Nitrogen dioxide: a variable attenuator for KrF laser radiation

Abstract

Nitrogen dioxide (NO2) in gaseous equilibrium with dinitrogen tetroxide (N2O4) is investigated as a variable attenuator for KrF excimer laser radiation (at 249 nm). A model for the physical chemistry of the equilibrium mixture is given and used to predict its absorption coefficient. Experimental data are presented, which confirm the model, and absorption cross sections of 6*10-24 m2 for NO2 and 87*10-24 m2 for N2O4 are deduced. Photobleaching of the gas was observed when it was exposed to high (1.7 J) pulse energies. This effect was found to be most severe at total absorbing gas pressures of approximately 5 kPa, for which there was a 60% increase in transmission after exposure to 60 laser pulses, with recovery over tens of minutes. The possibility of the NO2 and N2O4 concentrations taking a significant time to equilibrate in a fresh gas sample was investigated, and not found to cause a measurable change in the gas absorption.