Effect of permanent dipole moment of molecules onpositronium thermalization in the gases

正电子素（Ps）原子与原子或分子的散射是一个基本的过程，为散射理论提供了一个有吸引力的试验场。近年来，利用湮没辐射的角相关（ACAR）方法和SiO2气凝胶相结合，可以在1 eV以下的能量范围内提取Ps与原子或分子相互作用的定性信息，本项目中我们观测了Ne、H_2、N_2、CH_4和C_2H_4气体中Ps的热化过程。通过对实验数据的分析，我们确定了Ps与这些分子之间的动量转移截面：Ne为（10*7）×10^<-16>cm ^2，H_2为（17*5）×10^<-16>cm ^2，N_2为（26*8）×10^<-16>cm ^2，CH_4为（17*8）×10^<-16>cm ^2，C_2H_4为（23*7）×10^<-16>cm ^2。我们还比较了在neo-C_5 H_2中Ps热化过程<12>比在neo-C_5 H_2中慢<12>。这表明异C_5H_3的永久偶极<12>对Ps热化的影响很小。

The scattering of positronium (Ps) atoms by atoms or molecules is a fundamental process that provide an attractive testing ground for scattering theories. Recently, it has become possible to extract useful qualitative information on Ps interactions with atoms or molecules in the energy range below 1eV using the angular correlation of annihilation radiation (ACAR) method combined with the use of silica aerogel.In this project, we have observed Ps thermalization process in Ne, H_2, N_2, CH_4, and C_2H_4 gases. By analyzing the data, we determined the momentum-transfer cross sections between Ps and these molecules to be (10*7)×10^<-16>cm^2 for Ne, (17*5)×10^<-16>cm^2 for H_2, (26*8)×10^<-16>cm^2 for N_2, (17*8)×10^<-16>cm^2 for Ch_4, and (23*7)×10^<-16>cm^2 for C_2H_4. We have also compared the Ps thermalization process in neo-C_5H_<12> is slower than that in neo-C_5H_<12>. This indicates that the effect of permanent dipole of iso-C_5H_<12> is small for Ps thermalization.