Collision Cross Sections for Optical Plasma Diagnostics

The quantitative utilization of optical diagnostics in gas discharge plasmas requires knowledge of the collision processes of electronically excited states. This need has emerged prominently in the last two decades in the plasma community due to the increasing research on atmospheric pressure discharges for plasma processing technologies. Both passive (Emission Spectroscopy) and active (laser-aided) optical spectroscopy methods are heavily a?ected by all kinds of collision processes: rotational and vibrational energy transfers (RET and VET), elecronic quenching (QE), both with neutral or electrons as collision partners [1]. Collision-radiative (CR) models, or the interpretation of Laser Induced Fluorescence (LIF) outcomes, depend on the availability of cross sections and rate coecients relevant to electronic radiative states. There is a somewhat comprehensive knowledge about electronic quenching by neutrals. Data are available for He and Ar states [2,3,4] and emitting states of bi-molecular radicals, OH(A), NO(A), CH(A) with various colliders of interest for combustion research [5]. Less work has been done on VET collisions, a topic on which we contributed with a study on N2(C) [6] and on OH(A) [7,8]. Electronic quenching (and maybe VET) by collision with electrons is less addressed. These processes have been well studied for He n = 3, 4 sublevels [9,10,11,12,13]. Rate coecient values are of the order of 104cm3s1, as a result of both a larger cross-section and a higher average velocity than with heavy particle collisions. In He, where atomic collision rate coecients are of the order of some 1010cm3s1, electron collision quenching becomes competitive at ionization degrees of 106 105. No data are available for molecular states. However, there are clear indications of their importance also for molecules. The availability of collision data for electronic states is also an opportunity to develop new diagnostic stategies. A CR model has been deployed in [14] in an Ar discharge, to measure the electron density. LIF is more versatile, since it can selectively prepare one state and monitor its time evolution correlated to collision processes. Once rate coecients are known for the colliders, the concentration of the collision partner(s) can measured. It is the case of the LCIF (Laser Collision induced Fluorescence) [15], by which the electron density can be measured, or the more recent Collision Energy Transfer (CET)-LIF [16], by which it is possible to measure the gas mixture composition in a rapidly changing environment. In this talk we shall address these topics, with a focus on the contribution of our laboratory on the kinetics of electronically excited states.