15min:
HELIUM PRESSURE BROADENING OF HDO BETWEEN 2 AND 80~K.

T. J. RONNINGEN AND F. C. DE LUCIA, Department of Physics, The Ohio State University, Columbus, OH 43210.

Below 80~K the pressure broadening cross section for HDO-He collisions is predicted to be less than 10~Å2. This is quite small for a microwave transition and suggests that the line may, at low pressures, narrow with increasing pressure (Dicke narrowing). We have employed collisional cooling to measure the lineshape of the HDO 101-111 transition between 2 and 80~K. Initial results for the high pressure data indicate that the theoretical predictions for the cross section are valid. A more detailed analysis that investigates non-Lorentzian contributions to the lineshape will be discussed.