Ion–molecule reactions of CoAr6+ with nitrogen oxides N2O, NO, and NO2: 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 CoAr6+. In a collision with a neutral reactant the complex between Co+ and the collision partner is stabilized by evaporation of argon atoms. If CoAr6+ 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 N2O, NO and NO2 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 CoAr6+ are reactive.