基于发射和吸收光谱技术测量OH浓度变化受OH生成条件的影响

Influence of generation conditions on measurement of OH concentration variation using emission and absorption spectroscopic techniques

羟基自由基(OH)在大气化学反应、燃烧和等离子体中都具有诊断作用。以水汽及其混合气体为母体,通过辉光放电产生含OH的等离子体,同时测量OH吸收和发射光谱强度随OH生成参数的变化。高能电子碰撞解离水汽分子是产生OH的主要途径,在母体分子气压较低时,OH的发射光谱和吸收光谱强度都随水汽浓度增加而增加,且两种光谱技术测得OH浓度变化趋势一致。然而,当母体气压超过一定阈值时,气压持续增加会降低电子能量,减弱水汽分子的解离,从而导致OH吸收光谱强度随母体气压增加而减小。以纯水汽为母体分子时,压力增加产生的碰撞致使OH的A态布居增加,观测到OH发射光谱随水汽压力增加而缓慢增加。但含氮气母体分子中氮氧化物分子会与OH碰撞并发生反应,导致OH发射光谱强度随母体气压增加而减小。

Hydroxyl radical(OH)plays an important role in the atmospheric chemical reaction,combustion and plasma diagnosis.The plasma containing OH was produced through glow discharge with water vapor and its mixture as parent gases,and then the absorption and emission spectral intensities changing with the OH generation parameters were measured simultaneously.The dissociation of water vapor molecules by high-energy electron collision is the main way to produce OH.When the parent molecular pressure is low,both of the emission and the absorption spectral intensities increase with the water vapor concentration,and the variations of OH concentration measured by the two spectral methods are consistent. However, when the parent gas pressure exceeds a certain threshold, the continuous increase of parent gas pressure will reduce the electron energy and weaken the dissociation of water vapor molecules, resulting in the decrease of OH absorption spectral intensity. When pure water vapor is used as the parent gas, the collision caused by increasing pressure will increase the A-state population of OH, resulting in the slow increase of the emission spectral intensity with the water vapor pressure. However, nitrogen oxide molecules in nitrogen-containing parent molecules will collide and react with OH, resulting in the decrease of OH emission spectral intensity as the parent gas pressure increases.