IRMPD Spectra of Water Clusters and Strongly Bound Systems

Molecular spectroscopy provides us with very detailed information on properties of molecules, for example bond lengths between atoms, distribution of electrons in a molecule or its reactivity. Experimentally measured spectra might tell us that various structures (isomers) coexist for a given molecular stoichiometry or that a molecule decomposes through several reaction channels. The gained knowledge can be then employed in modeling various chemical processes, e.g., in atmospheric chemistry, reactivity studies or combustion. However, such information cannot be always easily extracted from the spectra, the principal limitation being theoretical analysis that is not trivial even for small molecules composed of several atoms. In the present project, we focus on infrared multiple photon dissociation (IRMPD), a spectroscopic technique during which a molecule absorbs infrared photons until a dissociation event is observed. In other words, we provide energy to the molecule in the form of irradiation and, when the energy reaches a certain amount, the molecule decomposes. Notably, the IRMPD spectra include information on the state of the target molecule before absorption of the IR photons as well as on elementary steps taking place during absorption of photons. The method is used in a wide range of applications, including biology. The IRMPD process is non-trivial especially in two molecular systems: 1) In hydrated molecules and ions in which decomposition proceeds over several stages, with water molecules desorbing from the cluster through complicated pathways. 2) In strongly bound systems that need a high amount of energy before they dissociate. In both cases, we are confronted with many degrees of freedom that make understanding of such spectra challenging. To model the IRMPD spectra, we will use methods of theoretical chemistry, describing nuclei and electrons through a mixture of classical and quantum physics. We will focus on the processes from the viewpoint of the molecule: How probable is it do absorb a photon coming, e.g., from a laser or the sun? In which state do we find the molecule at the given moment? How probable is molecular dissociation, with which fragments? This statistical approach will allow us to predict the fate of the molecule in every moment, getting as close to the actual process as possible. Cooperation with experimentalists is a crucial aspect of the project. We will collaborate with two experimental groups, the group of Prof. Martin K. Beyer at the University of Innsbruck and Dr. Joost M. Bakker at the Radboud University, Netherlands. They will provide us with the experimental input as well as an immediate feedback on the quality of our modeling approach. The cooperation will also enable us to suggest new experiments and challenge our modeling assumptions.