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Journal of Physics B Atomic Molecular and Optical Physics

Excitation of singlet and triplet He I states by He-He collisions

Y -Q Liu, Gebhard von Oppen

Published Jan 28, 1992• 6 citations
Publisher

Abstract

Excitation of the 1snd levels (n=4,5,6) of He I by 12 keV He-He collisions is investigated using an electric-field anticrossing technique. The measured relative substate population of the 1 D and 3 D levels reveals a remarkable difference between one-electron excitation, where only singlet states are excited, and two-electron excitation: the first process produces a strong alignment, whereas the latter yields a more isotropic and equal population of the 1 D and 3 D levels.

DOI
10.1088/0953-4075/25/2/001

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Excitation of the 1snd levels (n=4,5,6) of He I by 12 keV He-He collisions is investigated using an electric-field anticrossing technique. The measured relative substate population of the 1 D and 3 D levels reveals a remarkable difference between one-electron excitation, where only singlet states are excited, and two-electron excitation: the first process produces a strong alignment, whereas the latter yields a more isotropic and equal population of the 1 D and 3 D levels.
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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.
The anticrossing spectra of the $n=6,7,8 ^{1}D\ensuremath{-}^{3}D$ states of 3He^{3}\mathrm{He}3He have been observed. From these measurements precise values of the electrostatic singlet-triplet separation of these states have been obtained and are found to differ systematically from the accepted values of the 4He^{4}\mathrm{He}4He singlet-triplet separation by approximately 34 MHz. We have investigated mass-dependent contributions to both the energy-level structure and the magnetic-tuning characteristics of the states of interest. We have found no mechanism that can explain such a large difference in the electrostatic-energy separation of the singlet and triplet DDD states between the two helium isotopes.