Pulsed nozzle fourier transform microwave spectrometer: advances and applications

The pulsed nozzle Fourier transform microwave (PNFTMW) spectrometer was developed by Balle and Flygare [A new method for observing the rotational spectra of weak molecular complexes: KrHCl. J. Chem. Phys. 1979, 71 (6), 2723-2724 and 1980, 72 (2), 922-932] in 1979. The design, fabrication, and operation of this spectrometer are complicated and it has largely remained a research laboratory tool till now, though a portable spectrometer for routine analytical applications has been developed at the National Institute for Standards and Technology [Suenram, R.D.; Grabow, J.-U.; Zuban, A.; Leonov, I. A portable pulsed-molecular-beam Fourier-transform microwave spectrometer designed for chemical analysis. Rev. Sci. Instrum. 1999, 70 (4), 2127-2135]. However, the potential for extracting fundamental information about any chemical species, such as, molecules, radicals, ions, or weakly bound complexes between any of them including atoms, has been quite significant. It is evident from the fact that more than 25 laboratories around the globe have built this spectrometer, some in the recent past. Contributions from all these laboratories have widened the horizon of PNFTMW spectrometer's applications. This review summarizes the advances in design and the recent applications of this spectrometer. We also define an electrophore, as an atom/molecule that generates an electric dipole moment by forming a weakly bound complex with a species having zero electric dipole moment. The electrophore, thereby, enables structural determination using rotational spectroscopy, as in the case of Ar2-Ne, with Ne as the electrophore. Also, it can introduce a dipole moment about a principal axis where none existed before, such as in Ar-(H2O)2, enabling the observation of pure rotational transitions for several tunneling states