Hot Band Rotational Relaxation Time in Carbonyl Sulfide by Transient Infrared-Microwave Double Resonance
Leap, John William
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Permalink
https://hdl.handle.net/2142/69234
Description
Title
Hot Band Rotational Relaxation Time in Carbonyl Sulfide by Transient Infrared-Microwave Double Resonance
Author(s)
Leap, John William
Issue Date
1982
Department of Study
Electrical Engineering
Discipline
Electrical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Electronics and Electrical
Abstract
Rotational relaxation time (T) has been measured for pure Carbonyl Sulfide (('16)O('12)C('32)S) in a highly vibrationally and rotationally excited state (03('1f)0, J=24). A CO(,2) laser (P(,9)(24) line) was used to pulse excite the OCS gas: (01('1f)0, J=25) (--->) (03('1f)0, J=24). The resulting transient population difference between l doublet states, (03('1f)0, J=24) and 03('1e)0, J=24), was monitored with a microwave (7.4 GHz) probe signal. Individual transient infrared-microwave signals were analyzed and from the gain pulse shape it is concluded that population decay time (T(,1)) and polarization decay time (T(,2)) are approximately equal to a composite relaxation time (T). From the pressure dependence of T as pressure (p) changes from 0.04 to 0.3 Torr(5 to 40 Pa), it is concluded that (pT) = 32 ns(.)Torr = 4.3 (mu)s(.)Pa.
The CO(,2) laser included a transversely excited atmospheric pressure (TEA) gain section and a low pressure, selective, saturable absorber (NH(,3)) inside the laser cavity. The frequency shifted and modulated multimode pulse from the CO(,2) laser was abruptly truncated with a plasma shutter breakdown switch so that the OCS gas could be observed in the absence of further infrared excitation. The truncated laser pulse was used to transiently excite OCS gas in a 3-meter long arm of a WR90 waveguide bridge with pierced E plane bends. The microwave frequency was tuned to resonance with the l doublet transition and the bridge was adjusted to be sensitive to amplitude changes. Envelope detection of the amplified bridge imbalance yielded a video signal proportional to microwave gain. The signal to noise of individual transients was adequate and signal averaging was not used. Waveforms were recorded photographically from an oscilloscope. Values of relaxation time (T) were obtained from the transient gain pulse width by using a Bloch equation model for the gas and by considering the bandwidth limits of the video amplifiers.
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