M J Van der Wiel

Absolute oscillator strengths (10–60 eV) for the photoabsorption, photoionisation and fragmentation of H20s

Abstract Dipole oscillator strengths (cross sections) for the photoabsorption, partial and total photoionisation and photofragmentation of H 2 O have been obtained, over a range up to an equivalent photon energy of 60 eV, using electron-impact coincidence simulation techniques. The results are quantitatively equivalent to those that would be obtainable by photoelectron spectroscopy and photoionisation mass spectrometry with continuously tuneable light sources. The photoionisation efficiency is also reported. The results of the two experiments are combined to provide a quantitative picture of the dipole induced breakdown of H 2 O in the region up to 60 eV.

Absolute oscillator strengths for valence-shell ionic photofragmentations of N2O and CO2 (8–75 eV)

Abstract Dipole oscillator strenghts (cross sections) for photoabsorption, photoionisation and photofragmentation of N 2 O and CO 2 have been derived from electron energy loss and electron-ion coincidence measurements in the energy range 8 to 75 eV. The photoionisation efficiency from the ionisation threshold to 60 eV is reported for each molecule. These results have been combined with the partial photoionisation oscillator strenghts for valence-ionised N 2 O and CO 2 derived from earlier electron-electron coincidence experiments in order to investigate the dipole induced fragmentation of each molecule. Comparison is made with previous optical studies of the photofragmentation and photoionisation of N 2 O and CO 2 . Double ionisation in these molecules has also been examined.

Electron-ion coincidence measurements of CH4

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.

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.

Dipole oscillator strengths for the photoabsorption, photoionization and fragmentation of molecular oxygen

Abstract The photoabsorption, photoionization and fragmentation of O 2 have been studied using electron impact coincidence methods to obtain branching ratios and dipole oscillator strengths (cross-sections). The photoabsorption measurements cover the energy range 5–300 eV while the formation of electronic states of O 2 + (photoelectron spectroscopy) and the resulting ionic fragmentation (photoionization mass spectrometry) are both measured from close to threshold up to photon energies of 75 eV. The binding energy spectra of O 2 show peaks at 33, 47 and 57 eV in addition to those reported elsewhere in the literature. These peaks are assigned to multiple final ion states arising from photoionization of the inner valence orbitals. Structure in the O 2 + electronic state partial oscillator strength curves is in good agreement with recent theoretical work which predicts the existence of several shape resonances. A quantitative picture of the dipole-induced breakdown of O 2 is obtained for the energy range 12–75 eV. The photoionization efficiency is found to be constant above 20 eV.

Absolute oscillator strengths for the shape resonances near the K edges of N2 and CO

Absolute oscillator strengths for absorption near the K edge in N2 and the carbon K edge in CO have been derived from 8 keV electron energy-loss measurements. The resonance structure in the spectra is compared to results of the multiple-scattering model as applied by Dehmer and Dill (1976).

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.

Highly resolved electron spectra for resonantly enhanced four-photon ionization of no

Abstract Electron energy spectra of (2 + 2)-photon ionization of NO are reported. The intermediate resonance is A 2 + and scans are made over the rotational width of u = 0 and 1. A new type of spectrometer is used accepting 2π sr at a resolution of 15 meV. The vibrational distribution peaks at Δ v = 0 (Franck-Condon for direct ionization) but also at the highest energetically allowed Δ v . We present evidence for pre-ionization via a dissociative channel.

Absolute oscillator strengths (5-63 eV) for photoabsorption and ionic fragmentation of SF6

Dipole oscillator strengths (cross sections) for photoabsorption and photofragmentation of SF6 have been obtained over a range of energies up to an equivalent photon energy of 63 eV, from electron-ion coincidence measurements. The photoionisation efficiency from the ionisation threshold to 63 eV is reported. These results have been combined with photoionisation branching ratios for the six lowest energy states of SF6+ to obtain a model for the dipole-induced fragmentation of valence-ionised SF6. The relative contributions of dissociative and non-dissociative double ionisation have also been investigated.

Ionic fragmentation of SF6 ionised in the sulphur 2p shell

The ionic fragmentation of SF6 following excitations between 160 and 230 eV has been studied by the electron-ion coincidence technique. The observed fragmentation has been separated into valence- and inner- (S 2p) shell components. A constant ionic fragmentation pattern for all of the sulphur 2p ionisation continuum, including the pronounced structures at 184 and 196 eV, is demonstrated. The significance of this result to various descriptions of these structures is discussed. The absorption oscillator strength of SF6 from 5 to 230 eV, derived from forward-scattering electron energy-loss measurements at small momentum transfer, is also reported and compared to earlier optical data. Finally, the ion autocorrelation spectrum indicates the relatively large contribution of double dissociative-ionisation processes ( approximately 2% of all ionisation events for 8 keV electron impact) to the decay of highly excited SF6.