Two high-resolution techniques, slow photoelectron velocity-map imaging (SEVI) spectroscopy and infrared photodissociation (IRPD) spectroscopy, are used to probe cryogenically cooled anionic molecules and clusters. The systems studied here fall into four categories: astrochemical species, aromatic radicals, transition metal oxide clusters, and the reactive potential energy surfaces of bimolecular reactions.
Anion photoelectron spectroscopy (PES) is a versatile technique for studying transient neutral species via photodetachment of a stable anion. SEVI is a variant of anion PES which combines tunable laser energy and a velocity-map imaging detection scheme. Slow photoelectrons are preferentially detected, yielding sub milli-electronvolt (meV) spectra over a narrow electron kinetic energy range. SEVI yields detailed information on the geometry, vibrational frequencies, and electronic structure of the neutral molecule. Cryogenic cooling of anions in a radio frequency ion trap prior to photodetachment eliminates spectral congestion and allows for analysis of complex systems.
IRPD spectroscopy is a method complementary to cryo-SEVI for structural characterization of complex gas-phase ions. Anions (A−) are mass selected, collected in a cryogenic ring-electrode ion trap, and messenger-tagged with D2. The trapped ions are irradiated with intense, tunable IR light, then extracted into a time-of-flight mass spectrometer. The IR absorption spectrum of A− is constructed by measuring the depletion of the A−D2 species as the wavelength is scanned. IRPD spectra yield vibrational frequencies with meV precision. Geometries and vibrational modes are assigned through comparison with simulation.
Carbon clusters are structurally complex and of great interest in interstellar, plasma, and combustion chemistry. The cryo-SEVI spectrum of the C5 carbon cluster shows vibrational fine structure and subtle vibronic coupling effects not previously resolved. The cyanomethyl radical, CH2CN, is important astrochemically as an open-shell carbon-containing species, and its corresponding anion may be a carrier of a diffuse interstellar band. Temperature-dependent SEVI spectra show new rotational and vibrational structure of CH2CN, CH2CN−, and their deuterated isotopologs.
The ortho-hydroxyphenoxy (o-HOC6H4O) radical is a model for the photochemistry of larger biomolecules. Characterizing excited state surfaces and the dynamics of non-radiative relaxation pathways in these species can elucidate the chemistry subsequent to photoexcitation of biological systems. The SEVI spectra of the first three electronic states of o-HOC6H4O clarify the energetics and vibrational frequencies of these states. The deprotonated polycyclic aromatic hydrocarbon radicals α- and β-naphthyl (C10H7) and 9-, 1-, and 2-anthracenyl (C14H9) are intermediates in the combustion of organic matter and soot formation, and may be present in the interstellar medium. A gas-phase synthesis technique using trimethylsilyl-substituted precursors allows for the acquisition of isomer-specific SEVI spectra for these species. Detailed vibrational structure of the ground and first excited states of each isomer is reported and assigned.
Transition metal oxides catalyze many reactions fundamental to chemistry. Small gas-phase clusters can serve as models for catalytically active point defect sites on surfaces, which often demonstrate distinct bonding and stoichiometry from the bulk. The cryo-SEVI spectra of the ferromagnetic iron suboxide clusters Fe4O and Fe5O, model systems for oxygen-deficient metal oxide catalytic sites, illuminate the vibrational and electronic structure of these species. Bulk titanium dioxide is of great interest as a water-splitting photocatalyst to generate sustainable hydrogen fuel. We report IRPD spectra of the anionic clusters (TiO2)n− (n=3-8) and the reactive complexes of (TiO2)n− (n=2-4) with D2O. With comparison to calculations, these spectra confirm the most stable cluster geometries and provide detailed information on their vibrational structure. These results aid in understanding the size-dependent evolution of the properties and reactivity of (TiO2)n− clusters, and their potential utility as model systems for water-splitting catalysts.
Anion PES is one of the few ways to gain spectroscopic access to the transition states of neutral bimolecular reactive surfaces. In a transition state spectroscopy experiment, photodetachment of a bound anion similar in geometry to the desired neutral transition state can yield a spectrum showing structure very sensitive to the shape of the neutral potential energy surface. In particular, it is possible to observe discrete quantum resonances that are bound or quasibound along the reaction coordinate. Such resonances are an exceptional point of comparison between theory and experiment. Cryo-SEVI is used to study the benchmark F + H2 reaction through photodetachment of the FH2− anion. We report previously unresolved peaks that are attributed to long-predicted reactive scattering resonances. We also discuss a cryo-SEVI study of the seven-atom F + CH3OH hydrogen abstraction reaction based on photodetachment of CH3OHF−. This measurement reveals structure associated with a manifold of vibrational Feshbach resonances and bound states supported by the post-transition-state potential well. For both the F + H2 and F + CH3OH studies, high-level quantum dynamical calculations yield excellent agreement with experimental results, allow assignment of structure, and demonstrate the utility of cryo-SEVI transition state spectroscopy experiments as benchmarks for the study of increasingly complex bimolecular reactions.