We present scaled-energy spectra on helium Rydberg atoms in a static electric field. ͉M ͉ϭ1 states were studied in excitation from the 2 1 S 0 metastable state. Spectra were recorded for ⑀ϭϪ2.940(4), ⑀ϭ Ϫ2.350(4), both below the saddle point, and ⑀ϭϪ1.760(4), above the saddle point. Closed-orbit theory was applied to interpret the spectra. A recent extension to closed-orbit theory, incorporating core effects, was used. This significantly improved agreement between experiment and theory. ͓S1050-2947͑98͒02610-9͔

Spectra of Ba autoionizing Rydberg states (n = 60-80) in a varying external electric field, keeping the classical scaled energy ε constant, have been measured. Series converging to the 5d 3/2-as well as to the 5d 5/2-limit have been studied for three values of the scaled energy: ε = −2.94, −2.35 and −1.76, deep in the n-mixing regime around the classical field ionization limit (ε = −2.0). The experiments are interpreted using semiclassical closed-orbit theory by taking the Fourier transform of the experimental spectra. High resolution is obtained using CW laser excitation and large frequency scans. Effects of the large non-hydrogenic core of Ba are investigated by comparing the experimental Fourier spectra with hydrogenic closed-orbit theory for m l = 0-3 incorporating the 5d 2 1 G 4 metastable ground-state wavefunction and assuming only 5d-nf excitation.

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High-resolution scaled-energy spectra of barium Rydberg atoms in a static electric field are studied. Starting from the 6s5d 3 D3 metastable state we investigate Rydberg series converging to the 6s1/2 ionization limit. Spectra at scaled energies = -2.94, -2.35 and -1.76 are recorded and Fourier transformed. Hydrogenic closed-orbit theory, including recent ml = 0 corrections for actions around a bifurcation, reproduces the positions of the peaks but does not reproduce the intensities. A recent extension of closed-orbit theory, incorporating core scattering for nonhydrogenic atoms, is applied. In spite of the success of this technique in analysing scaled-energy spectra for light atoms with one (small) quantum defect the measured intensities in the barium case cannot be reproduced with this method.

From the results of Stark measurements on the (NH3)2 van der Waals complex formed in a molecular jet expansion, it was possible to determine the electric dipole moment for the G: ‖K‖=1 state. The partially quenched inversion in the complex gives rise to quadratic Stark effect. We find an electric dipole moment of ‖μ‖=(0.10±0.01) D in the ground state and an upper limit of 0.09 D for the dipole moment in the excited state. These small values give evidence that for the G: ‖K‖=1–1 states, the ‘‘antiparallel’’ (cyclic) structure is more likely than the hydrogen bonded one.