Electron-ion coincidence measurements of CH₄

Abstract

Measurements are reported of 10 keV electron scattering in CH4 at low momentum transfers, in coincidence with the various ionic products. Ion mass separation was performed in a time-of-flight system with total transmission for fragments with dissociation energies up to 20 eV. Results have been obtained on the oscillator strengths for the formation of all ionic products for energy losses from the IP up to 80 eV. The present results confirm earlier electron-electron coincidence measurements of the photoionization efficiency (see ibid., vol.8, no.18, p.3007 (1975)) and extend them to higher energy. The oscillator strengths of the (1t2)-1, (2a1)-1 and higher initial states of CH4 + could be extracted separately from the data.

Studies of the photoionization cross sections of CH4

“…The calculated cross sections agree well with the measured values of Backx et a/. 3 For example, they reproduce the sharp increase and prominent, but broad, maximum seen in the experimental cross sections within 5 eV of threshold. Experiments with adequate photoelectron kinetic energy resolution to resolve vibrational structure within these Jahn-Teller components near threshold could provide considerable insight into the photoionization dynamics of CH 4 • The agreement between the calculated and measured photoelectron angular distributions 10 is, however, less satisfactory.…”

“…3 The cross sections shown here, in both the length and velocity form, were calculated at the equilibrium internuclear geometry of methane and assume that the photoelectron spectrum is made up of three electronic bands with maxima at 13.5, 14.5, and 14.8 eV, respectively resulting from the Jahn-Teller distortion of the 2 T 2 ion of CH 4 • 9 The actual cross sections used in each of these intervals were obtained from the fixed-nuclei 2 T 2 ion calculation. This procedure builds in step-like behavior very near threshold and gives essentially the same cross sections above -18 eV as obtained assuming a single ionization threshold of 12.6 e V. From 14 to 16 e V photon energy there is a sharp increase in the 1t 2 experimental cross section from 25.8 to 45.2Mb.…”

Valence shell absolute photoabsorption oscillator strengths, constrained dipole oscillator strength distributions, and dipole properties for CH3NH2, (CH3)2NH, and (CH3)3N

“…The dipole (e,e) spectrometers and procedures used in the present work to determine the absolute low-resolution (~1 eV fwhm) (16)(17)(18)(19)(20) and high-resolution (0.048 eV fwhm) (21-23) oscillator strengths for the methylamines using dipole (e,e) spectroscopy have been described in detail previously. This technique involves high energy electron scattering at negligible momentum transfer and has been shown to provide data which are entirely equivalent to those obtained using conventional optical methods (24).…”

The proton formation from methane by dissociative ionization in the electron energy range of 25–40 eV

“…However, other mechanisms like processes (13) and (18) in table 2 could not be definitely ruled out.…”

“…The possible formation of the proton through transitions to unstable doubly ionized states of CH 4 , such as processes (24) and (26) listed in table 2 could probably be discarded because double ionization starts around 40 eV [13].…”