Kinetic processes determining the time dependence of vuv emission in He

We present an interpretation of the data by Bartell, Hurst, and Wagner (BHW) on the time resolved vuv emission from He excited by pulses of fast charged particles. Here we argue that the very large term, linear in pressure, observed in the decay rate of the He(2¹P) population represents He(2¹P) + He(1¹S)→He(2¹S) + He(1 ¹S) and that the dominant mechanism for this reaction is strong rotational coupling of the adiabatic states ¹Π_g(l ¹S+2¹P) and ¹Σ_g ⁺(1¹S+2¹S) in the region of internuclear separation near the potential curve crossing at R≃2.04 Å. The accurate prediction of the reaction rate, together with the fact that no other singlet states appear accessible at room temperature, leads to the conclusion that in nearly pure He most of the large He(2¹P) population is converted to He(2¹S) either by radiative transitions, the two-body process described above, or by the related predissociation of B ¹Π_g(1¹S+2¹P) when the molecule is formed in a high vibrational state. Thus, most of the continuum radiation comes from the A¹Σ_u⁺ and possibly the D ¹Σ_u⁺ molecules, and the time dependence of radiation from the A¹Σ_u⁺ molecule is rate-limited by decay of He(2¹S) by various collisional processes.