We describe the design, construction, and operation of a new type of microwave spectrograph which allows the measurement of the resonant transitions of transient or otherwise short-lived species. The spectrograph is composed of three parts: a Fabry–Perot cavity, a pulsed supersonic nozzle as a source for the sample, and the pulsed microwave Fourier transform method. Following a detailed discussion of the three above components in the spectrograph, the operation of the entire system is described and several examples are given.

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We have measured the dispersion of the gas phase depolarization ratio of Rayleigh scattered light for 12 linear and symmetric top molecules. Combining these data with known refractive index data we obtain the frequency dependence of the polarizability anisotropy. For all molecules studied we find that the polarizability anisotropy increases more rapidly with increasing frequency than the bulk polarizability. We have correlated this behavior with the oscillator strength and direction of the first electronic transition. We have also compared our zero frequency extrapolated anisotropies with the anisotropies determined from static fields with the Kerr effect and the Stark effect in microwave spectroscopy.

Equations are derived relating the spin—rotation constants and magnetic shielding for any type of molecule. In general, the spin—rotation constants depend on the equilibrium nuclear positions and a second-order term involving the excited electronic states. The spin—rotation constants are interpreted by a localized theory in terms of the average excitation energy, average value of (1/r3), and the charge-bond order matrix of the associated LCAO—MO functions.