Calculation and interpretation of the continuum radiation of hydrogen molecules

Calculations of the hydrogen molecular continuum (a3Σ+g → b3Σ+u, where the latter is repulsive) have been carried out in the wavelength range 120–600 nm, based on potential curves and reduced masses. They give transition probabilities, lifetimes and spectral intensities as a function of vibrational level population in the upper state a3Σ+g. Absolute radiation measurements in H2/He and D2/He low-pressure ECR plasmas in the wavelength range 170–300 nm are compared with results from these calculations. Using the Franck–Condon principle and the corona model for excitation, the relative vibrational population in the upper state is correlated with the population in the ground state, characterized by the vibrational temperature Tvib. The absolute intensities depend on the electron temperature Te via the electron excitation rate coefficients. Therefore, the shape of the continuum radiation reflects Tvib, and the absolute value is a function of Te. The results are in good agreement with those from other spectroscopic techniques (Te from He line intensities, Tvib from Fulcher band radiation) in the visible spectral range. This demonstrates that emission spectroscopy of the continuum radiation of H2 and D2 is a good tool for diagnostics of low-pressure plasmas. The calculations have also been carried out for other hydrogen isotopes (T2, HD, DT)