15min:

LIAM M. DUFFY, Department of Physics, Duke University, Durham, NC 27706; DAVID SKATRUD, Army Research Office, P.O. Box 12211, RTP, NC 27709.

For molecules with permanent electric dipole moments, the most rapid energy transfer mechanism is due to dipole-dipole collisions which induce J = +/- 1 transitions. Although this process is the dominant contributor to pressure broadening, little is known about the temperature dependence of the process. We have used a velocity selective time resolved pump/probe technique based on infrared-millimeter/sub-millimeter wave double resonance spectroscopy to study the temperature dependence of the dipole-dipole cross section of ¹³CH₃F. In these experiments we extract the collision cross section as a function of mean relative velocity without changing the rotational distribution of the collision partners. Our results show that the dipole-dipole collision cross section is largely independent of the mean relative velocity in the range between 400 and 900 m/s. On the other hand, we clearly observe a decrease in the cross section when the gas cell temperature is raised from 173K to 400K. These results strongly suggest that resonant collisions dominate the dipole-dipole collision cross section. We will discuss these finds in terms of the temperature scaling predicted by Anderson, Tsao and Curnutte theory.