Knowledge Bank

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 $NO2,NO,SO2,CO2,CO,O2,H2$ relative to that of $N2$ 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 $NO2\nu 1$ band has 15 times and $\nu 2$ band has 7.2 times larger cross section than that of $N2$, due possibly to resonance Raman effect at 3371 {\AA}.