Experimental techniques for cross-section measurements

The same experimental techniques were applied to measure effective cross sections for the electron-induced production of CO from condensed acetone [38], which again was attributed to the formation of TNI and their decay to neutral dissociative states. HREELS measurements have also been used to study electron-induced degradation of cyclopropane [260] and CFI3CI on graphite [261]. 

In evaluating the viability of a new experimental technique for studying ion-molecule reactions, a number of factors must be considered. Ultimately our aims are to measure relative cross sections for reactions as a function of both the internal energy of the ion and the collision energy. It is important that the collision energy can be varied down to = 10 meV where the rotational energy may be comparable with the translational energy. 

Experimental Techniques in Nonlinear Absorption Measurements of TPA yield nonlinear absorption coefficient/TPA cross section (compare Ref. [54] for further details). The nonlinear absorption coefficient [j is related to 8 as [i = 8NAcch x 10 3 (fi in cm/GW, /VA Avogadro s number with 6.02 x 1023 moP1, cch = chromophore concentration in mol/L).11 In general, the following techniques have been applied to quantify the TPA cross section[53, 54], which are complementary methods for determination of either [j or 8. 

Resonance parameters are determined from one or several cross-sectional measurements with high resolution, usually by the so-called area analysis. In the past few years, the experimental techniques to measure neutron cross sections in the resonance region have improved considerably. High-intensity sources for pulsed neutron beams and advances in neutron spectroscopy allow measurements with ever-increasing energy resolution, and thus it has been possible to identify resonance levels and to determine resonance parameters over extended energy ranges and with better accuracy. 

For a more detailed characterization of the IRP-method we will briefly discuss apparatus and experimental technique recently used to measure m- and j-dependent cross sections for the reaction K + HF(v=l,j,m) -> KF + H [7,9], Figure 1 shows a schematic drawing of the crossed molecular beams machine together with the optical set up employed to prepare HF via IRP. 

A tabulation of the ECPSSR cross sections for proton and helium-ion ionization of Kand L levels in atoms can be used for calculations related to PIXE measurements. Some representative X-ray production cross sections, which are the product of the ionization cross sections and the fluorescence yields, are displayed in Figure 1. Although these A shell cross sections have been found to agree with available experimental values within 10%, which is adequate for standardless PKE, the accuracy of the i-shell cross sections is limited mainly by the uncertainties in the various Zrshell fluorescence yields. Knowledge of these yields is necessary to conven X-ray ionization cross sections to production cross sections. Of course, these same uncertainties apply to the EMPA, EDS, and XRF techniques. The Af-shell situation is even more complicated. 

The use of synchrotron radiation overcomes some of the limitations of the conventional technique. The high brilliance of up to 10 ° photons s mm mrad /0.1% bandwidth of energy, and the extremely collimated synchrotron beam lead to a large flux of photons through a very small cross section (0.1-1 mm ). This allows measurements with samples of small volume if isotopi-cally enriched (with the relevant Mossbauer isotope, e.g., Fe). Measurements that were described earlier [4] and that require a polarized Mossbauer source now become experimentally more feasible by making use of the polarization of the synchrotron radiation. Additionally, the energy can be tuned over a wide range. This facilitates measurements with those Mossbauer nuclei for which conventional sources are available but with life times that are too short for most experimental purposes, e.g., 99 min for Co —> Ni and 78 h for Ga —> Zn. 

For the remaining of this chapter we will first describe the basic concept of this new technique, the details of our experimental setup, and the way to invert the measured data directly to the desired center-of-mass differential cross-section. Two types of applications will then be highlighted to illustrate the power of this exceedingly simple technique. We will conclude the chapter by comparing the technique with other contemporary modern techniques. 

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