CARS Application to Monitoring the Rotational and Vibrational Temperatures of Nitrogen in a Rapidly Expanding Supersonic Flow

Abstract

Using an excimer laser based CARS apparatus, single-pulse rotational and vibrational temperatures of nitrogen molecules have been measured in a rapidly expanding supersonic flow in which nitrogen molecules were not in thermal equilibrium. The supersonic flow facility should be installed outdoors, therefore an optical fiber was used in the CARS receiving optics to protect delicate instruments, such as a monochromator and an optical multichannel analyzer, from intense vibration caused by the flow. The optical fiber also worked as a medium producing a nonresonant CARS signal which was used to normalize the resonant CARS spectrum obtained in the flow. In this experiment the validity of the newly developed excimer laser based CARS system which could be operated up to 80 Hz repetition rate was proved, because the flow continued only for 1.5 seconds and a high repetition rate of the CARS system was important.