Cross section ionisation

Electron correlations show up in two ways in the measured cross sections. If the initial target state is well described by the independent particle Hartree-Fock approximation, the experimental orbital (6) is the Hartree-Fock orbital. Correlations in the ion can then lead to many transitions for ionisation from this orbital, rather than the expected single transition, the intensities of the lines being proportional to the spectroscopic factors S K... 

A common method of calculating the approximate photo-ionisation cross-sections is to use the Gelius intensity model [79]. Here the cross-section for a particular orbital, , is expressed in terms of atomic contributions based on population analysis. 

This compound also possesses a comparatively large ionisation potential (15.3 eV)163,164, and one of the largest known cross-sections for the capture of thermal electrons. The latter process has been studied in considerable detail by beam, swarm and microwave techniques104 165-170. The initial attachment gives rise to a vibrationally excited ion169,17°, viz. 

The reproducibility of the results from the standards is clear from the narrow spread of intensity ratios in Table 2. Readers are reminded, however, that the k-values will vary somewhat from one instrument to another because they are dependent not only upon the appropriate ionisation cross sections and fluorescence yields, but also upon the efficiency of the detection system. Values that are tabulated in commercial software packages should be used with caution. 

Fig. 15.22 On-line REMPI-TOFMS (at 266 nm) analysis of roast gas while roasting 80 g Ara-bica coffee, a The full-time-mass-intensity three-dimensional plot as recorded during roasting, b A time-intensity cross-section from a at a fixed time (medium roast level). The three phenolic VOCs, phenol (m/z 94), giraiacol (m/z 124) and 4-vinylguaiacol (150 m/z), are efficiently ionised at 266 nm. In addition, firrfurylacohol (m/z 96), dihydroxybenzene (m/z 110), indol (m/z 117) and caffeine (m/z 194) were also detected. (Adapted from [203])...

Basu, M., Mazumdar, P.S. and Ghosh, A.S. (1985). Ionisation cross sections in positron-helium scattering. J. Phys. B At. Mol. Phys. 18 369-377. 

Brauner, M., Briggs, J.S. and Klar, H. (1989). Triply-differential cross sections for ionisation of hydrogen atoms by electrons and positrons. J. Phys. B At. Mol. Opt. Phys. 22 2265-2287. 

When studying the absorption of increasing photon energy by an atom or ion initially in a given bound state, to be gradually excited until it becomes ionised, and to have afterwards the free electron increase its kinetic energy, there is no discontinuity in the oscillator strength spectral density at the ionisation threshold. An adequate theoretical calculation must reproduce such continuity, which may also be exploited to interpolate a value for the threshold photoionisation cross section. 

In Figure 2, correlation coefficients for each kxS plot are all better than 0.98, and, with few exceptions, the standard deviation for each data point (determined as an average of at least 15 individual analyses) is equivalent to the size of the symbol. For each element, the kxSi value is close to a theoretically determined value using reasonable estimates of ionisation cross-sections [32] ... 

If N electrons pass from cathode to anode (path length, /) through a gas of particle number density (n) and collision cross-section a (dependent on the type of gas and the electron energy), some of the particles will be ionised. 

Photocurrent spectra of a p-type GaN epilayer have also been measured [7], From the onset of photoconductivity spectra, it was suggested that metastable centres at 1.1, 1.40, and 2.04 eV above the valence bandedge were responsible for the PPC in Mg-doped GaN, and that Ga vacancies may be responsible for PPC in n-type GaN [7], The spectral dependence of the optical cross section of the PPC related impurities in n-type epilayers has also been measured and the results are shown in FIGURE 4, from which an optical ionisation energy of about 2.7 eV was obtained [8], The correlation between the... 

FIGURE 4 The relative optical absorption cross section as a function of photon energy, hv. The solid curve corresponds to the fitting result with o0p, oc (hv - Eopt) 5/(hv)3, from which one obtains the optical ionisation energy Eopt = 2.69 eV. After [8],... 

CH3C1 (90, 34 and 8 A2, respectively). These values suggest a similarity between these reactions and the corresponding ground state alkali atom reactions. The ionisation potential of Hg (3P2) is 4.974eV which is similar to those for the alkali atoms and so an electron jump mechanism is proposed for these chemiluminescent reactions of Hg (3P2 ) In contrast, the reaction of another spin-orbit state of metastable mercury with bromine, Hg (3P0) + Br2, has a much smaller chemiluminescent cross section [3 A2 compared with 150 A2 for Hg (3P2) + Br2] [406], which cannot be reconciled with an electron jump, suggesting the existence of a barrier to reaction of Hg (3P0) which is not present in the case of Hg(3P2). 

Our intention is not to calculate absolute values for photo-ionisation cross-sections, but rather to relate the cross-sections for the different states which may result from ionising the same orbital shell r. Thus we require a formula of the kind ... 

The following tables have been compiled using the formulae given in Sections 3 and 4. The numbers given are in fact the Z coefficients defined in Eq. (4), and are thus the intensities expressed in units of the one-electron cross-section for the orbital which is ionised. Cases where repeated states can arise, as discussed above, are marked with an asterix ( ) in these cases the total intensity given in the table may be distributed among two or more states. 

Inelastic collisions between electrons and atoms or molecules are of particular interest as they lead to excitation and ionisation states, which often induce dissociation of the molecule or enhance chemical reactivity. The general behaviour for electron energy of the electron-atom or molecule dissociative excitation or ionisation cross section is given by ... 

CT is a technique that uses ionising radiation and computer processing to generate cross-sectional three-dimensional images of the internal organs. In assessment of the hepatobiliary tract both an oral contrast agent and an intravenous contrast medium are required to visualise the bowel and blood vessels, respectively. In the context of hepatobiliary disease, CT is particularly useful in assessing the extent of mass lesions. 

Integral cross sections for selected electron-impact excitation and ionisation processes have been largely obtained by measuring optical excitation functions. These need to be corrected to a varying degree of accuracy for effects such as cascade contributions and photon polarisation. The details of the experimental procedures, sources of errors and data evaluation have been discussed by Heddle and Keesing (1968). 

If the excitation is to the continuum, then a number of multidifferential cross sections are possible. Consider, for instance, the single ionisation of an atom A ... 

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