J. Momigny

On the photoelectron spectra of HBr and DBr

Abstract The photoelectron spectra of HBr and DBr with He I (58.4 nm) are discussed. The photoelectron spectrum of DBr confirms the predissociation of the A 2Σ+ electronic state. A rough estimate of the predissociation rate is calculated.

The dissociative ionization of nitrogen

Energy distributions and appearance potentials of N ions produced by low-energy electron impact on N2 have been measured. At least five dissociation limits were detected either directly in the ionization efficiency curves of N or by extrapolation of the 1/2 slope straight lines drawn through the kinetic energy versus appearance potential energy diagram. Fine structure observed at low ion energy has been interpreted by the predissociation of at least two N 2 excited states one of which being the well-known C 2 3 u + state. One of the others could be a 2 5u state pre-dissociated to the dissociation limit at 24.34 eV. Close correlation between the results of the present work and photoelectron spectroscopic observations could be done over the electron energy range between 24.3 eV and 36 eV and assignments are discussed in detail.

DISSOCIATIVE AUTOIONIZATION AS A MECHANISM FOR THE PROTON FORMATION FROM METHANE AND METHANE-d4 BY LOW ENERGY ELECTRON IMPACT

Abstract The ion energy distribution and the electroionization efficiency curves of H + and D + from methane and methane- d 4 have been examined. The existence of thermal ions is unambiguously evidenced and the two lowest appearance thresholds of H + are measured at (21.3 ± 0.3) eV and (22.17 ± 0.1) eV. At both energies the protons have to be formed through dissociative autoionization. The present results closely parallel those of the dissociative excitation experiments on methane. The Jortner—Rice model for dissociation of polyatomic molecules is invoked to account for both the appearance of H + and high-Rydberg H atoms from CH 4 .

Correlation diagram approach to the dissociative ionization mechanisms of methanol

Abstract A systematic survey is made of the dissociation processes from the ground state and from the electronically excited states of CH3OH+. Most metastable processes occurring in the mass spectrum of CD3OH are studied also, including measurement of the kinetic energy release and appearance potentials. It is shown that if ionic species such as CH2OH+, CH2O+ and HCO+ are present in the mass spectrum of CH3OH, the isomers CH3O+, HCOH+ and COH+ are also present. The precursors of these ions having been identified by the metastable processes, correlation diagrams are used to show that, from the first ionization potential of CH3OH to 14 eV, the only exception being CH3O+ which results from an electronically spin-forbidden predissociation, the fragment ions can be considered to result from adiabatic decay of the ground state of CH3OH+. The potential barriers involved in these adiabatic processes result either from conical intersection or from avoided-crossing between the ground state of CH3OH+ and the electronically excited states occurring up to 14 eV. At higher energies, many non-adiabatic processes occur; for example, the appearance of CD2O+ and the electronically excited states of HCO+ and COH+ ions. It is shown additionally that kinetic energy releases observed in the collision-induced processes CH3OH+ → CH2OH+ + H and CH2OH+ → HCO+ + H2 are in good agreement with the mechanistic approach via the correlation diagram for the appearance of the non-collision-induced ion processes.

Mass Spectrometric Study of Ion-Pair Processes in Diatomic Molecules: H2, CO, NO and O2.

Abstract The appearance of ion-pair processes has been studied in H2, CO, NO and O2. The different threshold successively observed for each process have been interpreted. Using the results from the convolution of theoretical models of the ionization efficiency curves and the deconvolution of their first differential, it was possible:(i) to propose a method for the determination of the threshold energy of an ion-pair process, (ii) to show that, except for H2, the shape of the cross-section of an ion-pair process as a function of the electron excess energy is a step-function and (iii) to determine the kinetic energy distribution of the negative ions and consequently to form an idea about the relative position of the potential energy curves of the excited states of the molecule, decaying by ion-pair process, with respect to the ground state of the molecule. Our analysis suggests that, for CO and NO, some of the observed absorption bands, labelled by Tanaka as Pi, (i=1–5), might be related to the existence of some excited states of these molecules, correlated with an ion-pair as dissociation products.

Mass Spectrometric determination of the Electronaffinities of Radicals.

Abstract By mass spectrometric study of the appearance of C − 2 , CH − and C 2 H − from CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 3 X (X = Cl, F) the following electroaffinities have been determined: EA(CH) = (2.6 ± 0.3) Ev, EA(C 2 ) = (3.3 ± 0.2) eV and EA(C 2 H) = (2.1 ± 0.3) eV. Some evidence is brought to the exiatence of electronic excited states of these ions. An evaluation of these negative ions dissociation energy in their ground electronic state is given and is found to be of the order of magnitude of the corresponding isoelectronic molecular species dissociation energy.

A mass spectrometric photoionization study of CH3F. The CH+2, CH+3 and CH2F+ ion formation

Abstract The decay of CH3F+ in the CH+2, CH+3 and CH2F+ dissociation channels has been investigated by photoionization mass spectrometry using synchrotron radiation in the energy range 10–25 eV. The photoabsorption curve of CH3F and the ionization efficiency curves of CH+2, CH+3 and CH2F+ as well as the translational energy spectra and the kinetic energy versus appearance energy diagram for CH+3 were recorded. For the three fragment ions dissociative autoionization is a prominent formation process. Direct dissociative ionization is observed for the same ions through the decomposition of the CH3F+ ( A 2A1 + B 2E) state at 16.1 eV as well as presumably of the C 2A1 state at about 22.5 eV.

The dissociative electroionization of ammonia and ammonia-d3. I. The NH+ and NH2+ dissociation channels

Abstract The dissociative electroionization of NH3 (ND3), in the NH2+ (ND2+) dissociation channels, is investigated in the 15–50 eV electron energy range. The translational energy distributions of ND2+ (NH2+) exhibit several components, whereas NH+only shows a continuous, broad distribution. For both ions a kinetic energy versus appearance energy diagram is obtained and discussed. The predissociation of the Jahn-Teller split NH3+ (A, 2E) state produces both ions at their lowest threshold. At higher energies, dissociative autoionization (in the 22 eV region), dissociative ionization through the NH3+ (B 2A1 state in (the 24–30 eV region) as well as dissociation by Coulomb repulsion of doubly ionized states (in the 35–50 eV region) produce NH2+ (ND2+) and NH+ with large amounts of translational energy. Some arguments are brought forward to indicate that the former value of the NH2-H dissociation energy is overestimated.

Collision- and non-collision-induced predissociation in the appearance of S+ and CS+ ions from CS2 under electron impact

Abstract Metastable transitions in CS + 2 leading to S + + CS and CS + + S under electron impact on CS 2 ae reported. The predissociation processes able to explain the occurrence of these metastable transitions are discussed. An interpretation of some fast dissociation processes is suggested.

On the position of the electronic states of the C6H6+ and C6H5F+ ions

Abstract The sigma ionization potentials of benzene are shown to be 10.3 and 10.8 eV. These states are not detected by photoelectron spectroscopy using 21.21 eV photons. An assignment for the lowest electronic states of C 6 H + 6 and C 6 H 5 F + is given.