WAVELENGTH DEPENDENCE OF THE NONRESONANT PHOTOIONIZATION CROSS-SECTION OF A 2-ELECTRON ATOM NEAR THE IONIZATION THRESHOLD

We present the theoretical near-threshold photoionization cross sectio ns of He atoms from bound excited states, using a B-spline-based confi guration-interaction method. Our study has shown that in the absence o f a strongly energy-dependent feature such as a doubly excited resonan ce, the photoionization cross sections in the immediate vicinity of th e ionization threshold depend linearly on the wavelengths of the incid ent light. Qualitatively, following the quantum-defect theory, this li near dependence can be linked directly to the expected nearly energy-i ndependent scattering phase shift corresponding to the wave function o f the ionized electron at near-zero photoelectron energy.