High-resolution stimulated Raman spectroscopy of methane 13CD 4 in the pentad region

Abstract

We present the first Raman spectrum of 13CD 4 recorded at room temperature in the pentad region by inverse Raman spectroscopy, thus including the ν 1 ( A 1), 2 ν 2 ( A 1), 2 ν 4 ( A 1), and ν 2 + ν 4( F 1 + F 2) Q branches. It is noteworthy that the overtone bands 2 ν 2 and 2 ν 4 are observed for the first time in a methane-like molecule by a coherent Raman process. The wide frequency range investigated, covering 45 cm −1 in three parts, contains more than 300 lines with uncertainty less than 10 −3 cm −1 in most cases. These Raman data are combined with high-resolution infrared data in a weighted least-squares fit of the vibration-rotation constants of the pentad, thanks to a relevant partially reduced effective Hamiltonian including all the third-order interaction terms between the upper states of the five vibrational bands. These interactions within the pentad induce strong perturbations in the isotropic Raman spectrum. In particular the ν 1 band exhibits a complex structure, which results essentially from the second-order Coriolis interaction between the v 1 = 1 ( A 1) state and the v 2 = v 4 = 1 ( F 1) state. The analysis has been realized up to J = 18 in such a way that almost all of the Raman transitions were assigned up to J = 17. These observed Raman transitions are reproduced with an overall weighted standard deviation of 1.86 × 10 −3 cm −1, which is the order of magnitude of the accuracy of the experimental data.