Applications of Fourier transform microwave (FTMW) detected infrared-microwave double-resonance spectroscopy to problems in vibrational dynamics

A description of Fourier transform microwave (FTMW) detected infrared (IR) spectroscopy is presented. A series of measurements demonstrates the versatility of using a narrowband high Q cavity FTMW spectrometer as a rotationally resolved detector for molecular-beam infrared spectroscopy. The IR-FTMW spectrometer performance is characterized by comparing the spectrum of the acetylenic C-H stretch fundamental of 1-butyne to that obtained using a high-resolution electric resonance optothermal spectrometer (EROS). Two different measurement schemes, corresponding to IR excitation before or after the microwave polarization pulse, are compared. The ability to measure rotationally resolved spectra in the region of the first overtone of the acetylenic C-H stretch is demonstrated through measurements on tertbutylacetylene and trimethylsilylacetylene. One major advantage of the current technique is that large frequency scan rates can be achieved. This feature is useful for molecules exhibiting fast intramolecular vibrational energy redistribution (IVR) as shown by measurements of the highly perturbed acetylenic C-H stretch fundamental region of cyclopropylacetylene. The second strength of the technique is that the high-sensitivity of FTMW spectroscopy makes it possible to obtain the infrared spectra of molecules in low abundance in the pulsed jet expansion. This capability is exploited to measure the infrared spectrum of the ¹³C isotopomer of cyclopropylacetylene in natural abundance and to obtain the spectra of the trans and gauche conformational isomers of 4-fluorobut-1-yne. The technique is also well-suited to measuring the vibrational spectra of weakly bound complexes. The spectral simplification achieved by the double-resonance measurement has made it possible to identify weak perturbations in the acetylenic C-H stretch fundamental region of the H-C{triple bond, long}C-H-NH₃ complex. The acetylenic C-H stretch fundamental spectrum of propyne-ammonia has also been obtained using the technique.