The energy separations of closest approach were measured for the 1sn 1 × 3 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 1s4 and 1s5 configurations from the measured energy separations. The experimental results are compared with the calculated values.

The level-crossing technique is a valuable tool for precise measurements of energy separations at electric-field anticrossings. To exemplify this possibility, we measured the energy separations of electric-field singlet - triplet anticrossings of the 1s5d and 1s5f configuration of He I. The level-crossing measurements on the 1s5d anticrossings are more accurate than previous radiofrequency experiments by about an order of magnitude. The experimental results agree with calculated values.

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