Electron impact coincidence

Electron impact coincidence techniques have not been developed to the same extent as PIPECO. Nevertheless, reported measurements [125, 134, 135, 328, 394, 816] suggest that electron impact coincidence 

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Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

Coincidence experiments explicitly require knowledge of the time correlation between two events. Consider the example of electron impact ionization of an atom, figure Bl.10.7. A single incident electron strikes a target atom or molecule and ejects an electron from it. The incident electron is deflected by the collision and is identified as the scattered electron. Since the scattered and ejected electrons arise from the same event, there is a time correlation... 

Figure Bl.10.7. Electron impact ionization coincidence experiment. The experiment consists of a source of incident electrons, a target gas sample and two electron detectors, one for the scattered electron, the other for the ejected electron. The detectors are coimected tlirough preamplifiers to the inputs (start and stop) of a time-to-amplitiide converter (TAC). The output of the TAC goes to a pulse-height-analyser (PHA) and then to a nuiltichaimel analyser (MCA) or computer.

Figure Bl.10.11. Electron impact double ionization triple coincidence experiment. Shown are the source of electrons, target gas, tluee electron detectors, one for the scattered electron and one for each of the ejected...

Identification of compounds in the river water extracts was based on the coincidence of gas chromatographic retention times and on the equivalence of electron impact and chemical ionization mass spectra with those of authentic compounds. Quantitation was based on standard curves generated for selected compounds. 

It is apparent that in both cases energy E is deposited in [AB+ +eej] and that, as in the case of excitation, the photon energy is analogous to the electron energy loss. However, since there are now two electrons sharing the excess energy in electron-impact ionization, it is necessary to use time correlation (coincidence techniques) for the simulation of photoionization... 

Detailed studies of crT for helium and H2 were first reported by the Bielefeld group (Fromme et al., 1986, 1988). The arrangement used for this work was described in detail in section 4.3. Here we recall that ionization was distinguished from positronium formation, for which there is also a remnant ion, by detection of the scattered positron and the ion in coincidence. Normalization to electron-impact-ionization cross sections at intermediate energies was used to set the absolute scale since, for helium, convergence to the first Born values had been obtained. This approximation, as described in section 5.2, is independent of the sign of the projectile charge and thus predicts equal cross sections for positrons and electrons. 

Consider the possible effect of an electron impact on an H2 molecule. If the electron has less than 11 volts the only possible transition it may cause is to the 13S state. But applying the Franck-Condon principle we see that the probability of such a transition will be small except for those nuclear separations where the 13S curve nearly coincides with that for the 23S term. In other words we might expect a little but only a little atomic hydrogen to be produced below eleven volts since the 13S state is unstable. 

The translational energy releases reported in the literature for metastable ion decompositions are contained in Tables 1—7. Decompositions of positive ions occurring within an ion source are covered in Table 8 and decompositions of negative ions in an ion source in Table 9. Translational energy releases determined by photoion—photoelectron coincidence (PIPECO) appear, therefore, in Table 8. The results from the extensive series of electron impact (El) measurements [310] at ionizing energies close to threshold appear in Tables 8 and 9. Coverage of dissociations of diatomic ions is not exhaustive. 

The coincidence measurements discussed in the previous section were concerned with the total coincidence signal, i.e. the signal obtained when the decay of a particular ensemble of states is integrated over. These states are produced in a very short time ( 10 s) in electron impact excitation, and can sometimes evolve in a complicated way. In the absence of internal fields (e.g. the n P states of helium) each of the fm) states decays with the same exponential time dependence exp(—yt), and the coincidence technique can be used to yield the decay constant y of the excited state (see Imhof and Read, 1977, and references therein). However, if the excited state is perturbed by an internal (or external) field before decay, then the exponential decay is modulated sinusoidally giving rise to the phenomenon of quantum beats (Blum, 1981). 

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. 

Several groups have studied the transitions in chemical bonding for free Hg clusters. Cabauld et al. measured ionisation potentials by electron-impact ionisation for n<13 Rademann et a/. used a photoionisation and photoelectron coincidence technique to obtain ionisation potentials up to... 

Refinements in vuv spectroscopy W, aided by the development of synchrotron radiation (7 ) and equivalent-photon electron-impact ( ) tunable light sources, and closely related advances in photoelectron, fluorescence-yield, and electron-ion coincidence spectroscopy measurements of partial cross sections (9), have provided the complete spectral distributions of dipole intensities in many stable diatomic and polyatomic compounds. Of particular importance is the experimental separation of total absorption and ionization cross sections into underlying individual channel contributions over very broad ranges of incident photon energies. 

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