Study to realize the Bose-Einstein condensation of helium gas by means of laser cooling

本研究的目的是利用激光冷却的方法实现氦气的玻色-爱因斯坦凝聚（BEC）。对于BEC来说，氦原子必须在高密度和低温下被捕获，而被捕获氦原子之间的Penning碰撞是BEC最重要的障碍，而氦原子处于亚稳态激发三重态。因此，为了实现BEC，研究这一电离过程非常重要。在本研究中，虽然BEC没有成功，但对超低温下的Penning电离速率进行了详细的研究。特别是，实验和理论上首次发现，^4He和^3He在超低温下的电离速率有很大的不同。对于被激光冷却和捕获的He原子之间的Penning碰撞，有两种可能的情况：一种是两个He（2s^3S）原子之间的碰撞（S-S碰撞），另一种是He（2s^3S）和He（2p^3P）之间的碰撞（S-P碰撞）。对于s - s碰撞，0.5 mK时的Penning碰撞速率系数为(3.8*1.1)*10^<-10>cm^3/s，而^3He为(1.1*0.4)*10^<-9>cm^3/s，后者比前者大3倍。从我们的理论分析来看，这种巨大的同位素差异是由于在如此低的温度下，只有最低阶的部分波才会导致潘宁碰撞。在这种由单个部分波引起的碰撞中，量子统计对称性的差异明显影响碰撞截面。另一方面，对于S-P碰撞，尽管两种同位素的电离速率系数都比S-S碰撞大两个数量级，但只发现了很小的同位素差异。

The purpose of this study is to realize the Bose-Einstein condensation (BEC) of the helium gas by means of laser cooling.For BEC, helium atoms have to be trapped at a high density and a low temperature, and the most significant obstruction to BEC is the Penning collision between trapped He atoms, which lie in the metastable excited triplet state.Therefore, to realize BEC, it is important to investigate this ionization process.In this study, though BEC has been unsuccessful, the Penning ionization rate at an ultralow temperature has been investigated in detail.In particular, it has experimentally and theoretically been found, for the first time, that the ionization rate at an ultralow temperature is much different between ^4He and ^3He.For Penning collisions between He atoms cooled and trapped by lasers, there are two possible cases : one is a collision between two He(2s^3S) atoms (S-S collision), and the other is the one between He(2s^3S) and He(2p^3P) (S-P collision).For S-S collisions, the Penning collision rate coefficient at 0.5 mK has been found to be (3.8*1.1)*10^<-10>cm^3/s for ^4He and (1.1*0.4)*10^<-9>cm^3/s for ^3He, and the latter is as much as three times larger than the former.From our theoretical analysis, this large isotopic difference has been found to result from the fact that at such a low temperature the lowest order partial wave alone contributes to Penning collisions.In such a collision caused by a single partial wave, a difference in the quantum statistical symmetry explicitly affects the collision cross sections.On the other hand, for S-P collisions, though the ionization rate coefficients for both isotopic species are two orders of magnitude larger than for S-S collisions, only a small isotopic difference has been found.