Translational energy analysis

Using a guided ion beam instrument the translational energy dependent reaction cross sections of endothemiic fragmentation processes can be detemiined [32]. Modelling these cross sections ultimately yields their energy tln-esholds and a great deal of valuable themiochemical infomiation has been derived with this teclmique. Precision of 0.2 eV can be obtained for reaction tln-esholds. Bimolecular reactions can also be studied and reaction enthalpies derived from the analysis of the cross section data. 

Photofragment coincidence data were taken at several of the peaks in Fig. 1. Mass analysis of the fragments showed that only coincidences corresponding to channel (1), CH3 + CO (or CD3 + CO), were seen at all dissociation wavelengths examined. As discussed previously, the time and position data yield a coupled translational energy and angular distribution P(ET, 0), which can be written as... 

The photolysis of aromatic species with tetranitromethane in perfluoro alcohol solvent has been studied, in which the radical cations were observed by EPR spectroscopy.284 Photo-stimulated reaction of 1- and 2-haloadamantanes and 1,2- and 1,3-dihaloadamantanes with various carbanionic nucleophiles afforded products rationalized through an SrnI mechanism.285 286 Photolysis of the cycloadduct formed between a functionalized derivative of C6o and diazomethane has been shown to afford a pah of ling-opened structures (125) and (126) via a proposed biradical intermediate (127) (Scheme 19). The UV-photolytic fragments of /-butyl iodide (T and /-Bu ) have been ionized by resonance-enhanced multiphoton ionization for TOF mass spectro-metric analysis.287 A two-dimensional position-sensitive detector provided angular distribution and translational energy data. 

Product state analysis offers a flexible way to obtain detailed state resolved information on simple surface reactions and to explore how their dynamics differ from the behaviour observed for H2 desorption [7]. In this chapter, we will discuss some simple surface reactions for which detailed product state distributions are available. We will concentrate on N2 formation in systems where the product desorbs back into the gas phase promptly carrying information about the dynamics of reaction. Different experimental techniques are discussed, emphasising those which give fully quantum state resolved translational energy distributions. The use of detailed balance to relate recombinative desorption measurements to the reverse, dissociation process is outlined and the influence of the surface temperature on the product state distributions discussed. Simple low dimensional models which provide a reference point for discussing the product energy disposal are described and then results for some surface reactions which form N2 are discussed in detail, emphasising differences with the behaviour of H2. 

The retardation and deflection techniques represent the simplest methods of analysis using electrostatic fields. More sophisticated methods of electrostatic analysis are, of course, available and both cylindrical and hemispherical electrostatic analysers have been used to measure translational energies of ions formed by decomposition within ion sources [633, 667, 789, 790]. A Wien filter has been similarly employed [163]. 

Mass spectroscopy is a common detector in crossed-molecular beam experiments and has also been used in flow systems [17]. Measurements are normally restricted to an analysis of the identity of the reaction products and are not sensitive to the internal energy states of the products. The determination of product translational energy distributions in crossed-molecular beam experiments is usually achieved by placing some form of velocity analyser before the mass spectrometer (see below). 

For the reactions K + HBr and K + DBr, the KBr recoil energy distribution has been determined in a crossed-molecular beam experiment using a mechanical velocity selector. No difference was found in the form of the translational energy distributions for the two reactions for which a value of 0.30 may be derived. Although all the angular momentum appears in the product rotation, the moments of inertia for the alkali halides are large, which implies that the mean product rotational energy is quite small ( 0.21, 0.21 and 0.09 for K, Rb, Cs + HBr, respectively [3] these values are derived from the rotational temperatures obtained by electric deflection analysis). 

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