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

High-resolution energy distributions of ions, accelerated by the sheath at the powered electrode of a low-pressure 13.56-MHz gas discharge, have been measured. The observed spectra are compared to existing models. Excellent agreement between measured and calculated spectra is obtained. Detailed information on rf sheath behavior is derived from the observed energy profiles and from the measured total ion current densities towards the electrode surface. Analogous to the case of dc discharges, a decrease of sheath thickness is observed when a homogeneous variable magnetic field (0≤B≤315 G) is applied. However, the product of magnetic-field strength B and sheath thickness d is found to be independent of sheath voltage. This leads to the conclusion that in rf discharges, sheath contraction under influence of a magnetic field proceeds by a different mechanism than in dc discharges. It is suggested that the value of the product Bd is determined by the (virtually constant) temperature of the plasma electrons, rather than by the energy of secondary electrons that have been liberated from the electrode surface by ion bombardment. The decrease of sheath thickness d with magnetic-field strength B leads to a changing capacitive-voltage division of the applied generator voltage over the discharge. When the magnetic-field strength is sufficiently high, this may result in a sign reversal of the electrode self-bias voltage.

Ion energy measurements have been performed with an electrostatic parallel plate energy analyzer at the powered electrode of a 13.56–MHz rf discharge. Considerable splitting of the ion energy distributions is observed due to rf oscillations. Plasma potential, sheath thickness, and total ion current are derived from the observed energy profiles. Low-pressure operation of the plasma at several mTorr permits a collisionless sheath approximation and gives rise to well-defined energy spectra.

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