SEMICLASSICAL FORMULA FOR OSCILLATOR-STRENGTHS OF ATOMIC SPECTRA IN EXTERNAL FIELDS

In a previous Rapid Communication [Phys. Rev. A 56, R5 (1997)] we repo rted a semiclassical formula describing oscillator strengths for trans itions to a high Rydberg state of a hydrogen atom in an electric field . The formula relates the oscillator strength for an atomic transition into a Rydberg state to the angular distribution of outgoing electron waves and to the density of classical tori. Here we give a derivation of that formula and we suggest some generalizations. We compare the f ormula with experimental measurements of the absorption spectrum of hy drogen in an electric field. This absorption spectrum consists of quas idiscrete resonances superposed on a smooth background. We find that t his background has a hitherto unexplained structure that was visible b ut not recognized in the experiment. The background absorption is rela ted to electron orbits that directly escape from the atom, and its str ucture is related to the angular distribution of outgoing electrons.