We report on the determination of the reaction rate of photochemical reactions in the gas phase, taking NaHg as an example. The lifetimes as well as the reaction rates are determined by fitting the analytical solution of a simplified set of rate equations to the experimental measured time evolutions of the products of the photochemical reaction. In this way we also extracted the activation energies according to the Arrhenius relation for the NaHg molecule to be produced in their II1/2 and III1/2 electronically excited states, respectively.