Rayleigh Scattering of Excimer Laser Light from Some Simple Molecules at 193 nm and 248 nm: The Effect of Polarization upon Imaging Diagnostics

Abstract

Tunable excimer laser beams are increasingly used for imaging of combustion and/or flow systems. We (1) describe the need to use known polarization phenomena in interpreting diagnostics based on Rayleigh scattering (RS); (2) present relative RS cross sections for some important species at the most relevant wavelengths, 193 and 248 nm; and (3) measure the degree of linear polarization and the locking efficiency of the excimer beam, which are important diagnostic parameters that are otherwise difficult to obtain. Laser sheets, whose direction of linear polarization are adjustable, pass through gases. The Rayleigh light goes into intensified charge-coupled dervice (CCD) cameras. We report RS cross sections, normalized to those for N₂, at 193 nm for H₂, O₂, H₂O, CO₂, and CH₄, and at 248 nm for Ar, H₂, O₂, H₂O, CO₂, and CH₄. Even when these normalized RS cross sections are used, the diagnostics can be very sensitive to the laser beam's polarization state. For example, for CO₂, the normalized cross section changes by a factor of 4 with polarization. This usually unwanted sensitivity is greatest when an unmodified tunable excimer laser is used. The degree of polarization of the lasers, as well as their locking efficiency, is derived via RS from the spherical molecules Ar and CH₄.