Time-Dependent Studies of Vacuum-Ultraviolet Emissions from Helium

Proton excitation was employed to obtain new information on the vacuum-ultraviolet (vuv) emissions from helium gas. Time-dependent studies of the 584-{} resonance line and the vuv continuum (600-950 {}) have been made as a function of pressure. The decay rate for 584-{} resonance photons shows a type of pressure dependence which indicates that the 2¹P atomic state is destroyed much more rapidly by collisions than by radiation to the 2¹S metastable state. Most of the vuv continuum decays with two time components (fast and slow). The number of ions and the population of the lowest-ten excited states resulting when an energetic (4-MeV) proton loses a fraction of its energy in helium, and when the secondary electrons completely degrade in energy, have been calculated. The theoretical studies indicate that about 30% of the energy lost by a single proton goes into atomic excitation, where about one-half of these excited states are 2¹P states. These new results are combined with previous information to propose an energy pathways model for helium. We suggest that the 2¹P state is primarily destroyed by collisions to the D and B molecular states which are then the source of the fast vuv radiation (640-950 {}) and the precursor of slow radiation, respectively. In our model the metastable B molecular state stores the energy which is utilized in producing additional ionization (Jesse effects) when impurities are added to helium, in contrast with previous interpretations which assumed that the energy is stored in atomic metastable states.