Weber Research Group ^{Ion Spectroscopy}

1. Gas phase spectroscopy of "solution" species

Much of the behavior of important molecular species is only known in a condensed phase environment (mostly solutions), where interaction with the solvent changes some of the properties of those molecules. In order to elucidate the intrinsic properties of these molecules, and - by comparison with condensed phase data - characterize their interplay with their chemical environment (solvent or surfaces), the molecules have to be studied as isolated entities. This can be achieved using mass selected ion beams.

The deposition of energy into isolated molecules can lead to their fragmentation or (especially for anions) to a change of their charge state. Different from pure mass spectrometric studies, the application of laser spectroscopy affords the deposition of energy in a defined manner by choosing the laser wavelength and the absorbing chromophore. Investigation of fragmentation pathways at different photon energies can lead to new insight into the photophysics and photochemistry of these complex molecules. Based on these ideas, our group combines mass spectrometry with laser spectroscopy to characterize positively and negatively charged species.

Metal-ligand complexes: In this project, we gain a deeper insight into the electronic and geometric structures, and the inter- and intramolecular forces in transition metal complex ions. The experiments contribute valuable information e.g. on the electron donation/back-donation in metal- and metaloxide-ligand complexes and electron binding energies. In addition, they shed more light on the electronic structure of singly and multiply charged metalates [M_nX_m]^k- (M = metal; X = halide) as a function of coordination and size, where the spectra contain information on possible new routes for nanoparticle production. The interpretation of the experiments is aided by quantum chemical calculations.

Biomolecules: We are also interested in UV-induced photofragmentation of gas-phase nucleotides. These experiments provide a basis for a description of the intrinsic photophysical and photochemical properties of nucleotides in comparison to their properties in an aqueous environment. While nucleotides in the absence of solvent can be expected to behave differently from physiological conditions, this comparison may aid in a better molecular-level understanding of possible processes involved in UV photodamage to DNA and of the role of the solvent in condensed phase DNA photochemistry.  In addition, gas-phase experiments provide insight to the fragmentation processes that occur following the activation of oligonucleotides in mass spectrometry studies and may open new laser-based avenues in this area.

2. Cluster studies of partially solvated ions

 
The interaction of ions with their chemical environment plays a prominent role in many processes in physics, chemistry, and biology. Studies of gas phase ion-molecule complexes and cluster ions are well suited to contribute to the understanding of this interaction. On the one hand, the behavior of ligand molecules can be studied in binary complexes with different ions to learn about pairwise interactions, which are often masked in the condensed phase by many-body forces, fluctuations in the solvent shell, and by the presence of solvent molecules distant from the ions. On the other hand, the chemical environment of an ion can be built one molecule at a time in cluster ions. The change in the properties of these ionic systems as a function of cluster size sheds light on the factors dominating the microsolvation environment. In the limit of large cluster sizes, the bulk behavior can be approached, although very large clusters, containing several 10³ or 10⁴ solvent molecules may be necessary to reach this limit. In this project we study complexes of anions with common solvent molecules, such as water or benzene. Using infrared photodissociation spectroscopy, we can obtain interesting information on the structures of these clusters, hydrogen bonding, and mechanisms of solvent-mediated reactions.

3. Energy flow in molecules

All chemical reactions are governed by the nuclear dynamics of molecules, in other words, their patterns of vibrational motion, and any predictive theoretical treatment of chemical reactions needs to describe these motions. Therefore, the way in which vibrational energy flows through, and is redistributed in, molecules after excitation has significant impact on the understanding of chemical reaction dynamics, and for the prospect of coherent control of chemical reactions. Moreover, characterizing energy flow through nanoscale systems has become a critical issue as technology utilizing the progressively smaller sizes of electronic devices encounters the destruction limit of energy density. As a consequence, intramolecular vibrational relaxation (IVR) has long been a field of study. Most experiments in this field have dealt with the question how long it takes for energy to drain out of an exited degree of freedom. "Standard models" of IVR have been used with great success in the modeling of IVR in relatively small molecules. The extension of experiments to larger systems, however, remains a challenging and fertile area for experimental investigation. It is at this break in our current understanding where our approach will provide key data for interfacing the behavior of relatively small molecules accessible for fully quantum mechanical calculations with larger molecules.

We follow a new experimental approach to study the flow of energy as it drains out of a certain vibrational mode and arrives at a well defined place in a molecule. In this approach, we use model systems where the binding energy of an electron in a negatively charged molecule is less than the energies for certain vibrational transitions. This way, we can follow the flow of energy in a molecule by monitoring electron loss and analyzing the kinetic energy distribution of these electrons with high-resolution photoelectron spectroscopy.

4. Supramolecular chemistry and materials at ultrahigh pressures

 

Supramolecular chemistry, i.e., the study of non-covalent interactions between molecules, shows great promise in many fields, from soft condensed matter to materials chemistry and molecular machines. While many researchers investigate the behavior properties of supramolecular assemblies and explore various synthetic routes to new materials at ambient conditions, supramolecular assemblies at high pressure have been largely unexplored.

Very high pressures (in the range of 10⁴ atmospheres and higher) allow access to very unconventional thermodynamic parameters, and open avenues to radically different chemistry of supramolecular assemblies. This could result not only in the observation of completely new interactions in simple substances, but in the generation of fundamentally different chemical species from "ordinary" subunits. We investigate supramolecular assemblies based on aromatic compounds as a function of pressure, monitoring the changes in their electronic absorption spectra as pressure increases. The information encoded in the electronic spectra will enable us to understand the changes in the electronic structure of supramolecular assemblies and the weakly bonded nanostructures created by non-covalent interactions.