Reaction dynamics of metastable helium molecules and atoms near 4.2 K.

Abstract

Infrared absorption spectra from metastable helium atoms (2s $^{3}$S, 2s $^{1}$S) and molecules (a $^{3}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}^{+}$) were previously acquired by irradiating dense helium gas near 4.2 K with a pulsed proton beam [R. L. Brooks, J. L. Hunt, and D. W. Tokaryk, J. Chem. Phys. 91, 7408 (1989)]. The molecular spectrum was unusual because the observed rovibrational distribution within the a $^{3}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ state was far from thermal equilibrium. Three different rovibrational groups were observed: (i) v=0, N=1 (``thermal'' molecules); (ii) v=0, 9\ensuremath{\le}N\ensuremath{\le}21 (rotationally excited molecules); and (iii) 10\ensuremath{\le}v\ensuremath{\le}12, N=1 (vibrationally excited molecules). In this work, the time evolutions of members of these three molecular populations were studied both during irradiation and in the subsequent afterglow. In addition, the evolution of the 2s $^{3}$S--2p $^{3}$P atomic line was investigated. This study quantitatively explores the reaction dynamics of the metastable molecule in the gas phase near 4.2 K. Gas pressures between 100 and 750 Torr were used. Time-resolved data were taken with a transient-digitizer system and summed for several thousand cycles of the pulsed proton beam. The absorption measurements were converted to time-resolved number densities with the aid of theoretical transition moments. The analysis required that the data be fit to the solutions of sets of coupled differential equations with a nonlinear least-squares-fit routine. The results provide insight into the complicated reactions involved in generating the unusual molecular distribution and into the reactions between the metastable molecules, metastable atoms, and the background helium gas.