Integrated channel cross sections hydrogen

The difficulties associated with making an atomic-hydrogen target have precluded direct measurements of total cross sections for hydrogen. Estimates may be made by adding the best available estimates for the integrated cross sections of particular channels and the total ionisation cross section. 

To estimate the total cross section for hydrogen we use the n=l and 2 estimates above. For n=3 we interpolate in the direct measurements of integrated cross sections by Mahan, Gallagher and Smith (1976). For higher discrete channels we use the roughly-valid rule that integrated cross sections for principal quantum number n are proportional to n. Very accurate measurements of the total ionisation cross section have been made by Shah et al. (1987). These total cross section estimates are shown in table 8.3 in comparison with the three calculations we are considering. 

Table 8.2. Integrated cross sections (nal) for the Is, 2s and 2p states of hydrogen. Experimental values (EXP) are obtained as described in the text. Calculations are CCO, coupled channels optical (Bray et al., 1991c) CCC, convergent close coupling (Bray and Stelbovics, 1992b) vWW, pseudostate method (van Wyngaarden and Walters, 1986)...

Fig. 8.9. Integrated cross section for electron scattering to the 2s (above) and 2p (below) states of hydrogen below the n=3 threshold. The positions and quantum numbers of resonances are shown on the upper scale. Experiment, Williams (1988) solid curve, coupled channels optical (equivalent local) (McCarthy and Shang, 1992) long-dashed curve, pseudostate method (Callaway, 1982) short-dashed curve, 9-state coupled channels.

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