High-Resolution Photoelectron Imaging and Infrared Photodissociation Spectroscopy of Cold Negative Ions
Slow electron velocity-map imaging of cryogenically-cooled anions (cryo-SEVI) is a versatile spectroscopic technique that provides high-resolution detachment spectra of molecular ions, yielding insight into the vibrational and electronic properties of neutral species. This method provides orders-of-magnitude improvement in resolution over earlier measurements, and invariably reveals new subtleties in the resultant spectra. The cryo-SEVI apparatus has multiple ion-generation and photodetachment modes of operation which enable application to a vast range of molecular species, including organic radicals, reactive intermediates, and metal-oxide clusters.
While free radicals are generally highly reactive and difficult to isolate in a laboratory experiment, their corresponding anions are closed-shell, and thus cryo-SEVI is particularly well-suited for characterization of the vibronic structure of neutral radicals with relevance to combustion and atmospheric chemistry. In this thesis, several free radicals were probed by photodetachment of the corresponding anions. The cryo-SEVI spectrum of the tert-butyl peroxide anion showed detachment to two electronic states of the corresponding peroxy radical, giving a number of vibrational and electronic quantities regarding this atmospherically-relevant species. In addition to the peroxy radical, the heterocyclic aromatic radicals derived from hydrogen abstraction from furan and pyridine have been studied. These isomer-specific spectra showed interesting isomeric trends in their photoelectron angular distributions, providing insight into the charge distribution resulting from deprotonation of the parent heterocycle.
In a similar vein, the vinylidene anion (H2CC-) is used to obtain spectroscopic access to neutral vinylidene, a high-energy isomer of acetylene. The isomerization of vinylidene to acetylene on the neutral ground state surface has a remarkably low barrier, resulting in the potential for coupling between vinylidene vibrational states and highly excited levels of acetylene. The extent to which this coupling occurs, and the resultant lifetime of neutral vinylidene, has been the subject of some debate in the physical chemistry community. By performing cryo-SEVI experiments on the vinylidene anion and relating these results to a highly accurate ab initio potential energy surface, the state-specificity of coupling to acetylene was clearly established. The detachment spectra of vinylidene anions also showed other interesting spectroscopic effects, such as vibronic coupling between excited neutral states as well as resonant autodetachment from vibrationally excited anions.
Finally, cryo-SEVI has also been used to investigate gas-phase clusters which serve as models for the defect sites that constitute reactive centers on catalytic surfaces. These defect sites have geometries, stoichiometries, and charge distributions which differ from that of the rest of the surface, and can be challenging to probe in bulk experiments. Gas-phase metal oxide cluster anions thus provide model systems whose properties can be monitored as a function of cluster size and stoichiometry. To this end, two bare aluminum oxide clusters, Al2O2- and Al3O3-, have been characterized using cryo-SEVI. This work revealed electronically-mediated autodetachment from Al2O2-, and established the energy ordering of the close-lying Al3O3- isomers.
Following characterization of the bare cluster anions, it is of interest to characterize the products formed by reaction of metal oxide clusters with molecules of interest to catalysis, providing spectroscopic access to other parts of the potential energy surfaces of model catalytic reactions. To this end, cryo-SEVI was used to interrogate the product formed by reaction of TiO2- with a single H2O molecule, and comparison of these results to the cryo-SEVI spectra of bare TiO2- showed a similar energetic dependence of charge state as is observed for bulk water splitting on titania surfaces.
While cryo-SEVI provides invaluable information regarding neutral species via photodetachment of the corresponding anion, infrared photodissociation (IRPD) experiments may be used to structurally characterize the anions themselves, which can be particularly useful in cases where multiple low-lying isomers are expected. In an IRPD experiment, an ion of interest is complexed with a weakly interacting tagging species (such as Ar or D2) in a cryogenic ion trap, and the resultant cluster is irradiated with tunable infrared light. When the incident light is resonant with a vibrational transition of the ion, the tagging molecule is lost, and so monitoring the mass spectrum following irradiation provides a measurement of the vibrational spectrum of the bare ion. In this thesis, IRPD is used to observe the loss of a D2 tag from microhydrated acetate anions, CH3CO2(H2O)n D2, to determine the first steps in the structural evolution of the first solvation shell for this carboxylate anion.