MEASUREMENTS OF RAMAN CROSS SECTIONS FOR VARIOUS GASES IN THE POLLUTED ATMOSPHERE BY MEANS OF PULSED LASER-RAMAN AND PULSE-GATED PHOTON COUNTING METHODS
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Date
1972
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Ohio State University
Abstract
Feasibility and potentiality of the laser radar system which detects the Raman-shifted backscattering, either in non-resonant or resonant nature, from chemical contents of atmospheric pollutants to identify and to monitor remotely their concentrations have been $demonstrated.^{1-3}$ However, for the precise measurement of species concentration, the knowledge on the Raman scattering cross sections is always required. This paper presents the measured result and their comparison of the Raman cross sections of various molecules present in the atmosphere such as $NO_{2}, NO, SO_{2}, CO_{2}, CO, O_{2}, H_{2}$ relative to that of $N_{2}$ as a reference gas. We employed primarily a molecular nitrogen laser with 10mW average power at 3371 {\AA}, 20 kW peak power of 10 nsec duration and a repetition rate of 50 Hz. Recently, we have succeeded in the operation of transverse excitation atmospheric (TEA) nitrogen laser at 3371 {\AA} which is applicable to the field-use and also to the laboratory experiments. The Raman scattered radiation was observed in a direction perpendicular to the linearly polarized plane of the incident laser beam. The signal was processed by the pulse-gated photon counting method synchronized with the repetitive laser pulse, which is valuable for detecting extremely weak light signals. The accuracy of our measured cross sections is within 10\% and their values were found to agree well with the pre-laser result by Murphy et $al.^{4}$ and the laser result by Fouche et $al.^{5}$ for several gases. It is noteworthy that $NO_{2}\nu_{1}$ band has 15 times and $\nu_{2}$ band has 7.2 times larger cross section than that of $N_{2}$, due possibly to resonance Raman effect at 3371 {\AA}.
Description
$^{1}$ H. Inaba and T. Kobayasi, Nature 224, 170 (Oct. 1969) $^{2}$ T. Kobayasi and H. Inaba, Appl. Phys. Letters 17, 139 (Aug. 1970); Proc. of the IEEE. 58, 1568 (1970). $^{3}$ H. Inaba and T. Kobayasi, Paper presented to 26th Symposium on Molecular Structure and Spectroscopy N4, Columbus, Ohio, June 14-18, 1971. $^{4}$ W. F. Murphy, W. Holzer and H. J. Bernstein, Appl. Spectroscopy 23, 211 (1969). $^{5}$ D.G. Fouche and R.K. Chang, Appl. Phys. Letters 18, 579 (June 1971).""
Author Institution: Research Institute of Electrical Communication, Tohoku University
Author Institution: Research Institute of Electrical Communication, Tohoku University