Laser spectroscopy of electric-field 1s3d anticrossings of He I

Singlet-triplet anticrossings of 1s3d Stark sublevels of He I were investigated by inducing resonance transitions from the 1s2p ³P 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.