Infrared Photodissociation Spectroscopy of Mass-Selected Cluster Ions in the Gas Phase

This habilitation thesis describes my research activities at the Institute for Experimental Physics of the Free University Berlin, which I conducted in the group of Prof. Dr. Ludger Wöste in the period from September 1999 until December 2004. These research activities were the central part of, and financed by, the projects Asmis/Wöste of the Dedicated Research Center „Structure, Dynamics und Reactivity of Transition Metal Oxide Aggregates“ (SFB-546) and the Graduate School „Hydrogen Bonding and Hydrogen Transfer" (GK-788) of the German Research Foundation DFG. The central goal of this work was the development of novel experimental methods to characterize the structure of mass-selected gas phase cluster ions (see Chapter A). Infrared spectroscopy (Chapter B) has been a standard method for structural characterization of condensed phase samples for many decades. Its application to gas phase ions poses mainly two experimental challenges. First, the low number densities of ions attainable in the gas phase, roughly less than one million per cubic centimeter, generally prohibit direct absorption measurements. Second, most of the characteristic infrared transitions lie in the fingerprint region (500 to 2000 cm^-1) of the electromagnetic spectrum, a region which cannot be continuously covered with the required intensity using commercially available infrared radiation sources. To address these problems a novel, mobile tandem mass spectrometer was constructed (Chapter C.1), which allows trapping, cooling, and probing of mass-selected gas phase ions. The infrared photodissociation experiments (Chapter B.1) were performed at the FOM Institute for Plasma Physics Rijnhuizen (Nieuwegein, The Netherlands) using the free electron laser FELIX (Chapter C.2). In these experiments, FELIX is used as a monochromatic “Bunsen burner”, i.e., the ions are irradiated with intense infrared radiation of a specific wavelength. If the wavelength coincides with an infrared transition, the ion is resonantly heated and eventually breaks apart (Chapter B.3). The absorption is detected indirectly by measuring the fragment ion yield, resulting in a high selectivity and sensitivity. The measured infrared spectrum is a “fingerprint” of the molecular structure and its assignment is generally based on a comparison with the simulated spectra of possible candidates. The developed techniques were applied to two research areas. As part of the SFB-546 we were able to measure the infrared spectra of small vanadium oxide ions for the first time and, based on these, characterize their geometric and electronic structure (Chapter D.1). Unexpectedly, we were able to show a correlation between the spectra of a vanadium oxide surface and cluster ion cages of moderate size (~30 atoms). As part of the GK-788 we measured the first infrared spectra of model systems containing strong hydrogen bonds in the spectral region of the shared proton modes (Chapter D.2). The characterization of the spectral signature of the protonated water dimer H₅O₂⁺, also referred to as the „Zundel cation“, was particularly noteworthy. The failure to accurately model this infrared spectrum impressively demonstrates the difficulties of present day electronic structure theory in describing strongly coupled vibrational modes. The experimental work described in this thesis was a team effort and only possible as such. Parts of this work constitute the Ph.D. and Diploma theses of Mathias Brümmer, Sara Fontanella, Oliver Gause, Cristina Kaposta, Gabriele Santambrogio, and Carlos Cibrián-Uhalte