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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A new method of observing the rotational transitions of weak molecular complexes by combining the techniques of microwave Fourier transform spectroscopy, a Fabry–Perot cavity, and a pulsed nozzle source of molecules is described. High resolution and good sensitivity are achieved. Twelve isotopic species of the KrHCl molecule involving all combinations of the 86Kr, 84Kr, 82Kr, H, D, 35Cl, and 37Cl isotopes are assigned and the rotational constants, Cl nuclear quadrupole coupling constants, and centrifugal distortion constants are all given. The molecular structure indicates a linear equilibrium structure with the H(D) atom between the Kr and Cl atoms. However, the inverse moments of inertia measured here indicate large amplitude vibrations of the H(D) atom relative to the Kr–Cl internuclear line. Estimates of the Kr–Cl stretching frequency and force constants, which show considerable dependence on the isotopic species, are also given. Bending frequencies and force constants are also given.

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Rotational spectra have been observed for four isotopes of ArHBr and ArDBr and eight isotopes of KrHBr and KrDBr using a Fabry–Perot Fourier transform spectrometer with a pulsed supersonic nozzle as the molecular source. The rotational constants in the ground vibrational state ?0 with their centrifugal distortions DJ, as well as Br nuclear quadrupole coupling constants χa, are given. In addition, an important centrifugal distortion of the Br quadrupole coupling constant, Dχ, an indicator of the coupling between the radial and angular potentials, is given for ArHBr and KrHBr. The Br spin–rotation interaction c in ArHBr is also obtained. The results are: The molecular structures are consistent with a linear equilibrium geometry with the H(D) atom located between Br and the rare gas atoms. The complexes undergo large amplitude vibrations and estimates of the bending and stretching force constants and frequencies are given. By combining the bending, stretching, and their coupling, we have obtained the harmonic bending and stretching force constants (ks1 and kb1) and 6/12 Lennard-Jones potential parameters (ε and Re) for ArHCl, KrHCl, ArHBr, KrHBr.