G.R. Wight

K-shell excitation of CH4, NH3, H2O, CH3OH, CH3OCH3 and CH3NH2 by 2.5 keV electron impact

Abstract The regions around the respective carbon, nitrogen and oxygen K-edges of CH4, NH3, H2O, CH3OH, CH3OCH3 and CH3NH2 have been investigated by electron energy loss spectroscopy using a beam of 2.5 keV electrons. All spectra show a number of discrete peaks just below the K-shell ionization threshold. These discrete structures have been interpreted as being associated with the promotion of a K-shell electron to Rydberg orbitals which converge to the K-shell ionization threshold.

K-Shell energy loss spectra of 2.5 keV electrons in CO2 and N2O

Abstract Energy loss spectra of 2.5 keV electrons, scattered by CO 2 and N 2 O through small angles, have been studied in the regions around the carbon, nitrogen and oxygen K-edges. With the exception of the oxygen K-shell spectrum of nitrous oxide, the spectra are similar to those observed 1 for the diatomic molecules, N 2 and CO, in that the spectra are dominated by one very intense discrete transition while above the K-edge considerable structure is observed in addition to the normal K-continuum. This structure represents the simultaneous transitions of K-shell and valence shell electrons. The oxygen K-shell spectrum of nitrous oxide is unique in that the higher energy discrete peaks are more intense relative to the first discrete peak compared with the other spectra, while additional continuum structures are very weak. The observed spectra for the carbon K-shell of carbon dioxide and the terminal nitrogen K-shell of nitrous oxide are not in agreement with predictions based on the core analogy model which has previously been shown 1 to apply in the case of N 2 and CO.

Oscillator strengths (10-70 eV) for absorption, ionization and dissociation in H2, HD and D2, obtained by an electron-ion coincidence method

The gross spectral behaviour of the dipole oscillator strengths for absorption, ionization and dissociation of H2, HD and D2 has been measured over the energy range of 0 to 70 eV, using the technique of detection of forward-scattered 8 keV electrons in delayed coincidence with the ions. A quantitative collection of ionic fragments is ensured up to kinetic energies of 20 eV. The absorption results are in excellent agreement with those of Samson and Cairns (1965). By subtracting the total ionization spectrum from absorption, a region about 1 eV wide is found above the ionization potential where part of the oscillator strength goes into neutral formation. No appreciable isotope effect is observed in this region.

K-shell excitations in NO and O2 by 2.5 keV electron impact

Abstract Energy loss spectra of 2.5 keV electrons, scattered by NO and O 2 through small angles, have been studied in the regions of the respective nitrogen and oxygen K-edges. The spectra are similar to those for the diatomic molecules N 2 and CO in that they are dominated by the first discrete transition. The nitrogen and oxygen K-shell spectra of nitric oxide are in qualitative agreement with that expected from an O 2 and NF description respectively of the K-shell excited molecule. Similarly the ionization potential of the oxygen monofluoride (OF) radical, deduced from the oxygen K-shell spectrum of molecular oxygen, is in fair agreement with experimental and theoretical values.

Dipole term and first derivative at K=O of the generalized oscillator strength of He by keV electron impact

An approximate method is described for obtaining the derivative to K2 of the generalized oscillator strength for keV electron scattering at zero momentum transfer, over a large range of energy losses. The measured data enables one to reduce the systematical uncertainty in the derivation of optical oscillator strengths to below the 1% level. Results are presented for He over the spectral range of 19 to 65 eV. The data for the derivative are in satisfactory agreement with earlier electron scattering results at lower impact energy and extend over a sufficient range to allow the application of a sum rule for this term of the generalized oscillator strength.

Branching ratios and partial oscillator strengths for the photoionization of NH3 in the 15–50 eV region

Abstract The branching ratios for dipole ionization from the three valence orbitals of NH 3 have been determined up to 50 eV using an improved electron impac

Electron-electron coincidence measurements of CH4

A measurement of fast electron scattering from CH4 at low momentum transfers in coincidence with the ejected electrons is reported. A complete collection is made of ionization electrons ejected over the entire solid angle with kinetic energies up to 20 eV and a novel method has been developed for testing the transmission of the extraction system as a function of the initial kinetic energy of the electrons. The results are presented as optical data i.e. relative oscillator strengths for absorption (0 to 90 eV) and ionization (IP up to 27.5 eV) and the absolute photoionization efficiency.

Dipole oscillator strengths for excitation, ionisation and fragmentation of NH3 in the 5 to 60 eV region

Dipole oscillator strengths for absorption parent-ion formation and fragmention formation of NH3 have been obtained, using techniques of energy-loss electron spectroscopy and electron-ion coincidence. The data have been analysed using branching ratios for the three electronic states of NH3+ which have been derived from a recent electron-electron coincidence experiment. The given fragmentation patterns for the three ionic states were found to give a reasonable fit to the data. Discrete excited states converging on both the 1e and 2a1 ionic limits were found to give rise to some neutral formation.

K- and LII, III-Shell excitations in CS2 and COS by 2.5 keV electron impact

Abstract Energy loss spectra of 2.5 keV electrons, scattered by CS2 and COS through small angles, have been studied in the regions of the respective oxygen K, carbon K and sulfur LII, III(2p) edges. The carbon and oxygen K-shell spectra are dominated by the first discrete transition similar to the K-shell spectra of the “isoelectronic” molecules CO2 and N2O. However, the carbon K-shell spectra of CS2 and COS have features which may indicate the possible existence of an effective potential barrier in these molecules.

K-shell and valence shell excitations in CF4 by 2.5 keV electron impact

Abstract Energy loss spectra of 2.5 keV electrons, scattered through small angles by CF 4 , have been obtained in the region of valence, carbon K- and fluorine K-shell excitations. The carbon K-shell spectrum has features which may be associated with the existence of an effective potential barrier in the CF 4 molecule.