Detection of rovibrationally excited molecular hydrogen in the electronic ground state via synchrotron radiation

We describe an original setup named SCHEME (Source of exCited HydrogEn MolEcules) designed to study the recombinative desorption mechanisms of H atoms on a surface by means of high-brilliance monochromatic synchrotron radiation (SR). H atoms are produced on electrically heated filaments under vacuum (in the absence of any discharge) and subsequently recombine on a surface to produce excited hydrogen molecules, namely, H2(v″, J″). Once these levels are produced, they are probed in the vacuum chamber with 5-40 eV (248-31 nm) SR-photons. A preliminary test with Krypton has demonstrated the feasibility of SCHEME to obtain simultaneously VUV-synchrotron radiation induced fluorescence and VUV-absorption signals with room-temperature gas. In order to evaluate the hydrogen molecule dissociation induced by the heated filaments, the absolute density of H atoms has been measured by VUV-absorption. At 1800 K and 100 mTorr, we estimated a dissociation degree of 10-4, which corresponds to an absolute density of 1.8×1017 m-3. VUV-absorption spectroscopy is shown to be a valuable tool to detect H2(v″, J″) molecules up to v″ = 3. Possible improvements of the experimental setup, to detect higher v″ levels, are suggested in the end of this paper.