Electron impact excitation of H2:resonance excitation of B 1Σu+(Jj = 2, vj = 0)and effective excitation function of EF 1Σg+

Abstract

The electron impact emission function of the P(3) branchfor the (0, 4) band of the H2 B 1Σu+–X 1Σg+band system has been measured from threshold to 1800 eV. The emission functionexhibits structure indicating strong contributions from both resonance andnon-resonance excitation. The non-resonance component contains direct andcascade contributions. A combination of experimental and theoreticalconsiderations permits separation of resonance, dipole-allowed direct,dipole-allowed indirect, and dipole-forbidden excitation components for the Jj = 2,vj = 0 level of the B 1Σu+state. An effective excitation function for the EF 1Σg+–X 1Σg+band system has been obtained from a nonlinear least-squares analysisof the dipole-forbidden component of the B 1Σu+state emission function. The absolute value of EF 1Σg+–X 1Σg+cross section is established on the basis of earlier experimental results of Liu etal 1995 Astrophys. J. Suppl. 101 375–99 and 2002 Astrophys J. Suppl. 138 229–45and Abgrall et al 1997 Astrophys. J. 481 557–66 and 1999 J. Phys. B: At. Mol.Opt. Phys. 32 3813–38. A near-threshold apparent resonance excitation crosssection of (8.1 ± 3.2) × 10−18 cm2 is obtained for the B 1Σu+ (Jj = 2 andvj = 0). An EF 1Σg+–X 1Σg+Born cross section has been calculated from the electronic form factor ofKolos et al 1982a J. Chem. Phys. 77 1335–44. Analysis shows thatthe Born asymptotic shape function of the EF 1Σg+–X 1Σg+band system starts at ∼400 eV, a significantly higher energy than previouslyexpected. The excitation function is especially important for interpreting outerplanet atmospheric dayglow and auroral activity and can be used to infer energydeposition and heating rates.