  Absolute values of VUV-emission of a 2.45 GHz microwave-driven hydrogen
discharge are reported. The measurements were performed with a robust and
straightforward method based on a photodiode and optical filters. It was found
that the volumetric photon emission rate in the VUV-range (80-250 nm) is
$10^{16}$-$10^{17}$ 1/cm$^3$s, which corresponds to approximately 8%
dissipation of injected microwave power by VUV photon emission. The volumetric
emission of characteristic emission bands was utilized to diagnostics of
molecular plasma processes including volumetric rates of ionization,
dissociation and excitation to high vibrational levels and metastable states.
The estimated reaction rates imply that each injected molecule experiences
several inelastic electron impact collisions. The upper limit for the total
density of metastable neutrals ($2S$ atoms and $c^3\Pi_u$ molecules) was
estimated to be approximately 0.5% of the neutral gas density.
