RAYLEIGH-SCATTERING OF EXCIMER-LASER LIGHT FROM SOME SIMPLE MOLECULESAT 193 NM AND 248 NM - THE EFFECT OF POLARIZATION UPON IMAGING DIAGNOSTICS

Tunable excimer laser beams are increasingly used for imaging of combu stion and/or flow systems, We (1) describe the need to use known polar ization phenomena in interpreting diagnostics based on Rayleigh scatte ring (RS); (2) present relative RS cross sections for some important s pecies at the most relevant wavelengths, 193 and 248 nm; and (3) measu re the degree of linear polarization and the locking efficiency of the excimer beam, which are important diagnostic parameters that are othe rwise difficult to obtain, Laser sheets, whose direction of linear pol arization are adjustable, pass through gases, The Rayleigh light goes into intensified charge-coupled dervice (CCD) cameras, We report RS cr oss sections, normalized to those for N-2, at 193 nm for H-2, O-2, H2O , CO2, and CH4, and at 248 nm for Ar, H-2, O-2, H2O, CO2, and CH4. Eve n when these normalized RS cross sections are used, the diagnostics ca n be very sensitive to the laser beam's polarization state, For exampl e, for CO2, the normalized cross section changes by a factor of approx imate to 4 with polarization. This usually unwanted sensitivity is gre atest when an unmodified tunable excimer laser is used. The degree of polarization of the lasers, as well as their locking efficiency, is de rived via RS from the spherical molecules Ar and CH4.