光电子成像方法研究Xe时间分辨多光子电离过程

Time-Resolved Multiphoton Ionization Process of Xenon Investigated by Photoelectron Imaging Method

利用自行研制的离子成像检测器研究了Xe的飞秒时间分辨双色多光子电离过程.Xe的408nm多光子电离对比实验结果表明,该离子成像检测器与相应的进口产品具有相近的光电子能量分辨率.在272nm飞秒激光作用下,3光子电离产生能量为1.57和0.26eV的光电子,分别对应于Xe+的两个自旋态;在408nm飞秒激光作用下,还观察到第一级阈上电离产生的光电子.在双色飞秒时间分辨实验中,随着两束光相对时间的改变,光电子能谱出现了一系列的变化;随着两束光时间重合程度的增加,由双色多光子电离(3+1'或4'+1)产生的光电子信号逐渐加强;在第二束光的作用下,由第一束单色光产生的光电子出现能量红移,第二束光同时也导致中间态布居数减少.这种光电子能谱的红移现象反映了原子体系中激光场诱导有质动力势的时间分辨动态调制过程.

Femtosecond time-resolved multiphoton ionization dynamics of xenon was investigated using a homemade ion imaging detector. A comparison experiment comprising of the multiphoton ionization of Xe at 408 nm showed that the energy resolution of our homemade image detector was similar to that of a commercial detector. Under 272 nm femtosecond laser irradiation, photoelectrons with a kinetic energy of 1.57 and 0.26 eV, produced by three-photon ionization, corresponded to two different Xe＋ spin states, respectively. For the ionization at 408 nm, an additional first-order above-threshold ionization of Xe was also observed. In the two-color femtosecond time-resolved experiments, the photoelectron kinetic energy spectra varied with the delay time between the pump and the probe. The photoelectron intensities produced by 3＋1＇ and 4＇＋1 two-color multiphoton ionization schedules became stronger with an increase in the degree of overlap between the two laser beams. The kinetic energy of the photoelectrons produced by one-color multiphoton ionization showed obvious red shifts, which were modulated by the second laser beam. Depopulation of the excited states was also observed upon application of the second laser beam. The red shifts in the photoelectron kinetic energy spectra reflect the time-dependent dynamical modulation process of the laser induced ponderomotive effect in an atomic system.