Anticrossing induced by the motional Stark effect created by atoms moving perpendicular to a strong magnetic field have been observed in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. These anticrossings couple the $n{}_{}{}^1{}_{}{}^{}P$ state with the nominal $n{}_{}{}^1{}_{}{}^{}D$, $n{}_{}{}^1{}_{}{}^{}F$, $n{}_{}{}^3{}_{}{}^{}F$, and $n{}_{}{}^{1,3}{}_{}{}^{}\mathrm{H}$ states via first-order (for the ${}_{}{}^1{}_{}{}^{}D$) and second-order Stark effects. The theory is derived to explain the line shape in the second-order case. This theory, along with the previously existing first-order-effect line-shape theory, is used to obtain the zero-velocity crossing points. These values are used in a leastsquares fit to determine the zero-field intervals. The $n{}_{}{}^1{}_{}{}^{}P-{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ interval is determined precisely for $n=6, 7, \mathrm{and} 8$ and the $n{}_{}{}^1{}_{}{}^{}G-n{H}_{\mathrm{av}}$ interval is determined for $n=6\mathrm{} \mathrm{and} 7$. A power-series expansion establishes the $n{}_{}{}^1{}_{}{}^{}P$ energy levels with respect to the higher $\mathrm{nL}$ states with high precision.

• Received 20 November 1981

DOI:https://doi.org/10.1103/PhysRevA.27.895