Near Threshold Structure in the Atomic K-Shell Spectra for Ionization by Photons or Fast Charged Particles

Abstract

The differential optical oscillator strength df/dε for ionization, which results in an ejected electron of kinetic energy ε, is proportional to the photoionization cross section.¹ It has been known for some time^2–4 that df/dε does not necessarily decrease monotonically with ε from the threshold for outer atomic subshells. This is also true for the generalized oscillator strength df(K,ε)/dε for ionization at momentum transfer ℏK, when K is small. The quantity df(K,ε)/dε is the essential factor in the Born cross section for secondary-electron ejection by charged particles,⁵ and it tends to df/dε in the optical limit K → 0. The non-monotonic ε-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.