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A quantum cascade laser cw cavity ringdown spectrometer coupled to a supersonic expansion source
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Image of FIG. 1.
FIG. 1.

Overall experimental layout of the FP-QCL CRD spectrometer.

Image of FIG. 2.
FIG. 2.

Schematic of rigid armature mount designed to eliminate laser drift as the cryostat liquid nitrogen level changed. (a) Back mounting plate; (b) armature; (c) G10 washer; (d) QCL chip; (e) copper laser mount; (f) resistive heater; (g) temperature diode (hidden from view, but adjacent to resistive heater on backside of mount); (h) copper ribbon; (i) cryostat cold plate.

Image of FIG. 3.
FIG. 3.

View of the chamber showing how the ringdown cavity axis is perpendicular to the expansion axis from the supersonic source. The vacuum chamber is rendered translucent in the figure. The laser beam is shown entering the mirror mount and crosses the free-jet (shown as a cone) emerging from the source. (a) Cavity ringdown mirror mount; (b) PZT; (c) kinematic mirror mount assembly; (d) purge gas line; (e) roots blower; (f) supersonic expansion source.

Image of FIG. 4.
FIG. 4.

Two perspectives of the supersonic source mounted on a translatable plate attached to the back flange of the vacuum chamber. (a) Gas feedline to expansion source; (b) push-rod for adjusting nozzle distance from cavity axis; (c) height adjustment micrometer for the translatable plate; (d) rods for supporting source carriage; (e) source carriage plates; (f) Conflat blank used to seal source bore; (g) expansion source body; (h) pinhole nozzle machined into Conflat blank.

Image of FIG. 5.
FIG. 5.

Example scanning window of room temperature gas leaked into the chamber from . (a) Direct absorption scan (normalized) of . (b) Frequency readings from calibrated wavemeter. (c) Cavity ringdown spectrum of .

Image of FIG. 6.
FIG. 6.

Collection of calibrated overlapped spectra from 1196.5 to illustrating the prominence of the strong Q-branch features from all three isotopologues. All these spectra are of room temperature gas that has been leaked into the chamber.

Image of FIG. 7.
FIG. 7.

Comparison between recorded experimental spectra of and simulated spectra with all three isotopologues at two different rotational temperatures. (a) Experimental spectra from 1196.50 to with the nozzle 2.5 cm from the region probed, with the flow rates of the Ar only and set to 660 and 102 SCCM, respectively; (b) simulated spectrum composed of a linearly scaled coaddition from simulations (c) and (d); (c) simulated spectrum at with a Gaussian linewidth of ; (d) simulated spectrum at with a Gaussian linewidth of .


Generic image for table
Table I.

Compilation of spectroscopic constants obtained by fitting experimental spectra for all three isotopologues using PGOPHER. Only and were fit, with the uncertainties from the fit provided in parentheses next to the value. is taken from microwave studies, and shown for comparison to . The rightmost columns provide the average observed minus calculated for the fit, and the number of assigned lines for each isotopologue, respectively. The B and C rotational constants for all three isotopologues are not provided in the table because they were constrained to their values from microwave studies.

Generic image for table
Table II.

Comparison of the performance of previously reported QCL cw ringdown spectrometers with that of the current instrument.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A quantum cascade laser cw cavity ringdown spectrometer coupled to a supersonic expansion source