Using the information in a spectrum

Fourier transform techniques have become very powerful, particularly when used with pulsed sources of the sample. They are ubiquitous in studies of van der Waals molecules, in which two or more entities are very weakly bound together in the gas phase, such as HCT Ar, but are also used to determine structures of many other compounds, of which the mixed alkali halide dimer LiNap2 is an example. In a typical experiment, the sample is introduced into the cell in a pulsed supersonic jet expansion, the rotational temperature being reduced to a few Kelvin. A pulse of microwave radiation then aligns the dipole moments of the molecules, so that the sample is polarized on a macroscopic scale. The subsequent decay of this polarization is recorded, and Fourier transformation of the free induction decay yields the spectrum [9]. 

Other methods used to get rotational data from electronic and vibrational spectra, including doubleresonance methods, are described in the on-line supplement to Chapter 7. 

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