Aerodynamical acceleration and rotational-vibrational temperatures in seeded supersonic molecular beams

Abstract

Abstract The time-of-flight (TOF) technique was used to study the aerodynamical acceleration in seeded supersonic molecular beams of heavy molecules and light seeding gas. We also studied the correlation between the degree of aerodynamic acceleration achieved, and rotational-vibrational temperatures as measured using the laser-induced fluorescence (LIF) technique. The velocity slip (difference) between helium and hydrogen carrier gases and iodine and aniline heavy molecules was determined in free-jet expansion by TOF measurements and compared with rotational temperatures measured by LIF. The helium translational temperature was found to be abnormally high and dependent on teh heavy-molecule concentration, even at concentration as low as 400 ppm. In the case of iodine it was found that the rotational degrees of freedom were equilibrated with the helium or hydrogen seeding gas translational and slip temperatures, although this temperature was more than an order of magnitude higher than theoretical predictions obtained for the pure-gas expansion. In aniline, the rotational temperature is found to be higher than the gas-dynamic temperature and rotational relaxation is incomplete. The heavy-molecule kinetic energy increases linearly with the light-gas pressure up to = 50% of its maximum available kinetic energy. The possibility of making accurate heavy-molecule kinetic-energy measurements using the LIF technique is discussed. It is claimed that the existence of velocity slip can largely effect theoretical calculations concerning vibrational and rotational relaxation in seeded supersonic beams.