Pulsed technique for observing infrared emissions from ionic gas phase reactions at low reactant ion concentrations

A technique has been developed to detect infrared emissions from the produc ts of ionic reactions in plasmas. The technique employs dual-phase digital lock-in amplification and cold filtering to permit the detection of the wea k infrared chemiluminescence (IRCL) with a solid-state detector. A novel me thod of cleanly modulating plasma chemiluminescence by the pulsed introduct ion of reagent gases has been developed and implemented. This new technique has been tested by studying the well-characterized H-atom reactions, H+Cl- 2-->HCl(v=0-4)+Cl and H+NO2-->OH(v=0-3)+NO. Rotational and vibrational dist ributions have been measured for these two reactions and are presented and compared with previous determinations. Additionally, the associative electr on detachment reaction, H+Cl--->HCl(v=0-2)+e, has been studied, demonstrati ng that IRCL can be collected from reactions occurring at a low number dens ity approaching that of the plasma ionization (similar to 4x10(10) cm(-3)). The resolution, and hence, the information content of the collected emissi ons from this reaction have been greatly improved over previous work, with which our data are compared. (C) 2000 American Institute of Physics. [S0034 -6748(00)04904-2].