In electric fields 0 F 160 kV cm −1 , shift and splitting of the λ(1s2s 3 S-1s3p 3 P) = 389 nm line of He I were measured spectroscopically by crossing a beam of metastable He atoms with a frequency-doubled cw Ti:sapphire laser beam. We deduced the difference α sc = α sc (3 3 P) − α sc (2 3 S) = 4.20(3) MHz (kV cm −1 ) −2 of the scalar polarizabilities, the tensor polarizability α ten (3 3 P) = 84.0(7) kHz (kV cm −1 ) −2 and the energy separation of closest approach hν min = E(3 3 P 0 , 0 − ) − E(3 3 P 2 , 0 − ) = 8257(5) MHz • h of the Stark sublevels with m = 0 and negative reflection parity at F = 154(3) kV cm −1 . The polarizabilities agree with calculated values. By comparing the measured energy separation with theory, we deduced that the scalar polarizabilities of the 3 P 2 and 3 P 0 differ slightly by {α sc ( 3 P 2 ) − α sc ( 3 P 0 )}/α sc ( 3 P) = (1.8 ± 1.2) × 10 −4 .

By use of an rf electric resonance technique, the fine structure of the \s4f multiplet of He I and its splitting in a static electric field have been investigated. Our results for the fine-structure intervals v u between the l F 3 level and the 3 Fj levels are v l2 =232.8(2.0) MHz, v u =490.8(2.0) MHz, and vi3=704.3(l.0) MHz. Furthermore, a singlet-triplet mixing ratio 0=0.757(3) and the exchange interaction 2^ = 158(3) MHz have been derived from the measurements.

The fine structure of the 1 s 5f and 1 s 5 g levels of He I was measured using microwave spectroscopy. The helium atoms were excited by ion impact, and the eleven allowed 1 s 5f 2s+ 1F j_ 1 s 5 g 2s,+ 1G j, transitions near v,~ 15 GHz were induced and detected by measuring the 1 s 4d-1 s 2p or 1 s 3d-1 s 2p spectral-line intensities of the impact radiation as a function of the microwave frequency. The measured transition frequencies are in accord with theoretical values and, except for one transition frequency, with earlier experimental data. The existing discrepancy between these earlier data and theory could be solved.

Abstract. Helium atoms were excited by hydrogen-ion impact, and electric dipole transitions between Stark substates of the 1 s 4f multiplet of HeI were induced. Resonance signals were investigated at various frequencies 200 MHz < v < 800 MHz by scanning a static electric field F o < 1 kV/cm and observing the intensity of the impact radiation for the singlet or triplet I s 3 d--1 s 2p spectral line. From these measurements the following zerofield transition frequencies of the 1 s 4f fine structure were deduced: =490.6(O.4)MHz. Using calculated parameters of the magnetic fine-structure coupling, the exchange integral 2K= 158.0(0.6) MHz of the 1 s 4f configuration was evaluated.

Making use of the large tensor polarizabilities of excited He I levels, transitions between Zeeman substates of 1 snd 1D 2 levels have been induced by electric radiofrequency fields after ion-impact excitation of He atoms. Resonance signals have been observed near the Larmor frequency co=coL as well as near co = 2co L. These signals are interpreted as 2-quantum electric-dipole (2El) transitions and 1El transitions, respectively, between the Zeeman sublevels with M=0 and M= _+2. The interaction of this 3-state system {IM); M=-2,0, +2} with the applied external fields (magnetic field H~ and static and alternating electric field gx=g0+gx coscot) is discussed. The shape of the resonance signals has been evaluated in the rotating field approximation. In contradiction to the theory, the observed signals show a peculiar narrow structure at the center of the signal which is not yet completely understood. The experimental techniques developed are expected to be useful for investigations of Zeeman-, Stark-, fine and hyperfine splittings of excited He I levels.