Laboratory studies of uv emissions of H_2 by electron impact. The Werner- and Lyman-band systems

We report a laboratory measurement of absolute emission cross sections of both the Lyman bands (B^1Σ_u^+→X^1Σ_g^+) and Werner bands (C^1Π_u→X^1Π_g^+) of H_2 by electron impact over the energy range from threshold to 400 eV with the same optical system. We find the emission cross section for the B^1Σ_u^+→X^1Σ_g^+ transition at 100 eV to be (3.55±0.8) × 10^(−17) cm^2 (2.7 × 10^(−17) cm^2, direct excitation, 0.85 × 10^(−17) cm^2, cascading) and the emission cross section for the C^1Π_u→X^1Σ_g^+ transition at 100 eV to be (3.1±0.6) × 10^(−17) cm^2 (cascading is estimated to be not present). The cross-section ratio Qc/Qb for direct excitation is 1.21±0.30 at 300 eV in excellent agreement with published values for this ratio from theoretical calculations and experimental data of the optical oscillator strengths. We measure the cross section for cascading to the B state to be 24±10% of the total emission cross section both at 100 and 300 eV. We show that cascading increases to 51±20% of the total cross section of the B state at 20 eV. The vibrational population distribution of the B state is found to be a function of electron-impact energy as the importance of cascading relative to direct excitation changes with electron-impact energy