Robert Locht

The dissociative ionization in oxygen

Abstract Dissociative ionization in O 2 has been examined by means of a retarding potential lens combined with a quadrupole mass spectrometer. This particular arrangement made possible detailed analysis of the O + ions kinetic energy distribution and the measurement of threshold energies of O + ions with known kinetic energy. These two types of informations bring new experimental evidence, besides direct dissociative ionization of O 2 , for predissociation of the B 2 Σ g − state, the 2 Π u state and the c 4 Σ u − state of O 2 + . A new predissociation of the O 2 + ( 2 Δ g ) state has been found. At low ion-energy the predissociation of the 2 Φ u state or an autoionizing predissociation is observed.

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 .

The dissociative ionization of carbon monoxide

Abstract The dissociative ionization of CO by low energy electron impact into C+ + O (or O−) and O+ + C (or C−) is investigated using ion energy and mass analysis. The kinetic energy distributions are studied as a function of the impinging electron energy. The threshold energies of C+ and O+ are measured as a function of the released kinetic energy. The formation of the products in their ground level and with a small amount of kinetic energy mainly occurs through predissociation. The CO+ electronic states involve in these processes are tentatively identified. New evidences have been found for the importance of dissociative ionization via autoionization.

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.

The Photoionization of Ammonia revisited. The Vibrational Autoionization of NH3 and its three Isotopomers in the 10-12 eV photon energy range.

Abstract The photoionization efficiency curves of NH 3 and its three isotopomers have been investigated in the photon energy range of the first ionized state. From the analysis of the corresponding vibrational structure, wavenumbers (ω e ) and anharmonicity constants (ω e χ e ) are deduced. The detailed investigation of the abundant autoionization structure tends to show the adiabatic ionization energy to be 10.072 ± 0.010 eV for NH 3 , NH 2 D and NHD 2 and 10.083 ± 0.010 eV for ND 3 . All autoionization features were classified in vibrational progressions (ν 2 bending mode) belonging to n sa 1 (or n d) and n pe ( n = 5, 6, 7) Rydberg series. Vibrational autoionization occurs through Δν transitions up to −9. A qualitative analysis of the intensity distribution of these series strongly supports that transitions involving odd Δν values are favoured. This observation can be understood by applying group theoretical considerations to the theory of vibrational autoionization.

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 He(I), threshold photoelectron and constant ion state spectroscopy of vinylchloride (C2H3Cl)

Abstract Using synchrotron radiation, the threshold photoelectron (TPES) spectrum of C 2 H 3 Cl and constant ion state (CIS) spectroscopy of C 2 H 3 Cl, are reported. For comparison, the He(I) photoelectron spectrum has also been measured and reexamined. The threshold photoelectron spectrum has been measured between 9.0 and 25.0 eV, and the photon energy range of 9.9–12.0 eV has been investigated in detail. Many features have been identified and tentatively assigned with the help of the photoabsorption spectroscopic results [1]. These data were compared with a well-resolved He(I) photoelectron spectrum. The fine structure observed in the two first ionic states is assigned to progressions belonging partially to previously unobserved vibration normal modes. State-selected CIS spectra haa been recorded for the first vibronic states between 10.0 and 11.67 eV. They exhibit fine structure assigned to autoionization of Rydberg states.

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.