Beam flux

Fig. 11.6. Diagram depicting desorption ionization (MALDI, FAB or SIMS). The operating principles of the three techniques are similar. The initiating event is exposure of the analyte to a beam of photons, atoms or ions. In order to prevent damage to the fragile analyte molecules and enhance the conversion of the involatile molecules into gas-phase ions, a matrix is employed. For MALDI, the matrix compounds are UV absorbing compounds such as hydroxycinnamic acid. The most commonly used FAB matrix was glycerol and ammonium chloride was employed by some investigators in SIMS experiments (although at low ion beam fluxes molecular species could be effectively ionized for many analytes with minimal evidence of damage by the primary ion beam).

A schematic phase diagram of MBE growth is depicted in fig. 4 (Ohno 1998 Shen et al. 1999). Recently it was shown that metallic (Ga,Mn)As with x = 0.1 can be obtained by the use of a modified MBE growth technique at 7s = 150°C, migration-enhanced epitaxy (MEE), where the beam fluxes of source materials are precisely controlled (Sadowski et al. 2001a, 2001b). 

Figure 11 Experimental HD formation rates for H atoms incident on D-covered Ni(100) surfaces. The different initial D atom coverages are listed in the figure. The metal temperature is 120K. The beam flux, starting at t = 0, is 0.07 ML/s. Taken from Ref. [24].

Information from beam exposures of the sample can be obtained a number of ways. For surface adsorption probabilities greater than 0.03, the King and Wells method [16] can accurately be employed to determine initial adsorption probabilities of a gas species. In short, a King and Wells measurement monitors the partial pressure of the scattering chamber before, during, and after exposure of the sample to the beam flux. A typical reflectivity measurement can be seen in Fig. 2. 

Figure 5 Nucleation and growth kinetics of Pd clusters on MgO(l 00) from a TEAS study, (a) Series of nucleation kinetics curves for various substrate temperatures (atomic beam flux 1.1 x 1013 cm-2 s-2. (b) Arrhenius diagram of the saturation density, (c) Growth kinetics at various substrate temperatures. Atomic beam flux 1.1 x 1013 cm-2 s-2.

Many variants of the experiment described in figure 8.1 have been performed, and we shall encounter some of them later in this chapter. Perhaps the most important variant is that it is often possible to arrange the state selection so that resonant transitions result in an increase in the detected beam flux, ideally against a very low off-resonance background. This is known as the flop-in mode of detection, and it can be very sensitive. Again we shall meet examples of this later. 

A better compromise of intensity versus resolution can be obtained on synchrotron radiation sources since the beam flux is several orders of magnitude higher than with conventional sources, even for very small divergences. This is illustrated in Fig. 18, where satellites very close to a Bragg peak can easily be observed. However, the drawback is that the very small beam divergence leads to a very small diffracting reciprocal space volume thus making some observations more difficult. 

In the simplest measurement, data collected for two energies can provide a measure of the escape depth, the porosity and the open porosity fraction. The values are extracted by comparing the measurements to a set of calibration curves. With a positron beam flux of 0.2 pA, 100 locations on a wafer could be checked in 30 minutes. Mean lifetime measurements can be carried out with a pulsed positron beam. An intermediate time resolution of about 1 ns will be sufficient. A single measurement can be accomplished in about 1 minute. 

To calculate equation (14) correctly, the structure factor magnitudes must be expressed on the absolute scale, that is, relative to the scattering factor of a single electron under the same conditions. In practice, the observed //, and hence Fhki obs values are on an arbitrary scale, depending on the crystal size, primary beam flux, photon-multiplying effect of the counter, and so on, which are in practice impossible to estimate. The scale factor K, required to bring Fhki obs to the absolute scale, can be found by means of a so-called Wilson plot, 2i gives also the overall temperature factor B (see below) of the stmcture... 

Once the absolute cross section at a particular energy has been determined, a knowledge of the energy dependence of the response function C(Ei) and of the electron beam flux may be used to derive absolute cross sections at other energies and for inelastic processes (Williams and Willis, 1975). 

Another commonly-used normalisation procedure is to use the relative flow technique. In this method the elastic differential cross section for a particular species may be obtained by comparing the scattered intensity under the same conditions with that from another target with a known cross section. It is important to ensure, for both the gas under study and the reference gas, that the electron flux density and distribution, the detector efficiency, and the target beam flux distribution are the same for both gases during the measurement. 

The number of detected electrons is proportional to the incident beam flux, which is an accident of the experiment, not a property of the scattering process. The quantity that describes the scattering process is the differential cross section. da number of electrons detected oer unit time... 

Bimetallic clusters can be prepared by the same way in using two metal beams. However, controlling the composition of the clusters is not an easy task, because it depends strongly on the deposition parameters substrate temperature, beam fluxes, and (more annoyingly) the deposition time [37,38]. 

But what is measured in fact in neutron scattering is the differential scattering cross-section, dafcIQ. (q), which is defined as the number of neutrons scattered per second towards a detector in a certain direction per incident beam flux and solid angle. In the case of a liquid or a glass sample for which the average structure is isotropic, only the vector norms (r = r and q= q ) are relevant. 

Similar type experiments for the study of excitation in collision between neutrals have been hampered by the difficulty of detecting the weak beam fluxes which are expected in a differential scattering experiment. 

In addition to direct detection in photon-detected absorption or emission spectra, interference effects may be detected indirectly, for example by monitoring photofragment atoms or ions. Walter, et al., (2000) recorded spectra of the N2 (b Ej c1] caE+) <— a E v" = 0) transition. A molecular beam of the metastable N2 a// LE+ v" = 0) state is excited by a tunable laser to the b c c perturbation complex and transitions are detected by a decrease in the a"1 Eg (v" — 0) beam flux and by the coincidence detection of two N atoms on a time and position sensitive detector. The coincidence detection scheme... 

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