量子统计权重对原子场电离效率的影响

Influence of quantum statistical weight on efficiency of electric field ionization of atom

原子能级的量子统计权重(G)是原子的重要光谱参数,但在研究原子的电离过程中通常却为了简化问题而被忽略.本文在锂原子的三步光激发(PE)+电场电离(EFI)过程中计入了其影响,并发现其对原子EFI效率的影响显著.本文精心设计了一套锂原子的PE+EFI方案:首先,采用三台不同波长的脉冲激光器分三步将原子从基态激发到某一Rydberg态上,经过一段时间延迟后再施加脉冲电场将其电离.针对原子所经历的PE、零场和EFI这三个物理过程,本文对其物理机制进行了分析并建立了服从粒子数守恒的物理模型进而导出了显含G参数的普适的速率方程组.其次,通过Matlab编程,分别针对精心选定的两条激发路径2S_(1/2)→2P_(1/2)→3S_(1/2)→25P_(1/2,3/2)和2S_(1/2)→2P_(3/2)→3D_(5/2)→25F_(5/2,7/2)开展了数值计算.研究发现:PE+EFI过程的总体效率的上限既与激光参数无关,也不依赖于G参数的绝对值,而是决定于Rydberg态的G参数的分支比.总之,通过精选激发路径可以调控PE过程各相关态的布居率,并能适当提高PE+EFI过程的总电离效率,但却因受到Rydberg态布居率的限制而很难进一步提高.

The quantum statistical weight（G） of an atomic energy level is an important spectroscopic parameter, its effect on the atomic ionization process is, however, usually neglected for simplicity. In this work, the influences of the G parameters of the lithium atomic energy levels are taken into account explicitly for the first time in the study on the process of three-step photo-excitation（PE） ＋ electric field ionization（EFI）, which yields a significant effect on overall EFI efficiency of the PE＋EFI process. With a set of specially designed PE＋EFI schemes, the expected results are obtained. First, with a three-step PE scheme, the Li atom is excited by three pulsed lasers with different wavelengths,which are fired simultaneously, to one of the Rydberg states from its ground state, from which the Li atom is ionized by an electric-field pulse with a time delay in order to avoid the Stark effect. Based on the three physical processes the atom experiences the PE, none field, and the EFI processes, and a set of universal rate equations are established according to the conservation law of particle number with the knowledge of physical mechanism of the three different processes and the physical model set up for them, respectively. The G parameters of the four relevant bound energy states are displayed explicitly in the rate equations for the PE process to offer a clear viewabout their effect on the overall EFI efficiency of the PE＋EFI process. Secondly, the overall efficiencies of PE＋EFI process are calculated with the Matlab programming for the two specified excitation schemes, 2S_（1/2）→ 2P_（1/2）→ 3S_（1/2）→ 25P_（1/2,3/2）and 2S_（1/2）→ 2P_（3/2）→ 3D_（5/2）→ 25F_（5/2,7/2）.The overall EFI efficiency of PE＋EFI process is investigated not only by adjusting the laser parameters but also by comparing the results between the two different excitation schemes. In order to establish the relationship between the overall EFI efficiency and external field quantitatively, the dependence of population rate of the relevant bound states on various factors, such as laser and atomic parameters, is calculated systematically. The role of the G parameters of the relevant atomic energy levels played in the population rates is observed to determine which excitation scheme is better in terms of the population rate of the Rydberg state. Meanwhile, the spontaneous emission of the Rydberg state during the time delay between the pulses of electric field and laser is also evaluated to make a balance between avoiding the Stark effect and minimizing the spontaneous emission. Based on the analysis of the calculations, some new results are achieved.An enhancement of the overall EFI efficiency can be obtained by making a sophisticated design on the excitation scheme of the PE＋EFI process to optimize the G parameters. The time delay between the pulses of electric field and laser not only lets the atom experience a field-free time period, but also makes an upper limit for the population rate of Rydberg state due to the redistribution of atom among the four relevant bound states in the period. The upper limit is found to be dependent on neither laser parameters nor the absolute values of the G parameters, while only on the branching ratio of the G parameters among those bound states. The overall EFI efficiency is dominated by the population rate of Rydberg state, as the EFI process may ionize all Rydberg atoms once the strength of the EFI field reaches the EFI threshold of the Rydberg state. Hence, the key factor for raising the overall EFI efficiency is to enhance the population rate of Rydberg state in the PE process, which is a hard challenge due to the upper limit for the population rate of Rydberg state.