The present work is aimed at studying how spatially periodic modulations of the refractive index of the medium, i.e., laser-induced gratings (LIGs), generated in a gas mixture containing methane (CH4) by nanosecond pulses of resonant mid-infrared laser radiation, can be used to measure various gas parameters. It is investigated to what extent the temporal profiles of the LIG signals, recorded as the power of the diffracted by LIGs continuous wave probe radiation, are specific to the composition, pressure, and temperature of a selected buffer gas. This specificity is illustrated by the LIG signal profiles recorded in the experiments in different gas mixtures under various conditions. Experimental data show that large LIG signals can be obtained even in mixtures with CH4 concentrations as low as ∼100 parts per million due to the strong absorption of the excitation light and subsequent rapid, highly exothermic, and partner-dependent collisional energy exchange of the laser-excited molecules with the environment. These two factors ensure high LIG generation efficiency by a small number of CH4 molecules and high sensitivity of signal strength and profile to variations of gas parameters.

中文翻译：

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中红外产生含甲烷气体混合物中的激光诱导光栅信号作为成分、压力和温度的指示剂

目前的工作旨在研究在含有甲烷（CH）的气体混合物中如何产生介质折射率的空间周期性调制，即激光诱导光栅（LIG）4）通过谐振中红外激光辐射的纳秒脉冲，可用于测量各种气体参数。研究了 LIG 信号的时间分布（记录为 LIG 连续波探针辐射的衍射功率）在多大程度上特定于所选缓冲气体的成分、压力和温度。这种特异性可以通过在不同条件下不同气体混合物的实验中记录的 LIG 信号曲线来说明。实验数据表明，即使与 CH 混合，也可以获得大的 LIG 信号4由于激发光的强烈吸收以及随后激光激发分子与环境的快速、高放热和依赖于伙伴的碰撞能量交换，浓度低至百万分之100。这两个因素保证了少量CH的高LIG发电效率4分子和信号强度和轮廓对气体参数变化的高灵敏度。