphoton Ionization Probability

共振增强多光子电离光谱技术已成为研究原子、分子高激发态能级结构的重要方法。运用光和物质相互作用的速率方程理论,推导出四能级物质系统1+2+1双共振增强多光子电离概率的解析表达式,以此为基础,理论模拟了电离概率随激发光强、激光脉冲宽度和碰撞弛豫速率的变化,发现在1+2+1多光子电离机制中,电离概率随光强的增加而增大,继而出现单步、双步激发饱和的现象,直至饱和值1;继续增大光强,电离概率将围绕饱和值1窄幅振荡,振荡幅度随光强增加而增大。随激光脉冲宽度的增大,电离概率从零开始逐渐增大直至饱和值1。而随碰撞弛豫速率的增大电离概率以线性规律减小。

The technique of resonance enhanced multiphoton ionization（REMPI） spectroscopy has already become an important method for studying the energy level structure of atoms and molecules.Analytic expression of 1＋2＋1 double REMPI probability of four-level system has been deduced with the theory of rate equation.Based on the expression,the present paper simulated the variations of ionization probability versus laser intensity,laser pulse duration and collision relaxation rate.It was found that in the 1＋2＋1 ionization mechanism the ionization probability increases with laser intensity,until saturation phenomenon appears in one and two excitation steps.If laser intensity increases further,the ionization probability will oscillate around the saturation value,and the oscillation amplitude will increase with laser intensity.With regard to the influence of laser pulse duration and collision relaxation rate on the ionization probability,the results show that the ionization probability increases from zero to the saturation value 1 with the increase in laser pulse duration,while it decreases linearly with the increase in collision relaxation rate.