Abstract
A pulsed microwave spectrometer operating in the vicinity of 140 GHz for the detection of rotational transitions in gaseous molecules is described. The spectrometer incorporates a tunable Fabry–Perot cavity and a subharmonically pumped superheterodyne receiver for the detection of the molecular emission signals. A 70-GHz source supplying a high-efficiency frequency doubler which is pulse modulated at 30 MHz produces sidebands of sufficient power at 140 GHz to excite the molecules. The cavity is tuned to one of the modulation sidebands. The operation of the spectrometer is illustrated by the detection of emission signals from the 6(2, 4)–6(1, 5) transition of SO2 gas. The generation of the electric dipole analog of nuclear-magnetic-resonance (NMR) ‘‘spin-echo’’ signals by a π/2−π pulse sequence is also described.
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