Coincidence measurements electron-photon

Smith, A.J., Read, F.H. and Imhof, R.E. (1975). Measurement of the lifetimes of ionic excited states using the inelastic electron-photon delayed coincidence technique. J. Phys. B At. Mol. Phys. 8 2869-2879. 

Figure 4.48 Typical spectrum of electron-electron coincidences recorded with a TDC. The data refer to a situation in which the photon beam has no time structure. True coincidences are collected in the peak while accidental coincidences give a flat and smooth background. At indicates the coincidence resolving time and dt the time resolution of the time-measuring device. The two shaded areas represent accidental coincidences, measured on the left-hand side together with the desired true coincidences, but on the right-hand side separately (and simultaneously) in the full time spectrum.

For a correct analysis of photoionization processes studied by electron spectrometry, convolution procedures are essential because of the combined influence of several distinct energy distribution functions which enter the response signal of the electron spectrometer. In the following such a convolution procedure will be formulated for the general case of photon-induced two-electron emission needed for electron-electron coincidence measurements. As a special application, the convolution results for the non-coincident observation of photoelectrons or Auger electrons, and for photoelectrons in coincidence with subsequent Auger electrons are worked out. Finally, the convolutions of two Gaussian and of two Lorentzian functions are treated. 

If there is more than one particle emitted in the scattering process, and one or more of these (electron, photon, etc.) are detected in coincidence with the scattered electron, then the cross sections are also differential with respect to the energy and angular distributions of these secondary particles. Such measurements are called correlation measurements, since the cross section depends on the correlation between say the angles (or energies, or spin directions) of the final particles. 

The coincidence technique has been discussed in detail in section 2.4.4 and much of that discussion is valid for electron—photon coincidence measurements. The coincidence technique offers the important advantage of eliminating photon contributions from excited atoms produced by cascading rather than by direct excitation. This depends on the band pass of either the photon detector or electron detector being sufficiently narrow to isolate the excited state being studied. 

Quantum beats have been observed in a variety of experiments, particularly in beam—foil measurements. Teubner et al. (1981) were the first to observe quantum beats in electron—photon coincidence measurements, using sodium as a target. The zero-field quantum beats observed by them are due to the hyperfine structure associated with the 3 Pii2 excited state (see fig. 2.20). The coincidence decay curve showed a beat pattern... 

An interesting and alternative technique to electron—photon coincidence measurement of coherence effects in excitation processes is superelastic scattering from laser-excited targets. This technique, first developed by Hertel and Stoll (1974, 1978), can be thought of as the time inverse of the (e,e y) coincidence experiment. 

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. 

Brion94 and of Hamnett et al.m (2) the PES experiments of Samson and Gardner172 and Plummer et al.18 (3) the photofluorescence experiments of Judge and Lee189 and the recent extended measurements by Lee et al.190 and (4) a different type of measurement, the triple coincidence experiment by Backx et al.191 In this work coincidences were recorded between electrons of variable energy loss, CO+ ions, and photons from the process CO+(B22- A 22). The results of the various experiments shown in Fig. 39 indicate excellent quantitative agreement between the five different electron-spectroscopic methods. However, the photofluorescence measurements... 

The prerequisite for such a measurement is information about the starting time. This can be provided by the pulsed time structure of the primary photon beam (single-bunch mode for operating the electron storage ring), or by a reaction product of the photoprocess which is in coincidence with the emitted electron and can be another electron or a photon. More details on time-of-flight electron spectrometry are given in Section 10.1. 

If the accidental coincidences are calculated using equ. (4.103), ANacc- becomes negligible, because Iy and I2 are usually large numbers. However, due to the decreasing photon flux at an electron storage ring this equation would have to be applied at every instant of time. Hence, it is preferable to measure the accidental coincidences and desired total coincidences simultaneously.) With equ. (4.112)... 

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