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Journal of Physics B Atomic Molecular and Optical Physics

Precision measurements of electric-field 1snℓ singlet-triplet anticrossings of He I using radio frequency spectroscopy

O Reusch, C Dieste, Sandy Vanessa Garnica, Gebhard von Oppen

Published May 16, 2001• 4 citations
Publisher

Abstract

The energy separations of closest approach were measured for the 1s n ℓ 1 Λ× 3 Λ anticrossings of singlet and triplet Stark states of the 1s4d, 1s5d and 1s5f configurations of He I. Using a fast atomic He beam, a selective population of the upper state of the anticrossing was accomplished by starting with an initial selective excitation of singlet states by 12 keV He-He collisions followed by an adiabatic transit to the electric field of the anticrossing, where radiofrequency transitions were induced. From the resonance signals the energy separations of three 1s4d, four 1s5d and three 1s5f anticrossings were determined. We evaluated the off-diagonal matrix elements of the spin-orbit coupling for the 1s4ℓ and 1s5ℓ configurations from the measured energy separations. The experimental results are compared with the calculated values.

DOI
10.1088/0953-4075/34/11/307

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The energy separations of closest approach were measured for the 1s n ℓ 1 Λ× 3 Λ anticrossings of singlet and triplet Stark states of the 1s4d, 1s5d and 1s5f configurations of He I. Using a fast atomic He beam, a selective population of the upper state of the anticrossing was accomplished by starting with an initial selective excitation of singlet states by 12 keV He-He collisions followed by an adiabatic transit to the electric field of the anticrossing, where radiofrequency transitions were induced. From the resonance signals the energy separations of three 1s4d, four 1s5d and three 1s5f anticrossings were determined. We evaluated the off-diagonal matrix elements of the spin-orbit coupling for the 1s4ℓ and 1s5ℓ configurations from the measured energy separations. The experimental results are compared with the calculated values.
Singlet-triplet anticrossings of 1s3d Stark sublevels of He I were investigated by inducing resonance transitions from the 1s2p 3 P level to the 1s3d Stark states at electric fields 80 kV cm -1 <F z <100 kV cm -1 . The measurements were performed using a beam of metastable helium atoms perpendicular to a 588 nm laser beam from a tunable dye laser. From the energy separations of closest approach of four anticrossings we deduced the radial integrals h off 3p =(3660+or-200) MHz and h off 3d (652.6+or-1.0) MHz of the singlet-triplet off-diagonal matrix elements of the spin-orbit coupling for the 1s3p and 1s3d configurations. They agree with recently calculated values.
Two independent anticrossings have been observed between singlet and triplet levels in the He atom, one between the 71D7^{1}D71D and 73D7^{3}D73D states, the other between the 81D8^{1}D81D and 83D8^{3}D83D states. The states are excited by electron bombardment. Compensatory changes in the luminosity of the singlet and triplet states are detected as the levels are tuned through the anticrossing points by a magnetic field. The positions of the anticrossings have been analyzed to provide new values for these singlet-triplet separations accurate to \ensuremath{\sim}0.03 GHz. These results show the previous best optical determinations for the separations to be in error by over a GHz. The widths of the anticrossings are analyzed to provide the most direct measures of the spin-orbit coupling constant between a singlet and a triplet state yet obtained for any system. The measured values of these coupling constants are found to be several times larger than their corresponding diagonal value in the 73D7^{3}D73D and 83D8^{3}D83D states.
Anticrossing signals have been detected between n 1 D and n 3 D states of neutral He (n=5 to 7) using external electric field and internal singlet-triplet perturbations to couple the crossing states. Improved results for the n 1 D-n 3 D intervals are obtained: 34066.3(7.2), 20918.0(9.2) and 13632.8(5.3) MHz for n=5, 6, 7 respectively. This type of anticrossing also allows the study of the differential Stark effect, the anisotropic susceptibility chi A and the relativistic fine structure of the n 3 D states.
The method of singlet-triplet level anticrossing has been extended to the n=61,3Dn=6^{1,3}Dn=61,3D levels of He. Measurement of this anticrossing gives a highly accurate determination of the zero-field singlet-triplet separation and a less accurate determination of the antisymmetric part of the spin-orbit coupling perturbation between the n=61,3Dn=6^{1,3}Dn=61,3D states. The zero-field separation for n=6n=6n=6, although considerably more accurate, is in essential agreement with previous optical results, confirming the accuracy of the intercombination line measurements. The singlettriplet intervals calculated by many-body perturbation theory are also found to be in very good agreement with the experimental results.