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In the second period of this project, we have investigated several systems where negative charge plays an important role in aromatic molecules (nitroarenes, pentafluorobenzene and naphthalene-water clusters). We also followed an experimental avenue that opened itself to us during the investigation of nitroarenes and led to exciting results for the study of intramolecular relaxation in hydrocarbon molecules.

If the binding energy of an excess electron is lower than some of the vibrational levels of its host anion, vibrational excitation can lead to autodetachment. We used excitation of CH stretching modes in nitroalkane and nitroarene anions (where the excess electron is localized predominantly on the NO₂ group) to achieve vibrational autodetachment. We obtained data on nitroalkane anions of various chain lengths, showing that monitoring generation of neutral photoproducts by vibrational autodetachment from mass selected anions is a valid approach to the vibrational spectroscopy of such systems, going to nitroalkane anions at least the size of nitropentane. We compared spectra taken using vibrational autodetachment with spectra obtained by monitoring Ar evaporation from Ar solvated nitroalkane anions and found them to be very similar. In the neutralization spectra, vibrational resonances are superimposed on a background due to direct detachment. The spectra of nitromethane and nitroethane were assigned based on ab initio calculations. The results have been published in J. Phys. Chem. A 2008, 112, 7498–7506.

In the case of the nitroarene anions investigated (nitrobenzene and nitrotoluene), the electron affinities are on the order of 1 eV, which means that vibrational autodetachment from a molecule in its ground state is impossible by absorption of one photon in the CH stretching region (around 3000 cm^-1). Curiously, we still observe vibrational autodetachment, most likely from vibrationally hot molecules. The resulting spectra are again similar to those obtained by Ar predissociation of Ar-solvated anions, but somewhat broader. A particularly interesting similar case is the pentafluorobenzene anion, where the adiabatic electron affinity is estimated to be < 0.43 eV, but its precise value is unknown. We have used a recently constructed velocity-map photoelectron imaging spectrometer to measure the photoelectron spectrum of pentafluorobenzene at 1.16 eV photon energy and found only a very broad envelope without being able to resolve individual vibrational bands. This broad envelope is most likely due to a long and congested progression in several Franck-Condon active out-of-plane vibrational modes, since the anion is strongly deformed from the planar geometry. It is unclear, if the 0-0 band can be observed in direct photodetachment, since the Franck-Condon factors for this process may be very unfavorable. One way to potentially observe the 0-0 band and thereby determine the adiabatic electron affinity of pentafluorobenzene could be to use vibrational autodetachment on the CH stretching resonance and analyze the photoelectron energy distribution. Results from our work on vibrational autodetachment are presently in preparation for publication in an invited Feature Article in J. Phys. Chem. A.

A very interesting project in the past grant period has been to investigate the structure of naphthalene-water cluster anions. Naphthalene (Np) as the smallest polyaromatic hydrocarbon molecule presents an extended aromatic system to which water can dock. Some questions of importance in this context are where the water molecules prefer to bond and how flat the potential energy surface is. Neither of the constituents of the cluster has a positive electron affinity, but the dimer anion [Np-H₂O]^- exists. This complex is highly labile with respect to electron loss. Interestingly, excitation of OH stretching transitions of the water molecule led to loss of the electron from the complex (at least up to solvation with 6 Ar atoms), while CH stretching modes on the Np moiety did not, even for moderate Ar solvation. This demonstrates the importance of the water molecule in stabilizing the negative charge, which is probably mostly localized in the aromatic system. Preliminary ab initio calculations show several possible minimum energy structures. In the lowest energy isomer, the water ligand is bridging the two rings, tethered to two C atoms on either side of the central C-C bond of the Np moiety. Addition of the water molecules serves to greatly stabilize the charge. We were able to use Ar solvation to determine the spectrum of [Np-(H₂O)₂]^- and [Np-(H₂O)₃]^-, extending both the size range and the signal-to-noise-ratio of the data on this complex. Our preliminary findings suggest that the structures of the water cluster subsystems for two and three water molecules is very similar to those of pure water cluster anions, forming water networks on the graphenic surface of the Np moiety. The aromatic system of the Np moiety provides a “scaffold” for the excess electron, which otherwise would occupy a very large and diffuse purely dipole-bound state. The experiments on this system have been finished, but the interpretation is still ongoing. Currently, we work on ab initio calculations to conclude our search for candidate structures.

Funds from this grant were mainly used for the support of two graduate students. Holger Schneider graduated with a PhD in December 2008. A large part of his work has been supported by this grant, and thus has contributed enormously to his graduate education. He is now a postdoctoral worker at the Paul Scherrer Institure in Switzerland. Christopher Adams has taken over from Holger Schneider in this project. He has been in charge of the naphthalene-water cluster anion project and has been instrumental in the experiments on vibrational autodetachment. PRF funding has resulted in four publications so far, with at least two more in preparation. This has been the first grant during my time as an Assistant Professor, and it has been extremely valuable. It has enabled a considerable amount of my research program. In addition, this program has almost certainly helped to secure major funding in the form of an NSF CAREER Award, where a part of the experimental program builds on results from this PRF grant (investigation of anionic metal-ligand complexes, e.g. M^-...C₆H₆).