Molecular-beam spectrum of the 970 cm−1 Fermi triad of CF3CH3

Abstract

A tunable microwave-sideband CO2 laser is used with an electric-resonance optothermal spectrometer to investigate the infrared spectrum of CF3CH3 near 970 cm−1. A Fermi-coupled triad of states is observed, resulting from the interactions of 2ν6+ν11 and ν5+ν12 with the fundamental vibration, ν10, which is assumed to carry the oscillator strength in this region. The high resolution (∼3 MHz) of the spectrometer allows the observation of tunneling splittings associated with the ν6 torsional vibration. These splittings are used to identify the torsional character of the states observed. At the normal-mode level the ν10 and ν5+ν12 states are found to be nearly degenerate and interacting by an anharmonic matrix element of ∼3 cm−1. The lower-energy component of this diad exhibits torsional splittings of up to 400 MHz due to an anharmonic coupling of 0.70 cm−1 with the lower energy 2ν6+ν11 state which has an intrinsic tunneling splitting of ∼800 MHz. A fourth state, 3ν6+ν12, which has a still larger zeroth-order tunneling splitting, may also be affecting the torsional splittings of the observed states. The present investigation illustrates the utility of using resolved torsional splittings to unravel complex vibrational couplings in molecules.