Difference frequency laser spectroscopy of the ν3 band of the CH3 radical

The ν3 fundamental band of the CH3 radical has been detected in absorption with a tunable difference frequency laser. Zeeman modulation is found to be inefficient, because the spin‐rotation interaction is small. The molecular constants in the ν3 vibrational state have been determined with the molecular constants in the ground state fixed at the values determined from the analysis of the ν2 band. The main parameters thus obtained are the band origin ν0=3160.8212(12), B3=9.471 10(14), C3=4.701 67(15), (Cζ)3=0.345 88 (22), and q3=0.006 42(25), all in cm−1 with one standard deviation in parentheses. The transition frequencies and the molecular constants of the ν3 band may be useful in a search for interstellar CH3.