Near-threshold structure in the atomic K-shell spectra for ionisation by photons or fast charged particles

Abstract

The differential optical oscillator strength df/de for ionization, which results in an ejected electron of kinetic energy e, is proportional to the photoionization cross section.1 It has been known for some time2–4 that df/de does not necessarily decrease monotonically with e from the threshold for outer atomic subshells. This is also true for the generalized oscillator strength df(K,e)/de for ionization at momentum transfer ℏK, when K is small. The quantity df(K,e)/de is the essential factor in the Born cross section for secondary-electron ejection by charged particles,5 and it tends to df/de in the optical limit K → 0. The non-monotonic e-dependence, or structure in the spectral strength, is related primarily to angular momentum barriers to the escape of electrons near the threshold.1,6 These barriers are known to be especially important for continuum waves with angular momentum l ≥ 2 near the outer edge of the atom. Thus, it has been thought that the K-shell ionization should be unaffected by the barrier effect because the K-electron wave functions are quite compact and because the final state is limited to p waves only, by the dipole selection rule.