Frequency shifts of the lamb dip minimum in the helium-neon laser

The effects of various discharge tube parameters on the center frequency of the Doppler-broadened (
3s_{2}-2p_{4}
) 6328-å lasing transition in Ne
²⁰
and Ne
²²
have been measured. Frequency shifts from
17\frac{1}{2}
to 23 MHz/torr were measured in the discharge depending on the combination of helium and neon isotopes used. No measurable shift in Lamb dip frequency was found with discharge tube current or laser radiation intensity. In addition, an analysis is presented of the possible causes of the pressure-frequency shifts, the result being that they may be attributed to collisions between the excited neon atoms (3s
2
level) and ground-state helium atoms.