Doppler-limited spectra of the ν3 vibration of 12CH 4 and 13CH 4

The tetrahedral splittings in the P and R branches of the ν₃ band of natural methane have been examined with Doppler-limited resolution using a difference-frequency spectrometer. The spectra obtained by this difference-frequency mixer are compared to recent high-resolution grating spectrometer studies of ¹²CH ₄ and enriched ¹³CH ₄. The resolution, selectivity and precision are improved over the conventional methods. The mixing spectrometer utilizes tunable, narrow linewidth infrared radiation generated in the nonlinear optical crystal, LiNbO ₃ as the beat frequency between a CW argon ion and a tunable dye laser. This spectrometer covers the 2.2 to 4.2 μm infrared spectrum with an instrumental resolution of 5 × 10 ^-4 cm ^-1 and continuous scans up to ∼1 cm ^-1 and with ir power ∼1 μW.