一种校验带电粒子速度成像探测器的新方法

A new method to calibrate charged-particle velocity map imaging spectrometer

带电粒子(离子或者电子)速度成像技术可以精确测量带电粒子产物的速度分布和角分布,获得光谱级别分辨率的原子分子能级结构,广泛应用于光电离和光解离等分子反应动力学实验中.速度成像探测器的效率和信号相对强度对带电粒子速度分布信息的提取具有重要的影响.由于难以精确测量入射离子数,至今仍然没有直接校验速度成像探测器的方法报道.本文基于铷原子磁光阱搭建了一套离子速度成像实验装置,实现了入射离子数的精确测量及连续可调,并对冷原子的光电离过程、微通道板探测器采集的飞行时间信号、电荷耦合器件采集的荧光屏信号进行了校验,对可能影响速度成像信号相对强度的因素进行了检验.实验结果表明:当入射离子数低于1×10^(5)个/(100 ns)时,微通道板探测器的增益稳定,而荧光屏信号强度与入射其表面的电子数量的关系受荧光屏电压影响比较严重.同时,结果表明本方法可以有效校验带电粒子速度成像探测器的效率和信号的相对强度,对使用速度成像技术的实验具有一定的借鉴意义.

Velocity map imaging(VMI) technique has been widely used in the fields of atomic and molecular physics and chemical physics for determining velocity distributions of the charged-particles generated in photo-dissociation and ionization processes. Under VMI conditions,the charged-particles with same nascent velocity are projected to the same position on a two-dimensional detector,independent of where they are formed in the ionization volume. The kinetic energy and angular distribution of the incident charged-particles can be obtained by the information of the position and the relative intensity of the signals in the image on the phosphor screen,respectively. In the past three decades,promotion of the positional resolution of VMI detector has been well studied. However,few attentions have been paid for the calibration of the relative intensity of the signals in the image,which will influence the analysis of the angular distribution of the incident charged-particles.In the present work,the VMI technique was introduced into the experimental study of photoionization of cold (87)^Rb atoms in a magneto-optical trap(MOT). The signals from VMI detector,including the electron signal from the microchannel plate(MCP) detector and the image from the charge-coupled device,were acquired under the conditions of different incident numbers of ions. The results showed that the MCP detector has a constant gain performance when the incident ion number is less than 1 × 10^(5)/(100 ns),and the voltage applied on phosphor screen has a considerable influence on the response of phosphor to the incident electrons,which directly correlates with the relative intensity of the signals in the image. Therefore,this method can be applied to calibrate the VMI detector effectively,especially the relative intensity of the signals in the image,which offers a reference for related experimental researches.