Application of the DM formalism to the calculation of electron-impact ionization cross sections of alkali atoms

On the basis of the results of recent calculations of K-shell ionization cross sections (removal of a 1s electron) for 11 atoms [Deutsch et al., Int. J. Mass Spectrom. 177 (1998) 47] using the semiclassical DM formalism, we revisited our earlier calculations of electron-impact ionization cross sections of the alkali atoms Li, Na, K, Rb, and Cs whose ionization at low energies is also dominated by the removal of the lone ns electron (n= 2-6 for Li-Cs). We investigated the effect of (1) a slightly revised energy dependence in the low-energy region similar to the one developed for the K-shell ionization cross sections, and (2) a slightly revised set of empirically determined weighting factors on the overall cross section shape. A detailed comparison with available experimental data and with other calculated cross sections reveals that the revised cross section shape yields better agreement with experiment and with other calculations. Moreover, in accordance with predictions from Vallance and Harland [Int. J. Mass. Spectrom. Ion Processes 171 (1997) 173], we find a linear relationship between the presently calculated maximum cross section value σ_max and the polarizability volume α. This can be viewed as an independent justification of the reliability of the DM calculation for the alkali ionization cross sections and in particular for the use of the revised parameters.