Rotational and Torsional Analysis of the OH-Stretch Third Overtone in 13CH3OH

Abstract

We record double resonance spectra of the 4ν1 band of jet-cooled 13C-methanol using single rotational state selection in the ν1 fundamental and subsequent promotion of the selected molecules to the fourth vibrational level. We then detect transitions to the final excited states by infrared laser assisted photofragment spectroscopy (IRLAPS). The assigned A symmetry transitions reach upper states with K=0 and 1, and J from 0 to 5. For E symmetry, the transitions reach levels with K in the range −3 to 2 and J from 1 to 7. The rotation–torsional analysis determines a value for the torsional tunneling splitting of 2.8±0.4 cm−1 at v1=4. In a previous paper (J. Chem. Phys.110, 11 359–11 367 (1999)), we reported a trend of monotonically decreasing tunneling splittings in 12CH3OH for v1=0, 3, and 6 that we explained by a model that incorporates a linear increase in the torsional barrier height with OH stretch excitation. The 13CH3OH tunneling splitting for the 4ν1 band is in quantitative agreement with the trend found for 12CH3OH.