Y. Le Coat

Vibrational excitation via high-energy shape resonances in molecules: v=0 to 1 excitation in CO

The differential vibrational excitation cross section v=0 to v=1 has been obtained for CO in a crossed molecular-beam electron-beam apparatus in the 1-30 eV energy range. Besides the well known pi shape resonance at 1.8 eV, a broad and low-intensity peak centred at 19.5 eV is observed and attributed to a sigma shape resonance. Angular distributions within the sigma resonance energy show the contribution of the f wave (l=3).

Dissociative electron attachment on H2S: energy and angular distributions of H- ions

The authors have studied energy and angular distributions of H- ions produced by dissociative attachment on H2S in the 4.5-9 eV region by means of an electron impact spectrometer. H- formation proceeds via two distinct reactions. For the first one, the HS fragments, associated with the H- ions, are weakly vibrationally excited and the fraction of H- ions associated with the excited HS fragments increases with the incident electron energy. It is also shown that the H- ions associated with HSv=0 and those associated with HSv=1 fragments have different angular distributions. In the energy range of the second reaction, the HS fragments are in the ground vibrational state. In both cases, the HS fragments are found to be rotationally excited.

Clarification of oxygen bonding on diamond surfaces by low energy electron stimulated desorption and high resolution electron energy loss spectroscopy

In this work we correlate between O− electron stimulated desorption (ESD) from hydrogenated and bare polycrystalline diamond films exposed to thermally activated oxygen for incident electron energies in the 4–23 eV range and vibrational electron energy losses. By utilizing the previously established mechanism of low energy ESD of O− from diamond surfaces and high resolution electron energy loss spectroscopy carried out at an incident energy below the threshold for ion desorption, the surface bonding configuration of oxygen is discussed. Such analysis reveals formation of C=O and C–O–C bonds on the bare diamond surface, as well as some adsorbed CO molecules. The bonding configuration of the hydrogenated surface before and after exposure to oxygen is found to be dependent on the hydrogenation method: ex situ microwave plasma versus in situ treatment using a hot tungsten filament for gas activation. Oxygen-related features on the ex situ hydrogenated surface are attributed to formation of C=O and C–O–H bonds...

Low-energy electron-induced processes in condensed CF2Cl2 films

We report measurements of electron stimulated desorption yields and kinetic energy distributions for F− and Cl− anions from thin films of CF2Cl2 condensed on polycrystalline platinum at 20 K. Dissociative electron attachment processes cause a peak in the F− yield at an electron energy of ∼4 eV and a peak in the Cl− yield at ∼7 eV. After electron irradiation, a new dominant structure appears in the Cl− yield at 5 eV incident electron energy, and Cl− ions having high kinetic energy (>1 eV) are observed in the kinetic energy distributions. These changes are attributed to the synthesis of Cl2 in the condensed phase induced by electron irradiation of the CF2Cl2 film. This Cl2 formation is demonstrated by the observation in the Cl− yield of the Cl2−/Cl2 2Πu resonance state, and by Cl− kinetic energy distributions that are similar to those obtained from condensed pure Cl2 films.

Differential cross section for dissociative attachment in HCl: Cl- and H- formation

A modified electron impact spectrometer is used to study Cl- and H- formation in HCl by dissociative attachment. It is shown that the Cl- ion yield exhibits a small, new structure located at the maximum of the main peak in addition to the well known structure on the high-energy tail at the vibrational levels of HCl. The H- ion yield exhibits two peaks at 7.1 and 9.3 eV. The first one is structureless with a non-vertical onset and the angular distribution of the H- is characteristic of a d sigma wave. These results indicate that H- formation in the energy range of this peak proceeds via a repulsive 2 Sigma + HCl- state. The second peak, which proceeds via a 2 Pi HCl- state, exhibits a structure on the high-energy tail and the angular distribution of the H- ions is characteristic of a d pi wave. The structures in this peak are attributed to the interaction of the 2 Pi state of HCl- with the series of Feshbach resonances associated with Rydberg states of HCl.

Decay of secondary electron emission and charging of hydrogenated and hydrogen-free diamond film surfaces induced by low energy electrons

In this work, the decay of secondary-electron emission (SEE) intensity and charging of hydrogenated and hydrogen-free diamond film surfaces subjected to incident electron irradiation at energies between 5 and 20 eV are investigated. Electron emission curves as a function of incident electron energy were measured. For the hydrogenated films, it was found that the SEE intensity decays in intensity under continuous electron irradiation, albeit maintains a nearly constant onset. The decay in time of the SEE intensity was measured for various incident electron energies. From these measurements, the SEE intensity decay rate from the hydrogenated diamond surface was calculated as a function of incident electron energy and found to display a broad peak at ∼9 eV. The decay of the SEE intensity is explained as due to electron trapping in the near-surface region of the hydrogenated diamond films resulting in the formation of a depletion layer and upward surface band bending while overall charge neutrality is maintai...

Morphology effects in anion electron-stimulated desorption

Electron-stimulated desorption of O - anions from O 2 -covered multilayer condensed films of various molecules is investigated in the energy range 0-20 eV. A suppression of O - signal below 1 eV is observed from < 0.1 monolayer (ML) of O 2 deposited on either 4 ML of D 2 O or 6 ML NH 3 condensed at 15 K. A detailed study of O 2 /D 2 O target shows that the magnitude of the O - signal depends strongly on temperature (15-150 K) of D 2 O condensation on Pt, on the amount of condensed O 2 (0.1-4 ML) and on the D 2 O film thickness (4-100 ML). These results are shown to be related to different morphologies of the ice layer.

Negative ion resonances in CF4 probed by dissociative electron attachment

An electrostatic impact spectrometer is used to measure, between 3 and 10 eV, the kinetic energy, the angular distributions and the yield of negative fragment ions formed by dissociative attachment in CF4. The energy range and the symmetry of the resonances are determined as well as the dissociation processes and their dynamics. The first resonance state, observed around 6.8 eV, is attributed to the CF4- 3T2 shape resonance adiabatically dissociating in the two channels F-+CF3 and CF3-+F. The excess energy is shared between the fragment kinetic energies and the internal (vibrational) excitation of the trifluoromethyl fragment. The next resonance, located around 8 eV, is assumed to be of T1 symmetry. The authors suggest that this Feshbach resonance dissociates in F- ions by the sequential breaking of two C-F bonds through an intermediate CF3-* electronically excited negative molecular ion. The 2A1 shape resonance has not been observed in the dissociative channel.

Kinetic energy distributions for O− and metastable CO* produced by electron stimulated desorption from condensed CO2

Abstract We report kinetic energy (KE) distributions of O − anions and metastable CO * produced by 10–30 eV electron stimulated desorption from multilayer CO 2 films condensed on polycrystalline platinum at 20 K. The O − KE distributions are obtained from energy analysis with a hemispherical electrostatic deflector and the CO * KEs from time-of-flight measurements. Both anion and metastable energy distributions are broadened and the energy shifted is compared to those of the gas phase. The narrow (∼160 meV) and low energy (∼60 meV) peaked CO * KE distributions reveal that post-dissociation interactions play a minor role in the broadening, whereas the wide (>1 eV) KE distributions of O − result mainly from multiphonon excitation of the substrate by the departing stable fragment anion.

O− ESD dynamics from condensed N2O

Abstract Dissocative electron attachment processes have been observed in O − electron stimulated from multilayer films of N 2 O molecules condensed on polycrystalline platinum at 30 K. Kinetic energy distributions, fixed kinetic energy O − yields and O − yields are reported. The ion yields show dissociative attachment processes at 9 and 15 eV which are not observed in the O − formation for isolated N 2 O molecules. The kinetic energy distributions are broad and show that a value of about 0.5 eV is obtained for the most probable kinetic energy of O − ions whatever the incident electron energy. This indicates that the desorption dynamics are driven by the dissociation dynamics of the N 2 O − ∗ resonant states involved.