Reaction Dynamics of Metastable Helium Molecules and Atoms at 4.2 K

In 1989, infrared absorption spectra from metastable helium atoms (1s2s ^3S, 1s2s ^1S) and molecules (a ^3Sigma _sp{rm n}{+}) were acquired by irradiating dense helium gas at 4.2 K with a pulsed proton beam. The molecular spectrum was unusual because the observed rovibrational distribution within the a ^3Sigma_sp{rm n}{+} state was far from thermal equilibrium. Three different rovibrational groups were observed: (1) v = 0, N = 1 ("thermal" molecules); (2) v = 0, 9 <= N <= 21 (rotationally excited molecules); (3) 10 <= {rm v} <= 12, N = 1 (vibrationally excited molecules). In this work, the time evolutions of members of these three molecular populations, and of the metastable atoms, were studied both during irradiation and in the subsequent afterglow. In addition, the evolution of the bright molecular d ^3Sigma_sp {rm n}{+} to {bf b} ^3Pi_{rm g } and D ^1Sigma_sp {rm n}{+} to {bf B} ^1Pi_{rm g } emission bands, and the (1s2p ^3 P) - (1s3s ^3S) and (1s2p ^1P) - (1s3s ^1S) atomic lines was investigated. This study is the first to quantitatively explore the reaction dynamics of the metastable molecule in the gas phase at 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.