Temperature measurement from the translational kinetic energy release distribution in cluster dissociation: a theoretical investigation

Unimolecular dissociation of neutral and charged argon clusters is theoretically investigated in the context of calorimetric measurements. The temperature of the product cluster is estimated from the distribution of the translational kinetic energy released (KER), assumed to have the form f(epsilon) approximately epsilon(alpha) exp(-epsilon/k(B)T). Phase space theory (PST) in its orbiting transition state (OTS) version is validated by comparing its predictions to the results of large-scale molecular dynamics simulations. The temperatures estimated from the KER distributions are seen to be generally lower than the actual microcanonical temperature computed from independent Monte Carlo simulations of the product cluster at thermal equilibrium. On the basis of these deviations, the various approximations leading from the rigorous PST/OTS treatment to the assumed exponential form are critically discussed. In the case of Ar(n)(+) clusters, the use of a quantum diatomic-in-molecules Hamiltonian constructed from recent ab initio calculations reveals some possible inadequacies of the 1/r(4) ion/dipole interaction at intermediate distances due to some residual charge transfer.