Electron coincidence technique

The first arrangement uses photon—electron coincidence techniques. 

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. ... 

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... 

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 5. Comparison of measured values of Sorg between the ASTM and electron-optical techniques. Not all 56 points can be differentiated because some are coincident...

Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

The positrons that arrive at the formation foil share the time structure of the electron accelerator, giving 2 ps long pulses of about 104 slow positrons at 600 Hz. Since a 7-ray detector would be saturated, the coincidence technique cannot be used, giving an order of magnitude worse signal-to-noise ratio than that in the previous experiments (due to 7 scintillations in the Lyman-a photo-multiplier), but the higher data rate more than compensates for this in total time to reach a given precision. 

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... 

Recently in our laboratory we have initiated a program to study the photoabsorption processes of metal vapors throughout the UV and EUV region. Our research interests are (1) to obtain the absolute cross section measurement of atomic and molecular metal vapors, (2) to study the photoionization processes of molecular metal species, and (3) to study the photodissociation processes of molecular metal ions. Several experimental methods such as the heat-pipe absorption spectroscopy, photoionization mass spectroscopy, and electron-ion coincidence technique, will be used in the study. This report summarizes our first experiment using heat-pipe absorption spectroscopy. 

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. 

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). 

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... 

More recent experiments by Walker et al. [37] using linearly and circularly polarised light and ion TOP spectroscopy confirmed the existence of the knee but, more importantly, electron-ion and electron-electron coincidence experiments were performed. Using the electron-ion coincidence technique, it was found that the great majority of the Xe ions were produced in a sequential process. Moreover, direct double ionization was not observed in these electron-electron coincidence experiments. Charalambidis et al. [38] considered this long-standing question of direct double ionization in Xe and also concluded that it does not occur. That does not mean that correlated double emission cannot occur under appropriate conditions. 

The sensitivity of fragmentation to the nature of resonances was shown for tetramethylsilane by using the photoion photoion coincidence technique (Morin et al. 1986). In this technique, fragmentation channels are identified by the coincidences of pairs of ions in the time-of-flight mass spectrometer. For tetramethylsilane, two resonances were observed in the ion pair yield curves. The lower energy resonance, below the ionization edge, had been shown from electron spectra to decay into a one-hole state, but the production... 

Dupre C, Lahmam-Bennani A and Duguet A 1991 About some experimental aspects of double and triple coincidence techniques to study electron Impact double Ionizing processes Meas. Sci. Technol. 2 327... 

Another coincidence technique involves the use of a time-to-amplitude converter (TAC). A TAC is an electronic unit that converts the time difference between two pulses into a voltage pulse between 0 and 10 V. The height of the pulse is proportional to the time difference between the two events. The time spectrum of the two detectors is stored directly in the MCA. A time window is set around the coincidence peak (Fig. 10.30). A second window of equal width is set outside the peak to record accidental coincidences only. 

For ionization by cw lasers, quadrupole mass spectrometers are generally used. Their disadvantage is the lower transmittance and the fact that different masses cannot be recorded simultaneously but only sequentially. At sufficiently low ion rates, delayed coincidence techniques in combination with time-of-flight spectrometers can even be utilized for cw ionization if both the photoion and the corresponding photoelectron are detected. The detected electron provides the zero point of the time scale and the ions with different masses are separated by their differences Afa = ion — tQ in arrival times at the ion detector. 

The photoionization process is accompanied by the emission of electrons. The technique of photoelectron spectroscopy is based on a measurement of the kinetic energies of those electrons, either independently of the ion formation process or in coincidence with it. In photoelectron spectroscopy, the precursor atom or molecule is irradiated with light of a known energy sufficient to eject an electron from the material through the process... 

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