Multiple ionization of helium and krypton by electron impact close to threshold: appearance energies and Wannier exponents

We determined appearance energy (AE) values AE(X n + /X) for the formation of singly (He + ) and doubly charged (He 2+ ) He ions and multiply charged Kr ions Kr n + up to n = 6 following electron impact on He and Kr atoms using a high-resolution electron impact ionization mass spectrometer. The data analysis employs an iterative, non-linear least-squares fitting routine, the Marquart–Levenberg algorithm, in conjunction with either a 2-function or a 3-function fit based on a power threshold law. This allows us to extract the relevant AEs and also the corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and with spectroscopic AE values and the extracted exponents p are compared with other available experimental data and with the predictions of the various Wannier-type power law models. One observation is particularly noteworthy, namely the fact that none of the available experimental data seem to support the large values of ' p ' predicted by the Wannier–Geltman and the generalized Wannier law for n > 3.