Knowledge Bank

Below 80 K the pressure broadening cross section for HDO-He collisions is predicted to be less than 10 $\backslash AA2$. 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 $HDO101-1111$ 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.