ROTATIONAL SPECTRUM OF A DARK STATE IN 2-FLUOROETHANOL USING MICROWAVE RADIO-FREQUENCY-INFRARED MULTIPLE RESONANCE

Microwave/radio-frequency-infrared multiple resonance has been used with an electric-resonance optothermal spectrometer to characterize a weak 21.6 MHz perturbation in the infrared spectrum of the v(14) C-O stretching vibration of 2-fluoroethanol. The infrared spectrum of 2-fluoroethanol was recorded at a resolution of similar to 2 MHz using a tunable microwave-sideband CO2 laser. The spectrum is fit by an asymmetric-rotor Hamiltonian to a precision of 0.6 MHz, except for the transitions to the 4(13) upper state which are split into doublets by an interaction between the 4(13) level and a rotational level of a nearby background, or dark, vibrational state. Microwave/radio-frequency-infrared double and triple resonance reveals that the 4(13) level of the C-O stretching vibration is interacting with the 4(31) level of the dark state. The rotational constants determined for the dark state allow us to assign the perturbing state to the v(18)+4v(21) combination vibration. of the lowest energy conformer, where V-18 is the CCO bending vibration and v(21) is the C-C torsional vibration. From the weak Delta K-a=2 matrix element,between v(14) and v(18)+4v(21) it is possible to derive a J=0 anharmonic interaction between these states of similar to 3.5 GHz.