Observed Suppression of an E1 Allowed Transition in $^3$He

Many instances occur when hyperfine mixing enhances the strength of forbidden transitions, such as ground state to $^3$P$_0$ transitions in odd isotope alkaline earths. However, strong suppression of an allowed transition due to hyperfine mixing is rare. We have observed strong suppression of the E1 allowed 2$^3$S$_1$ F=3/2 to 3$^3$P$_1$ F=3/2 transition in $^3$He. In a theoretical model developed by Drake et al., this ``missing'' transition is attributed to hyperfine mixing of the 3$^3$P$_1$ F=3/2 and 3$^3$P$_2$ F=3/2 states. The theoretical model also predicts the magnetic field dependence of transition strength for the ten transitions between magnetic sublevels of 2$^3$S$_1$ F=3/2 and 3$^3$P$_1$ F=3/2. We present measurements of this dependence using laser spectroscopy at 389nm on a collimated beam of metastable $^3$He atoms.