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AMPLITUDE-MODULATED TIME-DOMAIN MICROWAVE MEASUREMENTS OF ROTATIONAL RELAXATION RATES, LINESHIFTS AND LINE POSITIONS

Please use this identifier to cite or link to this item: http://hdl.handle.net/1811/11362

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Title: AMPLITUDE-MODULATED TIME-DOMAIN MICROWAVE MEASUREMENTS OF ROTATIONAL RELAXATION RATES, LINESHIFTS AND LINE POSITIONS
Creators: Coy, Stephen L.
Issue Date: 1980
Abstract: We have performed both Stark-switched and PIN-switched measurements of the speed –dependence of rotational relaxation rates, lineshifts and line positions in the time domain. The PIN-switched method for transient experiments appears clearly superior, both in ability to measure accurately the speed-dependence of the relaxation rate, and in deriving the zero-pressure frequency intercept and the pressure-induced lineshift, with a factor of 10 improvement in the line position and shifts, and reduction I scatter in relaxation rate results. This improvement results from a variety of characteristics of those experiments. The amplitude-modulated experiments were performed in an X-band cell without Stark septum. The off-resonant power during the emission is essentially zero. There is no pickup from the operation of the Stark generator, and no drift or Jitter in the pulse width and position. In the PIN experiments, the pulse width is short compared to the spread in the relaxation times in the system at all measured pressure even for systems with as strong a speed-dependence as OCS. The ability to make the pulses short makes the meaning of speed-independent exponential fit results more clear, complementing the results for the speed-dependence. Pulse widths in the stark experiments cannot be made narrow enough over the full pressure ranges, which results in an initial velocity distribution which varies with the sample pressure. Only in the PIN experiments should the exponential fit results approach a well –defined average of the relaxation rate over velocities, facilitating comparison with calculations. Amplitude-modulation may also be used to measure relaxation rates for systems with a slow, or no, Stark effect, such as water, or oxygen. We will discuss experimental design and results for several systems.
URI: http://hdl.handle.net/1811/11362
Other Identifiers: 1980-TC-6
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