Spectroscopy and kinetics of ionic alkali halide excimers excited by a laser-produced plasma

Abstract

Vapors of five alkali halides, CsF, RbF, KF, CsCl, and NaI, have been excited by soft x rays from a laser-produced plasma. In the cases of heavy alkalis and light halogens (CsF, RbF, and CsCl), the emission of ionic excimer continua is observed. The upper state of the alkali halide molecule (AX) is a bound state correlating to A2+ and X−, which decays by emission of photons to a weakly bound state correlating to A+ and X. For lighter alkalis or heavier halogens the alkali halide bound states A2+X− predissociate, and the fluorescence yield decreases dramatically. No ionic excimer emission is observed for KF and NaI; however, their emission spectra are characterized by molecular halogen and atomic and ionic emission lines. The qualitative trends in fluorescence yield and ionic excimer predissociation are predicted correctly by molecular potential-curve calculations based on modified Rittner and Born–Mayer potentials. The ionic alkali halide excimer states are produced by direct photoionization by the soft x rays from the laser-produced plasma. For sufficiently small differences in the equilibrium internuclear distance of the upper and the lower electronic states, the vibrational distribution in the excited state remains, to a good approximation, Maxwellian after excitation even on a time scale for which collisions with the ambient gas can be neglected. In a harmonic approximation the vibrational temperature in the upper ionized state, Tν′, depends only on the lower-state vibrational temperature, Tν″, the vibrational quanta of the upper state, ωe′, and of the lower state, ωe″, and the relative equilibrium internuclear distance. If the upper and the lower states are at the same equilibrium internuclear distance, the simple relation Tν′ = (ωe′/ωe″)Tν″ is obtained.