Total electron yield signal

Surface EXAFS (SEXAFS) uses Auger or photo-electrons to detect the EXAFS signal. This ensures that this technique has a much higher surface sensitivity than EXAFS acquired using the total electron yield method. SEXAFS requires ultra-high vacuum and the detection instrumentation normally associated with Auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS) techniques. 

Alternatively, the total electron yield from the sample due to cascades initiated by the Auger processes, can be detected (Citrin, 1978). The signal is measured with a charmel-tron detector, or simply by measuring the photoionization current from the sample. This method has the peculiarity of probing a few thousand A under the sample surface (due to the limited electron escape depth) and can be useful in studying macroscopically layered structures, e.g. ion-implanted materials. 

Three types of measurements were performed in this study. First, photodissociation cross sections were measured, in which the total photofragment yield was measured as a function of dissociation photon energy. In these experiments, the electron signal generated by the microchannel plates is collected with a flat metal anode, so that only the total charge per laser pulse is measured. The beam block is 3 mm wide for these measurements. 

In the case of AEAPS, it is not necessary to detect individual Auger peaks, as the total secondary electron yield can be measured instead. The electron cascade within the material can act as an electron multiplier increasing the AEAPS signal. Hence an REA could be used or an electron detector of a type used in a scanning electron microscope. 

Yet further oxidation removes at least one more electron from each P cluster with an +90 mV to yield a protein oxidized by a total of at least eight electrons and with EPR signals from mixed spin states of S = I and S = I (42, 47). The combined integrations of these signals demonstrated that their intensity was equivalent to that of the FeMoco EPR signals in the same preparations. This provided the first evidence (47) that MoFe proteins contained equivalent numbers of FeMoco centers and P clusters and that P clusters contained 8 Fe atoms. Previously it had been considered that the P clusters were fully reduced Fe4S4 clusters and thus that there were two P clusters for every FeMoco center per molecule. 

In principle, the neutral desorbed products of dissociation can be detected and mass analyzed, if ionized prior to their introduction into the mass spectrometer. However, such experiments are difficult due to low ejfective ionization efficiencies for desorbed neutrals. Nevertheless, a number of systems have been studied in the groups of Wurm et al. [45], Kimmel et al. [46,47], and Harries et al. [48], for example. In our laboratory, studies of neutral particle desorption have been concentrated on self-assembled monolayer targets at room temperature [27,28]. Under certain circumstances, neutrals desorbed in electronically excited metastable states of sufficient energy can be detected by their de-excitation at the surface of a large-area microchannel plate/detector assembly [49]. Separation of the BSD signal of metastables from UV luminescence can be effected by time of flight analysis [49] however, when the photon signal is small relative to the metastable yield, such discrimination is unnecessary and only the total yield of neutral particles (NP) needs to be measured. 

Assuming that the detector signals are proportional to the masses of the products, the yield of DFBP is proportional to the absorbed dose however, the yields of the major products are less than proportional above 10-15 kGy. Some of the minor products show yields that are more than proportional at this range, and become more prominent at larger doses. If the detector sensitivity is assumed to be constant per monomer unit, i.e. the sensitivity of the tetramers is twice that of the dimer and 4/3 times that of the trimer, then the yield of all the products can be calculated. This total yield was found to be linear with the dose for the whole range studied (up to 25 kGy). Absolute yield can be measured only for DFBP, for which a standard exists, and it is 0.0465 molecule/100 eV. Measuring the formation of total polymer by the total ion current in the GC/MS gave a total yield of 1.7 molecules/100 eV, and similar yields are obtained by the electron capture detector. 

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). 

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