We present the first theoretical study of collisional dissipation of the alignment of a symmetric-top molecule (ethane gas) impulsively induced by a linearly polarized non-resonant laser field. For this, Classical Molecular Dynamics Simulations (CMDSs) are carried out for an ensemble of C₂H₆ molecules based on knowledge of the laser-pulse characteristics and on an input intermolecular potential. These provide, for a given gas pressure and initial temperature, the orientations of all molecules at all times from which the alignment factor is directly obtained. Comparisons with measurements show that these CMDSs well predict the permanent alignment induced by the laser pulse and its decay with time but, as expected, fail in generating alignment revivals. However, it is shown that introducing a simple requantization procedure in the CMDS “creates” these revivals and that their predicted dissipation decay agrees very well with measured values. The calculations also confirm that, as for linear molecules, the permanent alignment of ethane decays more slowly than the transient revivals. The influence of the intermolecular potential is studied as well as that of the degree of freedom associated with the molecular rotation around the symmetry axis. This reveals that ethane practically behaves as a linear molecule because the intermolecular potential is only weakly sensitive to rotation around the C–C axis.

中文翻译：

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激光诱导乙烷气体对准的碰撞耗散：重新量化的经典模型

我们提出了第一个理论研究的碰撞消散的线性极化非共振激光场脉冲诱导的对称顶分子（乙烷气体）的对齐方式。为此，对C ₂ H ₆进行了经典分子动力学模拟（CMDS）。分子基于对激光脉冲特性的了解以及输入的分子间电势。对于给定的气压和初始温度，这些提供了所有分子在所有时间的取向，从中可以直接获得取向因子。与测量结果的比较表明，这些CMDS很好地预测了由激光脉冲引起的永久对准及其随时间的衰减，但正如预期的那样，未能产生对准恢复。但是，结果表明，在CMDS中引入简单的重新量化程序“创建”了这些复兴，并且它们的预测耗散衰减与测量值非常吻合。该计算还证实，对于线性分子，乙烷的永久取向比瞬态复兴更慢地衰减。研究了分子间电势的影响以及与围绕对称轴的分子旋转相关的自由度的影响。这表明乙烷实际上起线性分子的作用，因为分子间的电势仅对围绕C–C轴的旋转敏感。