Pulsed-nozzle Fourier-transform microwave spectroscopy

The interaction of dihalogen molecules XY with different acceptors B quite often leads to vicious chemical reactions. In most cases, the 1 1 complexes are extremely short-lived. To investigate these prereactive complexes experimentally in a collision-free environment, pulsed-nozzle, Fourier-transform microwave spectroscopy has turned out to be the ideal technique. Legon and coworkers prepared a large number of these complexes and performed detailed rotational spectroscopic analyses. Several series of simple molecules... 

Legon AC (1983) Pulsed-nozzle Fourier transform microwave spectroscopy of weakly bound dimers. Ann Rev Phys Chem 34 275-300... 

Legon, A. C. and Willoughby, L. C., First-order Stark effect and electric dipole moment of HjP"HC N by pulsed-nozzle Fourier-transform microwave spectroscopy, Chem. Phys. Lett. 111,566-570(1984). 

A quite different approach to radiofrequency, microwave and infrared spectroscopy is that known as Fourier transform (FT) spectroscopy. As we shall see, this method of recording the spectra of transient molecular species is particularly appropriate in combination with the use of pulsed gas nozzles. For this reason it has proved to be a powerftd technique for the study of weakly bound dimer complexes formed in supersonic gas expansions. It has, however, also been used for the study of diatomic molecules, both... 

The CW microwave spectrometer just described is a typical frequency-domdim instrument. In the late 1970s it was demonstrated that pulsed /m -domain microwave spectroscopy could be practically performed in analogy to the techniques already well known in other fields such as NMR spectroscopy. Figure 2 depicts a block diagram of a modern version of a pulsed Fourier-transform microwave spectrometer. The particular instrument shown utilizes a Fabry-Perot cavity and a pulsed-gas nozzle, and is especially useful for detecting microwave... 

Fig. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency vm falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1 (is) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2 = 100 (is) compared with the relatively intense exciting pulse (half-life in the cavity t 0.1 (is). If detection is delayed until ca. 2 (is after the polarization, the exciting pulse has diminished in intensity by a factor of ca. 106 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at vm is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector at the beat frequency v - r , as a function of time, as in NMR spectroscopy Fourier transformation leads to the frequency spectrum [reproduced with permission from (31), p. 5631.

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