Pressure-scaling characteristics of femtosecond two-photon laser-induced fluorescence of carbon monoxide.
Pressure-scaling characteristics of femtosecond two-photon laser-induced fluorescence of carbon monoxide.
复制标题
飞秒双光子激光诱导一氧化碳荧光的压力缩放特性。
作者：
K. A. Rahman;V. Athmanathan;M. Slipchenko;T. Meyer;Sukesh Roy
宽带飞秒（fs）双光子激光诱导荧光（TP-LIF）的B1Σ+←X1Σ+， Hopfield-Birge系统的一氧化碳（CO）被认为与窄带纳秒激发相比有两个主要优点。它应该(i)最小化压力依赖的吸收线拓宽和移位的影响，以及（ii）产生压力无关的TP-LIF信号，因为分子碰撞导致的淬火增加的影响被数量密度的增加所抵消。然而，随着压力的增加，CO TP-LIF信号出现了非线性下降。在这项工作中，我们系统地研究了潜在的去激发机制的相对影响，包括碰撞淬火、正向激光、测试细胞窗口或气体介质对源激光的衰减以及2+1光电离过程。正如预期的那样，谱线加宽和碰撞淬火对压力缩放行为的影响较小，但TP-LIF信号的CO偏离理论主要有两个主要原因。首先，在高压下激发激光的衰减显著降低了探头体积处可用的激光辐照度。其次，2+1光电离过程随着数字密度随压力的增加而变得重要，并作为主要的去激发途径。这项工作总结了在高压下执行TP-LIF CO需要考虑的现象和策略。
Broadband femtosecond (fs) two-photon laser-induced fluorescence (TP-LIF) of the B1Σ+←X1Σ+, Hopfield-Birge system of carbon monoxide (CO) is believed to have two major advantages compared to narrowband nanosecond excitation. It should (i) minimize the effects of pressure-dependent absorption line broadening and shifting, and (ii) produce pressure-independent TP-LIF signals as the effect of increased quenching due to molecular collisions is offset by the increase in number density. However, there is an observed nonlinear drop in the CO TP-LIF signal with increasing pressure. In this work, we systematically investigate the relative impact of potential deexcitation mechanisms, including collisional quenching, forward lasing, attenuation of the source laser by the test cell windows or by the gas media, and a 2+1 photoionization process. As expected, line broadening and collisional quenching play minor roles in the pressure-scaling behavior, but the CO fs TP-LIF signals deviate from theory primarily because of two major reasons. First, attenuation of the excitation laser at high pressures significantly reduces the laser irradiance available at the probe volume. Second, a 2+1 photoionization process becomes significant as the number density increases with pressure and acts as a major deexcitation pathway. This work summarizes the phenomena and strategies that need to be considered for performing CO fs TP-LIF at high pressures.