Vibrational Excitation of Hydrogen via Recombinative Desorption of Atomic Hydrogen Gas on a Metal Surface

Abstract

Recombinative desorption of atomic hydrogen on the walls of a gas cell has been observed to populate vibrational levels up to u -9. The vibrational populations follow a Boltzmann distribution near 3000 K up to v * 3 and for higher levels the populations are well in excess of this temperature. These observations bring to light a new mechanism for vibrational excitation of H2 in volume H ~-ion sources.PACS numbers: 79.20.Nc, 34.80.Gs, 52.40.Hf The interaction of molecules with surfaces is a rapidly developing field where important advances are being made as a result of the use of gas-phase scattering and diagnostic techniques. In particular, the study of molecular vibration associated with surface interaction is becoming possible with the help of lasers. l The dynamics of hydrogen interacting with metals is of fundamental interest and the experimental evidence at the present time is that translational and vibrational temperatures of molecules leaving the surface are much higher than those associated with the surface temperature. 2 " 5 One aspect of this system is the recombination of atomic hydrogen from the gas phase with surface hydrogen atoms followed by desorption of the resulting molecule (recombinative desorption). Little is known about the dynamics of this process yet it may play a key role in gas discharges and interstellar chemistry.Here we report observations on recombinative desorption of atomic hydrogen on a hydrogen-covered metal surface which leads to hydrogen molecules in levels up to v =9. The vibrational population distribution follows a Boltzmann distribution near 3000 K up to u-3, higher levels having populations well in excess of this temperature. Furthermore, this distribution is not thermalized after several-hundred wall collisions.A technique has been developed for the determination of the relative vibrational populations of hydrogen molecules in a beam. The method is based on the dissociative attachment process which goes to the first limit at 3.72 eV, i.e., e + H2(u)-• H~+H. The cross sections for this process have been determined at threshold by Allan and Wong 6 up to v~4. Subsequently, theoretical models were adjusted to these observations and cross sections have been published for all vibrational and rotational levels. 7,8 These cross sections are characterized by sharp peaks at threshold whose magnitude increases rapidly from 10 ~2 1 cm 2 for v =0 to a few times 10~1 6 cm 2 for u-6 and remains roughly constant for higher levels. This process is further characterized by a strong isotope effect. The molecular dissociation time is in competition with the resonance lifetime, thus leading to progressively diminishing cross sections when one goes from H2 to HD to D2. Thus, crossing an electron beam of variable energy with a molecular beam of hydrogen and observing the H~ yield allows relative vibrational populations to be determined. The H2 beam effuses from a gas cell where the vibrational levels are populated by recombinative desorption of atomic hydrogen. The atomic hydrogen is generate...

Nonequilibrium rovibrational energy distributions of hydrogen isotopologues in an expanding plasma jet

“…Similar nonequilibrium distributions were found for H 2 in other kinds of gas discharges, 15,[23][24][25] but also for H 2 in the interstellar medium. 1,2,11 However, despite the similarity in distributions, the proposed mechanisms leading to nonBoltzmann state distributions are very different.…”

The Increase of Negative Hydrogen Ions Production in ECR Source by use of the Additional Low-Temperature Emitters of Electrons

“…Перспективным каналом колебательного возбуждения молекул H 2 является взаи-модействие между водородными атомными или молекулярными ионами H + 2 и H + 3 на металлической поверхности плазменной камеры [16,17]. В результате рекомбинации на поверхности атомов газовой фазы могут образовываться колебательно возбуж-дённые молекулы водорода H 2 ( ′′ ) [18,19]. Экспериментальные исследования [20] подтверждают существенный вклад поверхностных процессов на производительность источника отрицательных ионов.…”

4. The Role of Molecular Spectroscopy in the Vacuum Ultraviolet Region for the Development of a Negative Ion Source

“…In addition to the volume processes mentioned above, we cannot ignore the effect of chamber wall (filament and cesium depositions) because of the recombinative desorption (RD) [16][17][18]. This process also influences the population distribution of the vibrational states.…”