Düsseldorf 2007 – wissenschaftliches Programm
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MO: Fachverband Molekülphysik
MO 14: Molecular Clusters
MO 14.2: Vortrag
Montag, 19. März 2007, 16:45–17:00, 6D
Master Equation Modeling of Phase Transitions in Low Temperature Black-Body Infrared Radiative Dissociation (BIRD) of Hydrated Ions — Stephan J. Reitmeier1, O. Petru Balaj2, Mirko Gruber1, and •Martin K. Beyer3 — 1Department Chemie, TU München, Germany — 2DCMR-Ecole Polytechnique, Palaiseau, France — 3Insitut für Chemie, TU Berlin, Germany
Black-body infrared radiative dissociation (BIRD) of V(H2O)n+, n = 5 - 21, was studied with a newly developed temperature-controlled ICR cell in a temperature range of 200 - 300 K. Arrhenius plots of the observed unimolecular rate constants for the loss of water molecules exhibit the expected linear behavior, which confirms that the radiation temperature experienced by the trapped ions actually is the measured temperature of the cell walls. Analysis with the standard master equation modeling of a single reactant well yields activation energies for the loss of water molecules which are significantly lower than those calculated with density functional theory or literature values of the water binding energy to protonated water clusters. Ab initio molecular dynamics simulations of selected cluster sizes reveal that some hydrogen bonds which are present in the equilibrium geometry are broken at elevated temperatures. The energy of those hydrogen bonds is present as latent heat in the clusters, which helps to lower the activation energy of water loss. A multi-well master equation model is developed to describe the population of the different phases in the trapped ensemble of ions. Activation energies derived from single and multi-well master equation modeling are compared.