Dynamic properties of neutral excitations produced in electron-bombarded superfluid helium. II. Afterglow fluorescence of excited helium molecules

We have investigated the time dependence of the afterglow emission of pulsed-electron-bombarded superfluid helium, and in addition we have studied the effects of pressure on the steady-state intensities of the atomic and molecular emissions of the excited liquid. Our results indicate that two distinct mechanisms populate the excited states in the liquid. The first production mechanism basically determines the steady-state properties of the excited liquid and it is responsible for the rapid initial decay of the fluorescence during the first ${10}^{-8\mathrm{}}$ sec of the afterglow. This rapid decay time taken with the observed exponential decrease of the steady-state intensities with increasing pressure indicates that most of the excited states are nonradiatively quenched by the surrounding ground-state liquid. The second production mechanism results in a slowly decaying late afterglow fluorescence of the excited liquid. It is shown that this late time emission is due to the bilinear destruction of the long-lived ${\mathrm{He}}_2\left(a{}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}\right)$ metastable molecules which are produced also in the liquid by the electrons. It is shown further that most of the energy released during these bilinear reactions results in the population of the ${\mathrm{He}}_2\left(A{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}\right)$ state which subsequently radiates in the vacuum-ultraviolet region of the spectrum.