Rotational spectroscopy of the first excited state of the acetylenic C-H stretch of 3-fluoropropyne performed by infrared-Fourier transform microwave-microwave triple-resonance spectroscopy

The rotational spectra of 3-fluoropropyne in the ground and first excited acetylenic C-H stretch vibrational state have been measured. The pure rotational spectrum of the normal species and the ¹³C isotopomers were measured using FTMW-cwMW double-resonance spectroscopy based on the Autler-Townes (AC Stark) effect. The lineshape properties of this measurement make it possible to determine the transition strength, ΔJ-selection rules, and the relative energy ordering of the quantum states. The frequency accuracy of this technique is tested against a previous pure rotational study of 3-fluoropropyne. The rotational spectrum of vibrationally excited state was obtained through IR-FTMW-cwMW methods. In this technique a single-longitudinal-mode pulsed infrared laser source vibrationally excites the acetylenic C-H stretch with J-selectivity. The rotational spectrum of the excited state is then obtained by FTMW and FTMW-cwMW double-resonance methods. The excited-state measurements have a signal-to-noise ratio comparable to the pure rotational spectrum. The residuals in the excited-state fit are larger than those obtained in the ground-state fit. This greater deviation from a standard asymmetric top spectrum is most likely due to weak perturbations to the acetylenic C-H spectrum.