Peter W. Harland

Partitioning of excess energy in dissociative resonance capture processes

The translational energies of selected negative ions formed by dissociative resonance capture processes from the polyatomic systems NF3, BF3, CF4, C2F6, C3F8, and c‐C4F8 have been measured as functions of excess energy over the resonances. The excess energy in the molecular negative ion intermediate prior to dissociation has been calculated and partitioned into translational, vibrational, and, in some cases, electronic excitation of the dissociation products. The degree of vibrational activation in the intermediate state before dissociation is found to depend on the particular molecule under investigation and to vary from one dissociation channel to another. These observations are discussed in relation to theoretical concepts of dissociative resonance capture and given a qualitative explanation. The measurement of translational energy has led to a more complete interpretation of the states involved in the various processes and in computing ground state thermochemical properties of the decomposition products.

Erratum: Use of translational energy measurements in the evaluation of the energetics for dissociative attachment processes

The translational energies of negative ions formed by dissociative resonance capture processes from CO, NO, CO2, and SO2 have been measured as functions of excess energy. The sums of the translational energies of O− and the corresponding neutral from CO and NO were found to be equal to the electron energy above onset over a range of about 1 eV. At higher energies, the translational energies dropped down from the expected values because of loss of the more energetic ions to the walls. With CO2 and SO2 the total translational energy was always E*/N, where E* is excess energy and N the number of vibrational modes, 3 in each case. The measurement of translational energy has also helped in interpreting the states involved in the various processes and in computing the ground state thermochemical properties of the decomposition products.

Ionization cross-sections and ionization efficiency curves from polarizability volumes and ionization potentials

Abstract It has been shown by simple electrostatic considerations and confirmed using literature experimental data that a strong correlation exists between the maximum electron impact ionization cross-section and the square root of the ratio of the atomic or molecular polarizability volume to the ionization potential. The observed shape of the ionization efficiency curves for a wide range of molecules may be rationalized in terms of the interaction of an incoming electron wave with the molecule, which has an effective diameter determined from the polarizability volume. The peak in the curve occurs when the electron wavelength matches this diameter. An expression for the ionization cross-section at any incident electron energy, which is a function only of the molecular polarizability and ionization potential, has been deduced. The results of this treatment are in good agreement with experiment and the predictions are generally at least as good as those of more sophisticated theoretical treatments of the electron impact ionization process.

Enthalpies of formation for the isomeric ions HxCCN+ and HxCNC+ (x = 0−3) by “monochromatic” electron impact on C2N2, CH3CN and CH3NC

Abstract Enthalpies of formation determined from ionization efficiency curves and kinetic energy distributions for the ion series H x C 2 N + ( x = 0−3) from “monochromatic” electron impact on C 2 N 2 , CH 3 CN and CH 3 NC provide evidence for the retention of the carbon-nitrogen skeletal framework in two stable sets of isomers, H x CCN + and H x CNC + . The ionization efficiency curves for the C 2 N + ion from C 2 N 2 and CH 3 NC exhibit two thresholds, the first appearance energy corresponding to the formation of the lowest energy CNC + isomer with a second channel, ∼ 1.1 eV above the first appearance energy, corresponding to co-production of the more energetic CCN + isomer. The CNC + isomer is estimated to represent ∼ 30% and ∼ 60% of the C 2 N + ion populations produced by electron impact on C 2 N 2 and CH 3 NC, respectively, for electron energies 3–4 eV above the threshold.

Electron transfer to oriented molecules : K + CF3I and K + CH3I

K+ ions have been detected from the intersection of a beam of K atoms (5–30 eV) with beams of CH3I and CF3I molecules which had been oriented prior to the collision. Collisional ionization is found to be favored for both molecules when the fast K is incident at the I end of the molecule, even though the electrical polarity of the I end is different for the two molecules. For both molecules, the effect of molecular orientation is most pronounced near threshold (≊5 eV) and almost disappears at higher (30 eV) energies. For CF3I, the threshold for impact at the I end is ≊0.7 eV less than the threshold for impact at the CF3 end. We interpret these results using a ‘‘harpoon’’ mechanism in which the electron jump during the initial approach is probably independent of orientation, but as the charged particles separate, the electron may jump back to the K+. For impact at the I or ‘‘head’’ end, the I− is ejected backwards towards the incoming K+. This increases the final relative velocity of the ions and lowers the...

Absolute electron impact ionization cross-sections for the C1 to C4 alcohols

Absolute total positive-ion electron impact ionization cross-sections from a few eV above threshold to over 200 eV are reported for the C1 to C4 alcohols, including the isomers of propanol and butanol. Correlations between measured ionization cross-sections, ionization potential and molecular polarisability volume are explored and compared with data reported previously for perfluorocarbons, nitriles and mixed halocarbons. Small, but reproducible, differences between the isomers of the C3 and C4 alcohols have been used to estimate molecular volume polarisabilities for iso- and tertiary butanol. A C–OH functional group contribution to the total ionization cross-section has been deduced in line with bond contributions determined for a range of C–X (where X = C, H, F, Cl, Br, I, CN) bond types previously reported. The reproducibility of the measured cross-sections over the full energy range is better than ±2% and in some cases ±1%. Absolute cross-sections measured with the instrument are in excellent agreement with measurements for the inert gases and N2 made by several other groups claiming accuracies of around ±5%. Experimental data are compared with calculations using the Deutsch–Mark additivity method, the Binary Encounter Bethe method, and the polarisation model.

Absolute total electron impact ionization cross-sections for perfluorinated hydrocarbons and small halocarbons

Absolute total positive-ion electron ionization cross-sections from threshold to 220 eV are reported for a range of halogenated methanes and small perfluorocarbons (2–4 carbon atoms). Correlations between the measured ionization cross-section and related molecular properties, in particular the vertical ionization potential (or vertical appearance energy) and molecular polarizability volume, are noted. Contributions to the total cross-section from individual bonds are also determined. Cross-sections predicted using these ‘bond contributions’ are in agreement with experiment for a wide range of molecules to better than ± 10% accuracy, and in most cases to better than ± 5%. The experimental data are also compared with ionization efficiency curves calculated using the Deutsch–Mark (DM) and binary encounter Bethe (BEB) models.

Absolute electron impact ionization cross sections for , where X = H, F, Cl, Br, and I

Measurements of electron impact ionization cross sections have been made for methane and the series methyl fluoride to methyl iodide. The results for methane and methyl fluoride to methyl bromide have been compared with ionization efficiency curves calculated using Deutsch - Mark (DM) and binary-encounter-Bethe (BEB) methods, and also with the results of an ab initio model which gives the maximum cross section as a function of molecular orientation. In addition, the ab initio and DM methods have been used to calculate the steric ratios for the electron impact ionization of methyl chloride which have been compared with experimental measurements made previously.

Absolute Total Electron Impact Ionization Cross-Sections for Many-Atom Organic and Halocarbon Species

The experimental determination of absolute total electron impact ionization cross-sections for polyatomic molecules has traditionally been a difficult task and restricted to a small range of species. This article reviews the performance of three models to estimate the maximum ionization cross-sections of some 65 polyatomic organic and halocarbon species. Cross-sections for all of the species studied have been measured experimentally using the same instrument, providing a complete data set for comparison with the model predictions. The three models studied are the empirical correlation between maximum ionization cross-section and molecular polarizability, the well-known binary encounter Bethe (BEB) model, and the functional group additivity model. The excellent agreement with experiment found for all three models, provided that calculated electronic structure parameters of suitably high quality are used for the first two, allows the prediction of total electron-impact ionization cross-sections to at least 7% precision for similar molecules that have not been experimentally characterized.

Absolute electron-impact ionization cross sections for a range of C1 to C5 chlorocarbons

Absolute total electron-impact ionization cross sections from threshold to 220 eV are reported for the formation of positive ions from a range of chlorocarbons (one to five carbon atoms), including all chlorine-substituted methanes and ethanes. Correlations between the measured ionization cross section, ionization potential and molecular polarizability volume are explored and compared with data for the perfluorocarbons and mixed halocarbons. A C-Cl bond additivity cross section determined previously for mixed halomethanes has been refined to fit the experimental data for the higher chlorocarbons. Maximum cross sections predicted using bond additivity contributions are shown to be in agreement with experiment for a wide range of molecules to better than ±10% accuracy, and in most cases to better than ±5%. The experimental data are compared with the predictions of the Deutsch-Mark and binary-encounter Bethe models.