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Broadband absolute absorption measurements of atmospheric continua with millimeter wave cavity ringdown spectroscopy
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View: Figures


Image of FIG. 1.
FIG. 1.

Block diagram of the experimental setup.

Image of FIG. 2.
FIG. 2.

Empty resonator loss per pass as a function of frequency. Single scan, taking about and containing ringdowns is shown. For the mirror separation, is equivalent to 1% loss per pass.

Image of FIG. 3.
FIG. 3.

Triggering scheme. The sum of the ramp voltage and the SVC voltage is applied to the BWO, resulting in a rapid frequency jump of the source frequency.

Image of FIG. 4.
FIG. 4.

Typical amplitude and mode voltage separation variations of the resonator modes. Higher order FP modes are clearly visible, especially at the high frequency end of the scan.

Image of FIG. 5.
FIG. 5.

Absorption of of water vapor. The five lines used for the frequency calibration are marked.

Image of FIG. 6.
FIG. 6.

Subtraction of two consecutive scans (grey) and two groups of 50 scans, averaged (black), of the empty resonator.

Image of FIG. 7.
FIG. 7.

Baseline structure of the empty resonator loss. Fifty scans averaged, Fourier transform processed. The structure on the zoomed part of the graph is due to the detector reflections.

Image of FIG. 8.
FIG. 8.

Absorption of of pure nitrogen as a function of frequency. Single scan, no averaging (grey), 66 point averages (black markers), and the fit (solid black line).

Image of FIG. 9.
FIG. 9.

Fifty scan-averaged absorption of of pure nitrogen as a function of frequency. Systematic effect in the form of the standing waves of varying periods can be seen after the averaging.

Image of FIG. 10.
FIG. 10.

Typical small scale structure of the frequency-voltage characteristic of a BWO. Mode separation as a function of the resonance mode number for two empty resonator scans.

Image of FIG. 11.
FIG. 11.

Single scan, 66 points averaged, absorptions of of oxygen (+), nitrogen (∎), and dry air (▵) as a function of frequency. Solid and dashed lines are frequency squared fits of the nitrogen and dry air absorptions, respectively.

Image of FIG. 12.
FIG. 12.

Fifteen points average of the room temperature moist air absorption with water partial pressure of (about 5% relative humidity). Solid line is a fit to the Van Vleck-Weisskopf line shape with addition of a frequency squared continuum absorption term. Dry air absorption is shown to demonstrate the relative size of the effects. Residual of the fit is presented above.

Image of FIG. 13.
FIG. 13.

FT processed loss of the resonator filled with of Ar, after subtracting the empty resonator loss. No averaging (grey) and 250 points average (▴).

Image of FIG. 14.
FIG. 14.

Absorption of of nitrogen. Vacuum used for baseline subtraction (grey). Ar used for baseline subtraction (black). In both cases FT processed 50 scan averages are shown.

Image of FIG. 15.
FIG. 15.

Moist continuum absorption dependence on the product of water and dry air pressures at . Two days of data are shown with the same temperature and close starting water pressures (at of the total moist air pressure).

Image of FIG. 16.
FIG. 16.

Moist air absorption with partial water pressure. The solid line is a fit to the Van Vleck-Weisskopf profile and a quadratic in frequency continuum absorption term. Only frequencies with absorption below one half of the peak absorption (∎) are used in the fit. The rest of the points (◻) are presented on the same graph, showing no signs of the line profile distortion related to insufficient detecting bandwidth.

Image of FIG. 17.
FIG. 17.

The magnitude of the room temperature nitrogen absorption fit coefficient as a function of nitrogen pressure squared. Solid line is a linear fit of the data.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Broadband absolute absorption measurements of atmospheric continua with millimeter wave cavity ringdown spectroscopy