Shift of a molecular resonance peak of a helium-neon laser with a methane absorption cell due to increase in methane pressure

A red frequency shift of a resonance peak due to the F²₂ component of the ν(3) [P(7)] transition in the ¹²CH₄ molecule was observed experimentally when the methane pressure was increased. The rate of this shift decreased rapidly at high methane pressures and the shift itself was attributed to the magnetic hyperfine structure of the molecular transition just mentioned.