Knowledge Bank

Rotationally and vibrationally excited hydrogen molecules are measured using Laser Induced Fluorescence (LIF) spectroscopy in the vacuum-UV. The laser radiation is generated via the Stimulated Anti-Stokes Raman Scattering (SARS) process, and is tunable between 120 nm and 230 nm. Up to 10 Anti-Stokes beams are simultaneously directed into the hydrogen plasma expansion. This so-called multiplex excitation makes an accurate wavelength calibration of the pump laser imperative. This calibration is performed by simultaneously recording the VUV-LIF signal from the plasma expansion and the well-known two-photon laser induced fluorescence spectrum of nitric oxide in a gas $cella$. The density of hydrogen molecules in the ro-vibrationally excited energy levels is calibrated on the basis of previously performed Coherent Anti-Stokes Raman Scattering measurements. Population distributions are measured in an expanding plasma to determine the production mechanism of the ro-vibrationally excited hydrogen molecules. In this contribution we present results on the population distribution of $H2r,v$ from $v=2$ to $v=6$ in the electronic ground state. The ro-vibrational population distribution in the expanding hydrogen plasma can be described with a two temperature Boltzmann distribution. The population distribution in the low rotational levels ($J\le 5$) in the measured vibrational states can be described with a temperature of 800 K, while the higher rotational levels are populated according to a vibrational-like temperature of 3600K. In the $v=2$ state rotational excitation is observed up to $J=19$. A possible generation mechanism for the super-rotational excitation is the association of atomic hydrogen at the reactor wall.