CARS WITH SUPERSONIC MOLECULAR BEAMS

Author Institution:This work demonstrates the feasibility of Coherent Anti-Stokes Raman Spectroscopy, CARS, using supersonic molecular beams. The low rotational temperatures in these beams reduce the gas phase spectra of molecules to a few but very intense lines. The tight focusing and short duration of the CARS experiment allow probing in relatively small free jets. A modest rotary pump is adequate in obtaining a supersonic free jet extending 3 to 5 mm from a nozzle of 60 $\mu m$ diameter. CARS Q-branch spectra of oxygen have been measured with a resolution of $0.2 cm^{-1}$ and compared with calculated spectra to determine the rotational temperatures. $T_{Rot} =55$ K was found for a stagnation pressure of 0.4 atm and a distance of 0.5 mm to the orifice. Adding 4 atm helium as seeding gas reduced $T_{Rot}$ to 15 K. The additional cooling produced by the seeding gas increased the intensity by a factor of 7. This peak intensity then was comparable to that of a static sample at 2.3 Torr. At a distance of 1 mm, $T_{Rot}$ was less than 11 K. The excellent signal/noise ratio allowed the measurement of spectra of 60 Torr oxygen in 600 Torr helium, conditions that cosrrespond to an $O_{2}$ pressure of about 0.1 Torr at 0.5 mm in the jet. These results show that the CARS technique is quite promising for the study of densities, temperatures and rotational relaxations in supersonic jets. Experiments with other diatomic and polyatomic molecules will also be described