Ion-molecule reactions of CoAr with nitrogen oxides NO, NO, and NO: Measuring absolute pressure by shock-freezing of the collision complex

A new method to determine the absolute pressure in an ultra-high vacuum apparatus is tested using ion molecule reactions with CoAr. In a collision with a neutral reactant the complex between and the collision partner is stabilized by evaporation of argon atoms. If CoAr reacts at the collision rate, the absolute pressure can be determined by comparing the experimental collision rate with the collision rate calculated from average dipole orientation theory. The experimental results with NO, NO and NO do indeed show that the collision complex is frozen out. Comparing the rates of primary, secondary and tertiary reaction products suggests that not all collisions of CoAr are reactive. © 2010 American Institute of Physics.