Michel Tronc

MEDIUM ENHANCED, ELECTRON STIMULATED DESORPTION OF CF3- FROM CONDENSED CF3I

Abstract It is shown that (a) the cross-section for electron stimulated desorption of CF 3 − from condensed CF 3 I is enhanced by more than 2 orders of magnitude with respect to the corresponding gas-phase dissociative electron attachment cross-section and (b) CF 3 − is ejected from the CF 3 I film into vacuum with surprisingly high kinetic energy. These observations are explained by a preferential orientation of the molecule at the surface and a particular, medium enhanced, desorption mechanism which is based on the conversion of an open-channel resonance into a closed-channel (Feshbach) resonance when solvated.

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).

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.

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.

Shape-resonance-enhanced vibrational excitation of BF3 by electron impact

Vibrational excitation of BF3 has been studied in an experiment with crossed electron and molecular beams from 1 to 20 eV. The differential inelastic cross sections are dominated by broad maxima which are attributed to shape resonances with a2 and e symmetries and which have their counterparts in the boron and fluorine K-shell photo-absorption spectra of the molecule.

Non-adiabatic transitions observed in H-formation by dissociative electron attachment on HBr

Differential cross sections and angular distributions of ions formed by dissociative electron attachment in HBr have been obtained, by means of a modified crossed-beam electron impact spectrometer, in the 5-10 eV energy range. The yields of H- ions associated with both Br(2P32/) and Br(2P12/) atoms exhibit two peaks in the energy range 5-7 eV and 7-10 eV, respectively. The first peaks in the 5-7 eV energy range have an angular distribution characteristic of a d sigma wave indicating that H- ions in this energy range are formed via a 2 Sigma HBr- resonant state. For the two higher peaks the angular distributions of H- ions are characteristic of d pi waves in agreement with 2 Pi HBr- resonant states. The first peak of the H- ions associated with Br(2P12/) atoms shows a non-diabatic transition from the 2 Sigma 1/2HBr- resonant state to the H-+Br(2P12/) dissociation limit which is due to a strong dynamical (d/dR) coupling between the two Omega =1/2 HBr-1 resonant states. This coupling occurs at a large internuclear separation of the H- and Br fragments as shown by the angular distribution of H- ions. A d/dR coupling calculation does not account for the experimentally measured branching ratios.

Low-energy electron dissociative attachment processes in HgBr2

Br- formation in HgBr2 has been studied by means of two cross-beam electron impact spectrometers, one equipped with a trochoidal electron monochromator and a magnetic mass analyser and the other one equipped with two 127 degrees electrostatic energy filters and a quadrupole mass analyser. It is found that the Br- cross section in the energy range 0-7 eV exhibits two peaks, at zero energy and 4.15 eV respectively. The energy distributions of Br- ions are obtained in the energy range of the second process, different interpretations are discussed.