H2O laser discharge diagnostics with UV OH emission

A spectroscopic technique for measuring time-varying gas kinetic temperatures with submicrosecond time resolution is described. The temperature in H2O discharges is determined from instantaneous OH emission spectra at ∼3064 Å. The temperature rose linearly from 350 K to ∼2000 K at the end of a 4-μs 150-A current pulse through H2O. This temperature rise tends to switch off the gain and shorten the lsser pulse. Helium, when added, limited the temperature rise to ∼200 K and lengthened the laser pulse. Previous observations that the laser medinm becomes lossy after the end of the current pulse is shown to be due to the observed temperature rise. Models of pulsed lasers should include temperature, and hence time-dependent rate constants, to be accurate. Time-varying gas kinetic temperatures in pulsed CO2-N2-He laser plasmas, e.g., in TEA CO2lasers, could be determined by this technique using the 3371-Å emission from the second positive system of nitrogen.