Visualization and Measurement of Rarefied Gas Flow using O_2-PLIF

氧分子的吸收带很宽，从紫外光的114 nm到红外光的1580 nm。主要的Schumann-Runge吸收带对应于基态CHI^3SIGMA_8^-到激发态BETACHI^3SIGMA_u^-的跃迁，在175~250 nm之间有离散吸收带，在130~175 nm之间有连续吸收带。已知的是，许多排斥势能曲线跨越并预解离了β状态。由于强烈的预解离速率使得荧光的寿命缩短到10^&lt；-12&gt；s数量级，因此可以假设荧光在分子间碰撞之前产生。在本研究中，我们研究了O_2-LIPF(激光诱导预解离荧光)对稀薄气体流动的成像能力。由于氧气是空气的主要成分之一，O_2-LIPF可以在没有示踪剂的情况下对高速风洞中的气流进行可视化。然而，由于用O_2-LIPF显示室温以下的流场是非常困难的，所以它只适用于高粘度和燃烧流动，而不适用于低温下的高速流动。在本研究中，利用ArF准分子激光器(193 Nm)和高灵敏度的CCD摄像机，利用O2-LIPF对稀薄氧气流动进行了可视化。在设置长曝光时间(1200秒)和充分抑制激光散射的情况下，利用O2-LIPF可以成功地对室温以下的氧气流动进行成像。

Oxygen molecules have the absorption bands in the broad range from 114 inthe ultra violet to 1580 nm in the infrared. The dominant Schumann-Runge absorption band system corresponds to the transition of the ground state CHI^3SIGMA_8^-to the excited state BETACHI^3SIGMA_u^-, and has the discrete absorption bands from 175to 250 nm and the continuum ones from 130 to 175 nm. The BETA state has been known to be crossed by many repulsive potential curves and to be predissociated. Sincethe strong predissociation rate makes the lifetime of the fluorescence short in the order of 10^<-12> seconds, it is possible to assume that the fluorescence yields prior to the intermolecular collision.In this study, we investigate the capability of imaging of rarefied gas flow using O_2-LIPF(Laser-Induced Predissociative Fluorescence). Since oxygen isone of main component of air, O_2-LIPF may make it possible to visualize the air flow in the high speed wind tunnel without the tracer. However, because the visualization of the flow field below the room temperature using O_2-LIPF is verydifficult, it has been applied only to the high entalpy and combustion flow^<(1)>, not to high speed flow with low temperature. In this study, the rarefied oxygen flow is visualized by O_2-LIPF, using an ArF excimer laser (193 nm) and a high-sensitive CCD camera. If long exposure time (1200 seconds) of the CCD camera is set and the scattered laser beam is suppressed sufficiently, the oxygen flow below the room temperature using O_2-LIPF can be imaged successfully.