Slow Photoelectron Velocity-Map Imaging and Infrared Photodissociation Spectroscopy of Cryogenically-Cooled Ions

Both slow photoelectron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) and infrared photodissociation (IRPD) spectroscopy are employed to probe anions and cations, respectively. Such techniques are capable of providing insight into the vibrational, electronic, and geometric properties of these small molecules, which are facilitated by the high resolution afforded by each method. Further, recent developments have enhanced the abilities of cryo-SEVI to probe vibrationally excited anions (IR-cryo-SEVI), and plans are underway on further improving the cooling abilities of the machine (cryo-cryo-SEVI), allowing for an expanded range of viable molecular targets. In the meantime, there is a vast array of viable molecular species accessible to both cryo-SEVI and IRPD, with systems studied here falling into the categories of free radicals, interstellar species, and metal oxide clusters./nAnion photoelectron spectroscopy (PES) is a powerful technique for studying transient neutral species, owing to the ease with which stable anions are photodetached to access these states. Cryo-SEVI is a high-resolution variant of anion PES that exploits the resolving properties of velocity-map imaging by employing a tunable laser source to achieve sub milli-electronvolt (meV) resolution for many species. This is further enhanced by the cryogenic cooling of anions in a radiofrequency ion trap prior to photodetachment, greatly improving spectral clarity and giving access to a larger array of systems./nThe systems capable of being studied by cryo-SEVI, however, are limited by the ability to cool them sufficiently. To this end, development of a second ion trap has begun, allowing for the study of larger species, especially metal oxides, which are of considerable temperature entering the trap. Installing this second trap should then give access to larger clusters, as well as allow for the introduction of a reaction gas to study how such species react with small molecules, possibly elucidating catalytic reaction mechanisms./nMeanwhile, IRPD spectroscopy, a complementary method to cryo-SEVI, can readily characterize the structures of large metal oxide clusters. Here, cations (I$^+$) are mass selected, collected in an ion trap, and messenger-tagged with He. These species are then irradiated with intense, tunable IR light and extracted into a time-of-flight mass spectrometer to determine the depletion of I$^+$He as a function of photon energy. IRPD spectra then yield vibrational frequencies with comparable resolution to cryo-SEVI, allowing for the determination of geometries and vibrational frequencies when compared with simulation./nCarbon and silicon carbide clusters are structurally complex species of great interest in interstellar, plasma, and combustion chemistry. Cryo-SEVI spectra of C$_7$ and C$_9$ allow for the extraction of previously unresolved vibrational frequencies, as well as evidence of vibronic coupling effects to numerous electronic excited states. Small silicon carbides are important astrochemically as a number of them have been observed in interstellar space, though the relative energetics of many of these species are in question, as there exist multiple low-lying stable isomers. SEVI spectra of 4-atom silicon carbides (Si$_3$C, Si$_2$C$_2$, and SiC$_3$) shed light on this energetic ordering, elucidate new vibrational frequencies in these species, and observe the first Si$_2$C$_2$ structure with a permanent dipole./nAmong the free radicals studied are the nitrate radical (NO$_3$) and the hydroxy radical (OH). Cryo-SEVI spectra of NO$_3$ reveal the extent to which vibronic coupling shapes this molecule's vibrational structure, quelling a controversy surrounding the position of the $\nu_3$ mode of this species. Study of the hydroxy radical was facilitated by recent development of IR-cryo-SEVI, wherein anions are vibrationally pre-excited prior to photodetachment, allowing for the probing of previously inaccessible regions of the neutral potential energy surface. This method, showcased by the photodetachment of vibrationally-excited OH$^-$, results in newly allowed features to arise in the spectra of this molecule as well as characterization of the anion's vibrational frequency without the use of a messenger-tag, as is in IRPD./nTransition metal oxides serve as a catalysts for many fundamental reactions in chemistry, with the active site often occurring at molecular-scale defects. Given the challenge of studying such active sites, it has become commonplace to use small gas-phase clusters as models for these defect sites, which have the benefit of being easy to produce and tractable for theoretical comparison. The cryo-SEVI spectra of ZrO$_2$ reacting with H$_2$O revealed the coexistence of two structural isomers of the product, arising from a ``hot'' ion distribution ``frozen in" to the cold, trapped population. Comparison of the electron affinities of this and the titanium analogue of the system, as well as the un-reacted clusters, provides insight into the reactivity of these clusters. Further, the IRPD spectra of (NiO$_m$)(Al$_2$O$_3$)$_n$(AlO)$^+$ with $m$ = 1-2 and $n$ = 1-3, a model for Ni/Al$_2$O$_3$ - industrial catalyst for oxidative dehydrogenation with high selectivity, are presented. Comparison with theory shows that the structures formed lead to under-coordinated Nickel centers that may elucidate the catalytic mechanism of bulk Ni/Al$_2$O$_3$. Finally, cryo-SEVI spectra of NdO characterize the energetics of this species, including detachment transitions to high-lying excited states that may help explain previous observations from atmospheric release experiments.

Advisor: Neumark, Daniel M