Microcanonical analysis of the kinetic method. The meaning of the "effective temperature"

An analytical expression for the "effective temperature" parameter in the C ooks kinetic method is derived using classical Rice-Ramsperger-Kassel (RRK) theory for the microcanonical unimolecular dissociation rate. The approxim ate expression is appropriate for metastable ion dissociation experiments a nd high ion source temperatures. The effective temperature is directly prop ortional to the well depth (complexation energy) of the dissociating cluste r ion, is inversely proportional to the number of oscillators of the cluste r (vibrational degrees of freedom), and also depends on the product of the reaction frequency (preexponential factor) and the instrumental time window of the experiment. Numerical simulations using classical RRK rates with a detailed kinetics treatment are compared with the analytical expression for the effective temperatures. For fast dissociations (shallow cluster well d epth or small number of oscillators), threshold effects cause significant c urvature in the kinetic method plots. The implications of these results on the accuracy of relative thermochemical measurements by the kinetic method are discussed. (C) 2000 Elsevier Science B.V.