Observation of Exceptionally High Vibrational Excitation of Hydrogen Molecules Formed by Wall Recombination

Abstract

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<V') as a detection technique. Therefore, two independent detection techniques have been used by now to establish the presence of vibrationally highly excited molecules in the gas effusing from a nearly closed box containing hydrogen has and heated filaments. The vibrational excitation is attributed to recombinative desorption of hydrogen atoms on the cold walls of the discharge chamber. The atoms result from dissociation of molecules on the hot filaments. The main distinction between our work and that of Hall et ai 8 is our ability to detect atoms in addition to molecules. This way we can demonstrate the very close relation that exists between those two types of particles.Other experiments dedicated to a study of recombinative desorption of hydrogen 9 " 11 used permeation through the sample to obtain a wall covered by atomic hydrogen. In these cases only v " = 1 has been detected. With use of various wall materials, the population of v " = 1 was found to be in excess of the value expected on basis of the wall temperature.The discovery that walls catalyze the population of high vibrational states will bear on different fields like catalysis and plasma or interstellar chemistry, because reaction rates are known to increase an order o...

Dynamics of H2 Eley-Rideal abstraction from W(110): Sensitivity to the representation of the molecule-surface potential

“…Such results gave comparable results with former experiments. [6][7][8] However it is worth mentioning that the population of excited levels higher than v = 3 is very small (<1.5 10 −3 ). To compare with experiments, vibrational state distributions have been computed by thermal averaging normal collision energy resolved results in the limit of normal and total energy scaling.…”

Reactions of Gas-Phase Atomic Hydrogen with Chemisorbed Hydrogen on a Graphite Surface

“…The chosen gas temperature is near the lower end of the filament temperature used in experimental studies. 37 Thus the temperature condition mimics H(g) produced in a tungsten filament chamber interacting with a graphite surface, which is maintained at room temperature. The reactive event takes place on a subpicosecond time scale.…”

Internal, translational, and angular momentum product state distributions of CuF molecules desorbing associatively from copper surfaces

“…This mechanism has recently been invoked to explain the results of three independent studies of the recombination of H atoms at metal surfaces (3)(4)(5). The high vibrational product excitation has been modelled with quantum mechanical calculations by Kratzer and Brenig (6).…”