Novel pathways to the assignment of the third rare gas excimer continua

Abstract

High quality, time correlated optical emission spectra of argon in the wavelength range from 110 to 300 nm, and 0 to 128 ns time interval, recorded at the Munich Tandem accelerator, using heavy ion beam excitation with 2 ns beam pulses were measured in order to clarify the origin of the so called third rare gas excimer continuum. Experiments were performed at argon pressures between 230 and 1500 mbar. The spectra clearly show several distinct peaks, emerging at different time delays after excitation. These spectral maxima are interpreted, as arising from excimer emissions by separate radiating species, formed by gas kinetic processes. All wavelength spectra obtained could be reproduced by fitting a limited number of Gaussian functions with fixed center-wavelengths and widths to the data. Six distinct maxima, appearing at four different times after the excitation pulse, could be identified. Besides the 128 nm peak of the 2nd continuum and the `left turning point' region around 155 nm, emission maxima were found at 177 nm, 188 nm, 199 nm, 212 nm, 225 nm, and 245 nm. A novel interpretation of the reaction pathways, leading to the various emission bands within the third continuum is proposed, essentially by combining the pathways proposed earlier by other authors. In this approach, the emission at 188 nm, occurring at early times and low pressures, is assigned to the decay of doubly charged dimers Rg 2 2+. The emission at 199 nm, with a build-up time longer than 10 ns at 500 mbar, could then be attributed to the Rg 3 2+ molecule. A transition to Rg 2 +* states will then occur by potential curves level crossing. The emission bands at 177 nm, 212 nm, and 225 nm are attributed to this Rg 2 +* molecule. The 245 nm continuum, appearing only at late times and high pressures, can then be attributed to the optical decay of Rg 3 +* clusters.