The gas dynamics of a pulsed supersonic nozzle molecular source as observed with a Fabry–Perot cavity microwave spectrometer

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Abstract
The gas dynamics of a pulsed supersonic nozzle molecular source are investigated by using a pulsed Fabry–Perot cavity microwave spectrometer to obtain free induction decay signals from rotational two‐level systems in the gas expansion. An equation is derived giving the time domain emission signal line shape as an integral over the active molecular distribution in the beam. The Doppler splitting phenomenon is discussed in detail. Experimental line shapes are deconvoluted to give molecular velocities, dephasing times, and density distributions. We find that the density distribution of active molecules from the the pulsed nozzle varies rapidly in time, starting with a depletion on the nozzle axis at short times after the nozzle is opened, and changing to on‐axis concentration at longer times. Results obtained with the gas nozzle axis oriented at angles ranging from 0 ° to 90 ° with respect to the direction of propagation of the microwaves are reported.