Spectroscopic Evidence for Slow Vibrational and Electronic Relaxation in Solids. The Vegard–Kaplan and Second Positive Systems of N2 in Solid Rare Gases

Abstract

The spectra emitted from N2-doped rare-gas solids at low temperatures, irradiated with x rays, have been investigated in detail. The molecular N2 emission seen in the spectral range 2300–5000 Å consists of the Vegard–Kaplan (VK) A 3Σu+ → X1 Σg+ and the second positive C 3Πu → B 3Πg band systems. The latter is observed only in solid Ne. Forbidden transitions of N atoms and at times of impurity O atoms are also seen. Both N2 band systems show emission from υ′ > 0, establishing a relatively slow vibrational relaxation rate in the A 3Σu+ and C 3Πu states in solid rare gases. For the VK system the relative intensity of emission from υ′ > 0 is strongly dependent on the rare-gas host and on the N2 concentration. No υ′ > 0 is seen for a Ne host, whereas in Ar, Kr, and Xe the relative intensity of υ′ > 0 increases with decreasing N2 concentrations, with a maximum of υ′ = 6 being found in these three hosts. Decay times for isolated VK bands were measured and appeared exponential within experimental error. From the measured lifetimes it is concluded that the vibrational relaxation time in the A 3Σu+ state of N2 in solid rare gases lies between 0.4 and 3.3 sec. The effect of temperature on the relaxation rate was found to be small over the range 1.7°–30°K. The measured lifetime of the A 3Σu+ state in solid Ne is 3.3 sec, which lies within the range of values reported for the purely radiative lifetime of gaseous N2. A close connection between the processes discussed in this paper and the detailed elementary reactions of radiation chemistry is pointed out.