Radiofrequency spectroscopy inside a laser cavity: Nuclear quadrupole resonance and A1-A2 splitting of CH3Br

The highly sensitive method of infrared-radiofrequency double resonance inside a CO₂N₂O laser cavity has been applied to the observation of “pure” nuclear quadrupole resonance and direct A₁-A₂ transitions of CH₃Br. More than 150 quadrupole resonances have been observed for ¹²CH₃⁷⁹Br, ¹²CH₃⁸¹Br, ¹³CH₃⁷⁹Br, and ¹³CH₃⁸¹Br using direct double resonance as well as collision-induced satellites. Accurate hyperfine constants, i.e., the quadrupole coupling constant eqQ and its rotational dependence χ_J and χ_K, Hougen's coefficient χ_d, and the spin-rotation constants C_N and C_K have been determined for the ground state. The eqQ in the excited states ν₃, ν₆, ν₃ + ν₆, and 2ν₆ have also been determined. About 60 direct l-doubling transitions have been observed, which has enabled us to determine the l-doubling constant q_v(1,1), its rotational dependence δq_v^J(1,1) and the asymmetry parameter for the quadrupole coupling constant eqQη for the ν₆ and ν₃ + ν₆ states. A set of direct A₁-A₂ transitions in the 2ν₆ state with K = 1 and l₆ = ?2 and that in the ν₆ state with K = 2 and l₆ = ?1 have been observed. The rovibrational and isotope assignments of the observed A₁-A₂ are helped by the existence of quadrupole hyperfine structure and many collision-induced satellites. The rovibrational assignments of CH₃Br transitions which are coincident with laser lines are summarized.