Quasi-cw inverse Raman spectroscopy of the ν1 fundamental of 13CH4

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Abstract
A Doppler‐limited Raman spectrum of the symmetric stretching fundamental (ν1) of 13CH4 has been recorded using high‐sensitivity ’’quasi‐cw’’ inverse Raman spectroscopy. The band is very different in appearance from ν1 of 12CH4, due mainly to a much smaller value of ΔB for the heavier species, which causes many of the transitions to overlap near the band origin. Line assignments have been made for J?11. The spectroscopic constants have been determined from the frequencies of 25 resolved transitions and a computer synthesis of the largely unresolved central region. The observed 12C–13C isotope shift, 1.04±0.02 cm−2, does not agree completely with predictions based on any available set of anharmonicity constants, but it does indicate that a simplified calculation based on a Dennison‐type approximation for the anharmonicity constants of isotopic species is not valid. The large difference in ΔB between the two species is attributed to the effect of Coriolis interaction with the nearby combination level ν2+ν4.