Zero-phonon transitions in O2−, S2−, Se2−, and SeS− molecules dissolved in alkali halide crystals

Abstract

Zero-phonon transitions of O2−, S2−, Se2− and SeS− molecules in alkali halide crystals were studied by their emission and excitation spectra at 2°K. Most of the information came from emission spectra, which contain large numbers of zero-phonon lines. These lines usually form a single ground-state progression, but extra progressions also appear in most S2− spectra, and in the spectrum of O2− dissolved in KBr. The spectroscopic constants ν00, ω0″, ω0″x0″ and Δ G1/2″ were calculated for all observed progressions, and the Δ G1/2″ values compared with measured Raman frequencies of the molecules. By measuring the effects of uniaxial stress on the emission spectra it was shown that the optical dipole of the emission transition is parallel to the molecular axis, that the axis is parallel to 〈 111〉 directions for O2− in NaCl, KBr and KI, and parallel to 〈 110〉 directions for all other systems, with the exception of the extra emission of O2− in KBr. This dipole could not be so well characterized, except to say that it must lie in a {100} plane. All the emissions are probably due to a Πu2→Πg2 transition. Stress reorientation in the excited state was measured for O2− in KBr crystals. Stress and strain coefficients were calculated for many zero-phonon emission lines; most of the results are in agreement with a linear theory.