Relative densities of H atoms and Yiiiv") molecules in vibrational levels up to i/'=5, effusing from a metal box containing heated tungsten filaments, were detected by a resonance-enhanced multiphoton ionization technique. The atom production is compatible with dissociation of molecules at the filament. The formation of Hiiv") is attributed to an Eley-Rideal-type reaction, in which a free atom recombines with an adsorbed atom at the cold wall, followed by desorption. Above 2800 K we observe an abrupt drop in the atom and the Hiiv") densities. It is ascribed to the effects of annealing on the filament surface.PACS numbers: 79.20. Nc, 33.80.Rv, 52.50.Dg, 82.65.Yh It is commonly accepted that in a hydrogen volume source the negative hydrogen ions (H~) are formed by dissociative attachment of slow electrons to vibrationally excited molecules Wiiv") 1 ' 1 where v" is the vibrational quantum number. The vibrational excitation is attributed to electron-impact excitation of molecules (in v" -0) by the fast primary electrons in the discharge. 4 To gain information on these processes one is interested in the measurement of the distribution of vibrationally excited molecules. 5 We have chosen resonance-enhanced multiphoton ionization (RMI) 6 as the method to study the discharge in our volume source. RMI is a powerful method because it allows the efficient detection of (metastable) molecules in specific rotational and vibrational states, 7 as well as atoms.The present Letter is concerned with the discovery that with the filaments heated, but the discharge turned off, we have been able to detect vibrationally excited molecules with v " up to 5. In a similar experiment, Hall et a/. 8 observed the formation of H2(i>") molecules with v" up to 9, using dissociative attachment of slow electrons to H2

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We have determined densities of negative hydrogen ions in a discharge by a laser detachment technique. We measured the electron density, the electron temperature, and the positive ion density using a Langmuir probe. We also performed extraction measurements. Combination of H− density measurements and extraction measurements yields information about the H− drift velocity. It was found that the velocity scaled with the square root of the electron temperature. All measurements were done as a function of discharge voltage, discharge current, and gas pressure. The densities are compatible with a semiquantitative model in which H− is produced by dissociative attachment of plasma electrons to vibrationally excited molecules and destroyed by wall collisions at very low pressure and collisions with H atoms, positive ions and/or hot thermal electrons at higher pressure.

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