铷离子-铷原子混合阱飞行时间谱的拟合和仿真模拟

Analysis and simulation of time-of-flight spectrum in Rb+-Rb hybrid trap

离子-原子混合阱是研究带电粒子-中性粒子低温反应的理想平台,直接甄别反应产物最准确的方法是带电粒子飞行时间谱,飞行时间谱峰的强度、位置(飞行时间)和宽度给出了相应带电粒子的强度和动能(温度)等信息.本文通过分析和模拟铷离子-原子混合阱中的飞行时间谱,获得了不同荷质比的离子绝对强度和温度等信息.具体说,首先使用Gumbel型极值分布函数飞行时间谱的谱峰,获得谱峰强度、位置和宽度等信息.然后对实验建模得到耦合的原子数和总带点离子的速率方程,用这些速率方程拟合实验数据,并结合实验测量到的绝对原子数,获得绝对的离子数强度.由此提供了一种标定探测器(本文使用的是微通道板)的方法.改变电离激光的波长和强度得到的标定因子是一致的,表明了这种方法的可靠性.此外,利用COMSOL Multiphysics模拟实验的飞行时间谱,仿真模拟结果表明离子动能大,谱峰宽度窄.本文对飞行时间谱的强度和宽度分析为冷原子光电离过程的离子-原子反应碰撞和带电粒子温度弛豫奠定了基础.

The time-of-flight mass spectrum of charged particles,which are created through two-step cw-laser photoionization of laser-cooled ^(87)Rb atoms in an ion-neutral hybrid trap,is quantitatively investigated to further facilitate the study of Rb^(+)-Rb reactive collisions.A microchannel plate(MCP)is used to detect charged particles,and two spectral peaks corresponding to the ^(87)Rb^(+)ions and the product ^(87)Rb_(2)^(+) of the Rb^(+)-Rb reaction were observed in the time-of-flight spectrum,respectively.The two peaks overlapped with each other and both showed an asymmetric profile.The information about the intensity,position,and half-width of the peak for a specific ion species was derived by fitting the time-of-flight spectrum with the probability density function of the Gumbel distribution.Then the relative ion intensity was converted into absolute ion number through the following steps.The rate equation of the total number of ions,which includes the number of atoms,the calibration factor of MCP,and the effective decay rate of ions in the ion trap,was established by modeling the photoionization of atoms.Combined with the absolute number of atoms measured by absorption imaging,the calibration factor in converting the ion intensity into the ion number was derived and the relative ion intensity was converted into the absolute number of ions.This provides a method of calibrating the MCP.The reliability of our calibration method was proved by the fact that the calibration factor in converting the intensity measured by MCP into particle number is independent of the duration of photoionization,the intensity and wavelength of the ionizing laser.Moreover,in order to explain the relationship between the peak width and temperature of the corresponding ion species,the time-of-flight spectra of the ions trapped in the ion trap were simulated by using COMSOL Multiphysics.The simulation results demonstrated that the large ion kinetic energy results in a narrow spectral peak.In sum,we quantitatively analyze and simulate the time-of-flight spectrum of the photoionization of cold atoms in the Rb^(+)-Rb hybrid trap.The absolute number of ions is obtained by the intensity of the spectral peak,and the width of the spectral peak is related to the temperature of the ions.These results lay a foundation for the in-depth analysis of the ion-atom reaction collision and charged particle temperature relaxation in the photoionization of cold atoms,and thus further elucidating the subsequent collisional dynamics of ultracold plasmas.