The CH4
+ electroionisation efficiency curve and the CH3
+, CH2
+, CH+ and C+ appearance potential curves are presented. The data were obtained by an apparatus of very high sensitivity with an improved electron monochromator. The CH4
+ ionisation potential is measured to be 12.63+or-0.02 eV. The results show that some previously reported appearance potential values should be corrected, in some cases by several electron volts. This is attributed to superior apparatus sensitivity. Ion-pair formation is clearly observed for the CH3
+ and CH+ fragments. The CH+ and C+ appearance potentials have been detected down to adiabatic threshold. The stable fragment CH2
2+, is also observed.
Abslact. Electron impact excitation of the resonance levels of Ne, Ar, Kr and Xe has been studied for electron scattering angles up to 5O'and impact energies between 30 and 80 eV. T h e P, and Pd Stokes parameters have been measured in each case so that the influence of spin in the excitation process could be studied through evaluation of poo. the relative spin-flip cross section. After careful account was taken of various depolarizing effects due Particularly to the finite volume of the interaction region and, in the c a m of Kr and Xe, to nuclear spin, very good agreement has been found with theoretical predictions thus resolving a previously reported discrepancy. No evidence has been found for spin-flip under the experimental parameters used in this study, even for the heaviest target studied.
Threshold region ionization efficiency curves of ethane, of all singly charged fragments consisting of at least one carbon atom, and of [Formula: see text] and [Formula: see text] are presented and analysed. An energy filtered monoenergetic electron beam apparatus capable of producing very high signal-to-noise ratio data was used to study these ions. The C2H6 ionization energy is evaluated to be 11.57 ± 0.02 eV and structures due to competition with [Formula: see text] fragment production are observed at higher energy in the [Formula: see text] curve. As a result of the high apparatus sensitivity several of the measured appearance energies are lower (some by several eV) than previously reported. [Formula: see text], [Formula: see text], and [Formula: see text] ion pair formations are observed. The [Formula: see text] ion pair formation, previously reported by several studies, is shown to be obscured by the isotopic [Formula: see text] fragment signal.
Electron-polarised photon coincidence measurements have been carried out using Kr and Xe targets. For the first time photon polarisation analysers positioned both in and perpendicular to the scattering plane have been used. Data are presented for electron scattering angles up to 30" and incident energies of 50eV (Xe) and 60eV (Kr). The magnitude of the spin flip observed in the excitation process is most significant for Xe. Comparison is made with other experimental and theoretical work.
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