Bifurcations of closed orbits in singlet helium Stark spectra

We measured constant-scaled-energy M=0 spectra of helium Rydberg atoms (n=5 5 to 80) in an electric field, allowing for a test of closed-orbit theory u nder conditions where bifurcations from the uphill and downhill orbits are important. Recent advances in the theory (uniform approximation) were imple mented for a comparison with experimental data. The observed recurrence spe ctra could be explained in detail as fur as the actions of all hydrogenic o rbits and the intensities of most recurrences are concerned. Additional pea ks in the recurrence spectra that correspond to orbits with an action equal to the sum of two hydrogenic orbits were attributed to scattering at the H e+ core. This was confirmed in a closed-orbit calculation including the qua ntum defects in helium. Small changes in the recurrence strength of the hyd rogenic orbits were found both in experimental recurrence spectra and in th e closed-orbit calculation for helium, and could be ascribed to the effect of cure shadowing. [S1050-2947(99)07104-8].