title: Scaling laws for the photoionization cross section of two-electron atoms.
abstract: The cross sections for single-electron photoionization in two-electron atoms show fluctuations which decrease in amplitude when approaching the double-ionization threshold. Based on semiclassical closed orbit theory, we show that the algebraic decay of the fluctuations can be characterized in terms of a threshold law sigma proportional to |E|(mu) as E --> 0(-) with exponent mu obtained as a combination of stability exponents of the triple-collision singularity. It differs from Wannier's exponent dominating double-ionization processes. The details of the fluctuations are linked to a set of infinitely unstable classical orbits starting and ending in the nonregularizable triple collision. The findings are compared with quantum calculations for a model system, namely, collinear helium.
authors.affiliation: School of Natural Science, Kumoh National Institute of Technology, Kumi, Kyungbook 730-701, Korea.
authors.forename: Chang Woo
authors.initials: CW
authors.lastname: Byun
authors.forename: Nark Nyul
authors.initials: NN
authors.lastname: Choi
authors.forename: Min-Ho
authors.initials: MH
authors.lastname: Lee
authors.forename: Gregor
authors.initials: G
authors.lastname: Tanner