Roger Azria

Formation of HS− and DS− by Dissociative Attachment in H2S, HDS, and D2S

We have studied the formation of HS− by dissociative attachment in a total ionization chamber, with a RPD electron gun. The onset of the ionization efficiency curve (1.45 ± 0.1 eV) is equal to the minimum energy necessary for the formation of HS− (1.55 ± 0.03 eV). Nevertheless the curve does not exhibit the usual shape characteristic of a vertical onset process. The cross section at the maximum is σ (HS−/H2S)=1.7 × 10−18u2009cm2. We measured the isotope effects separately, in a mass spectrometer, and we found that the cross section in D2S is much smaller: σ (HS−/H2S) /σ (DS−/D2S)=25 ± 3. We tried to reproduce the experimental results by a semiempirical calculation, using the expression of OMalley for the cross section and parameterizing the final state potential curve of the negative ion. We have shown that the experimental results are consistent with the assumption of a very shallow bound potential in the Frank–Condon region. Quantitative calculations are impossible because the formula of OMalley is incorr...

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.

Reactivity induced at 25 K by low-energy electron irradiation of condensed NH3-CH3COOD (1:1 mixture!

Chemical reactivity is observed following electron irradiation of a binary mixture of ammonia (NH(3)) and acetic acid (CH(3)COOD) at 25 K, without any subsequent thermal activation, as evidenced by vibrational high resolution electron energy loss spectroscopy (HREELS). Analysis of the HREEL spectra and comparison with infrared and Raman data of different molecules are compatible with glycine formation in its zwitterionic form. The onset for electron induced reaction is found to be at about approximately 13 eV. The mechanisms may involve NH radicals interaction with CH(3)COOD molecules. Then glycine formation does not imply any displacement of reactants, so that it involves only NH(3) and CH(3)COOD neighboring molecules.

Post-dissociation interactions in ESD: The 18O−C16O surface reactions induced by 4–10 eV electrons

Abstract The 18O−ue5f8C16O ion—molecule istope-exchange reaction in the condensed phase is induced and controlled by electron impact on a C16Oue5f818O2 mixture film deposited on a Pt substrate. The measured efficiency of this reaction is shown to increase with 18O− energy. The data allow us to understand the post-dissociation mechanism in O− ESD from condensed CO.

Mechanism for resonant O- electron stimulated desorption from condensed O2. Non-adiabatic transitions and dynamics

Absolute kinetic energy distributions and yields associated with ground state 3P and excited state 1D oxygen atoms have been obtained for O− anion electron stimulated desorption from condensed O2 in the electron energy range 6–15 eV. The observed yields are understood as resulting essentially from dissociative electron attachment reactions via the two lowest 2Σ+g O−2 resonance states through adiabatic and non-adiabatic transitions to the limits O−(2P) + O(3P) and O−(2P) + O(1D). The kinetic energy distributions show the prominent role of electron multiple collision processes and post-dissociation interactions of the O− anions in the condensed phase.

Chemistry induced by low-energy electrons in condensed multilayers of ammonia and carbon dioxide.

We have investigated by means of HREEL spectroscopy electron induced reactivity in a binary CO2 : NH3 ice mixture. It was shown that the interaction of low energy electrons (9-20 eV) with such mixtures induces the synthesis of neutral carbamic acid NH2COOH and that flashing the sample at 140 K induces the formation of ammonium carbamate. The products have been assigned by FTIR spectroscopy of a CO2 : NH3 mixture heated from 10 K to 240 K. A mechanism involving dissociation of NH3 molecules into NH2* and H* radicals is proposed to explain the product formation.

Reactions in condensed formic acid (HCOOH) induced by low energy (<20 eV) electrons

The interaction of low energy (< 20 eV) electrons with a five monolayer (ML) film of formic acid (HCOOH) deposited on a cryogenically cooled monocrystalline Au substrate is studied by electron stimulated desorption (ESD) of negatively charged fragment ions. A comparison with results from gas phase experiments demonstrates the strong effect of the environment for negative ion formation via dissociative electron attachment (DEA). From condensed phase formic acid (FA) a strong H desorption signal from a resonant feature peaking at 9 eV is observed. In the gas phase, the dominant reaction is neutral hydrogen abstraction generating HCOO- within a low energy resonance, peaking at 1.25 eV. ESD studies on the isotopomers HCOOD and DCOOH indicate effective H/D exchange in the precursor ion at 9 eV prior to dissociation. The evolution of the desorption signals in the course of electron irradiation and the features in the thermal desorption spectra (TDS) of the electron irradiated film suggest the formation of CO2 at electron energies above 8 eV.

O− electron stimulated desorption from O2 in CO and N2 matrices

Abstract It is established in experiments with CO and N 2 matrix films that O − electron stimulated desorption from condensed O 2 at electron energies below polar dissociation proceeds via dissociative attachment from three different resonant states, namely the 2 Π u and two Σ + configurations of O 2 − .

Synthesis of Cl2 induced by low energy (0–18 eV) electron impact to condensed 1,2-C2F4Cl2 molecules

The interaction of low energy electrons (0–18 eV) with C2F4Cl2 molecules condensed in multilayer amounts on a cryogenically cooled Au substrate generates Cl2, as can be seen from the energy and temperature dependence of the Cl− desorption signal. The cross section for Cl2 formation exhibits two pronounced resonant features with maxima near 0 and 10 eV, dropping to essentially zero in the energy range between the resonances (near 3 eV). The energy dependence of the reaction cross section qualitatively follows that of Cl− desorption, which itself can be correlated to dissociative electron attachment (DA) processes in the gas phase. While at 10 eV excited negative ion resonances and electronically excited states of neutral C2F4Cl2 may be involved in the process, in the low energy region (<2 eV) the reaction can nonly be initiated by dissociative electron attachment. Possible reaction pathways are discussed.

Zero kinetic energy ions in dissociative attachment on triatomic molecules: S−/OCS, O−/CO2

Abstract The zero kinetic energy yield of S − ions in OCS and O − ions in CO 2 , produced by dissociative electron attachment through the lowest shape resonance, has been measured in a quadrupole mass spectrometer with electrostatic filter for translational energy analysis. The relative cross sections for S − and O − ion formation associated with CO fragments excited up to the vibrational level υ = 4 are obtained.