Laser magnetic resonance spectroscopy of normally forbidden transitions: Electrostatic fine structure of then=9,L=1−8He4singlet states

Forbidden transitions have been observed from the $7{}_{}{}^1{}_{}{}^{}S$ state of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ to the manifold of states, $n=9,L=1-8\mathrm{}$. These transitions are tuned into resonance with C${\mathrm{O}}_2$ laser lines by an applied magnetic field of 0-140 kG. Transitions to the odd-parity states are characterized by a velocity-dependent, motional Stark effect line shape which has been described previously. Transitions to the even-parity states have, in addition, a velocity-dependent transition moment creating a more complex line shape whose form is derived in an adjoining article. Analysis of the positions of over 20 observed resonances allows a determination of the zero-field separations of all the $n=9,L=1-8\mathrm{}$ states of He, and the quantum defects for He singlet states with $L>\mathrm{}0\mathrm{}$.