SPECTROSCOPIC STUDY OF VIBRATIONAL-RELAXATION AND COOLING OF RARE-GASEXCIMERS FORMED IN A DIRECT-CURRENT DISCHARGE WITH SUPERSONIC EXPANSION

Emission spectra (in the region 100-200 nm) of the excimers Ar-2 Kr-2 , and Xe-2* produced in a dc jet discharge with supersonic expansion were used to determine vibrational temperatures of the excimers, and t o analyze the process of excimer relaxation from high vibrational leve ls near dissociation of the A (3) Sigma(u)(+)(1(u)) states to the lowe st levels. The observed narrow bandwidths of the second continua yield ed population distributions with at least 80% in the upsilon=0 levels at thermal equilibrium, for all three gases, corresponding to vibratio nal temperatures of 280, 95, and 82 K for Ar-2+, Kr-2 and Xe-2* respe ctively. Changes in relative intensities of the first and second conti nua with pressure were monitored by optical pumping of atoms in the 1s (5) metastable level [precursor of the excimer A (3) Sigma(u)(+)(1(u)) stale], and the resulting difference spectra were analyzed in terms o f vibrational relaxation to the upsilon similar to 0 levels. A simple model based on collisional relaxation was developed to calculate the V UV band spectra. Comparisons of the Ar-2 spectra with the model showe d that vibrational relaxation accounted for the observed difference sp ectra, up to plenum pressures of similar to 7 atm. Departures from thi s model at higher pressures for Ar-2, and particularly the observed b ehavior of Kr-2 and Xe-2* difference spectra, indicated that populati ons in the lowest vibrational levels were affected by additional proce sses, and not only by vibrational relaxation of excimers formed by ass ociation. (C) 1995 American Institute of Physics.