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 wi th an electric-resonance optothermal spectrometer to characterize a we ak 21.6 MHz perturbation in the infrared spectrum of the v(14) C-O str etching vibration of 2-fluoroethanol. The infrared spectrum of 2-fluor oethanol was recorded at a resolution of similar to 2 MHz using a tuna ble microwave-sideband CO2 laser. The spectrum is fit by an asymmetric -rotor Hamiltonian to a precision of 0.6 MHz, except for the transitio ns to the 4(13) upper state which are split into doublets by an intera ction between the 4(13) level and a rotational level of a nearby backg round, or dark, vibrational state. Microwave/radio-frequency-infrared double and triple resonance reveals that the 4(13) level of the C-O st retching vibration is interacting with the 4(31) level of the dark sta te. 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 vibrat ion 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 deri ve a J=0 anharmonic interaction between these states of similar to 3.5 GHz.