Physics
Scientific paper
Dec 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986fdhs.nasa...31p&link_type=abstract
In NASA. Langley Research Center Future Directions for H sub x O sub y Detection p 31-32 (SEE N87-15528 07-46)
Physics
Absorption Spectroscopy, Chemical Analysis, Hydroxyl Radicals, Optical Measurement, Continuous Wave Lasers, Laser Spectroscopy, Optical Paths, Photon Density, Ultraviolet Absorption
Scientific paper
The long path absorption spectroscopy (LPA) measurement of OH utilizes the strong UV absorption spectrum of the molecule with well resolved rotational lines around 308 nm. The light source consists of an Ar ion laser pumped CW dye laser, the output of which is frequency doubled to emit a spectrally smooth light profile of 0.1 nm FWHM at 308 nm. This is considerably broader than the sharp (0.0018 nm wide) absorption features of OH and also wider than the total spectral region (0.06 nm) scanned around the OH lines. For field measurements a Cassegrain-type telescope expands the small diameter of the original laser beam by a factor of 50 to about a 0.25-m diameter in order to reduce the divergence of the beam as well as its mean photon flux density, thus reducing the maximum self-generated OH concentration to 40,000 cu. cm. The light is passed into the open atmosphere and reflected at about 5 km distance resulting in a 10-km light path length. A telescope focuses the returning light onto the entrance slit of a monochromator the single exit slit of which is replaced by a mechanical scanning device operating at a scan repetition rate of 6.6 KHz. For a typical measurement (1h duration) some 20,000,000 scans are averaged, rendering the contribution of atmospheric turbulence to the spectral noise negligible.
Ehhalt D. H.
Platt Ulrich
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