Coincidence experiments electron impact excitation

It can be seen that electron—photon coincidence experiments with polarised electrons permit the investigation of spin effects in electron impact excitation of atoms at the most fundamental level. It can lead to direct information on both exchange effects and spin—orbit effects in the excitation mechanism. The information on the population of the magnetic sublevels can be visualised by charge-cloud distributions. These can tilt significantly out of the scattering plane for incident electrons transversely polarised in the scattering plane. 

Autoionization spectra resulting from specific resonances can be obtained by electron-electron coincidence measurements (Haak et al. 1984 Ungier and Thomas 1983, 1984, 1985). To associate a fr.rgmentation pattern with a particular core hole excited state and a particular autoionization or Auger decay channel, a double-coincidence experiment must be done using electron impact excitation. The energy of the scattered electron must be determined, the energy of the emitted electron must be detennined, and the ions produced in coincidence with these two events must be determined. The difficulties inherent in these kinds of experiments have been aptly summarized by Hitchcock (1989), If you can do it by photons, don t waste your time with electron-coincidence techniques. ... 

This method has been employed in many areas of atomic physics. Imhcff and Read have used electron-photon coincidences to measure helium lifetimes thus ensuring the complete absence of cascade processes from affecting the measurement. Pochat et al have measured differential cross-sections for electron impact excitation of n = 4 and 5 states of helium using the decay photons of appropriate wavelengths to uniquely specify the coincident scattered electrons. In addition to several other similar examples, it has also been employed for particles other than photons and electrons e.g. between two electrons as in the (e, 2e) experiments and between ions and photons in ion-atom collision experiments. 

Quantum beats have been extensively studied in beam foil spectroscopy, but only recently have results been reported for their observation following electron impact excitation.For beats to be observed the excitation to the participating levels must be coherent and the modulation period of the decay must be greater than the instrumental resolution of about 10 s obtainable in electron photon coincidence experiments. Since the internal relaxation times from fine or hyperfine interactions of coherently excited states are T 10" s and beats between... 

Evaluated In a photoionization mass spectrometric study from the difference in the thresholds for PHJ from PH3 and PHJ from PHg [1 ]. This value is only 4 kJ/mol higher than the almost coincident values calculated with the 4th-order Moller-Plesset perturbation procedure [1, 8] and with the generalized valence bond model [9]. - From the electron impact ionization of PH3 and PHg. - From the fluorescence excitation of PH3 photolysis fragments. - From the appearance potential of PHJ in electron impact studies of PH3 and the ionization potential of PHg. - From the appearance potential of PH2 from PH3 (2.2,2.3 eV) and the electron affinity of the PH2 radical (1.25 eV, see p. 62) [5] for earlier results (D<326 kJ/mol), see [10]. - From the highest populated rotational-vibrational level of HF, which is produced in a hydrogen abstraction reaction of PH3 with F atoms in a flowing afterglow experiment [6] for earlier results, see [11]. - Literature value based on the upper limit D<326 kJ/mol. 

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