KSEF RDE: High Resolution Pulsed-Field-Ionization Ion Spectroscopy of Transition-Metal-Oxide Clusters

I. INTRODUCTION Transition metal oxides are widely used as both catalysts and catalytic supports in petroleum refining and other industrial processes. However, catalyst development is still carried out by try and error, and the rational design of new catalysts with predictable properties is a long-term goal requiring both basic and applied research. To help understand chemistry occurring at the surface of heterogeneous catalysts, an approach termed molecular surface science' has gained considerable attention in surface science2 and gas-phase cluster chemistry. ~ This approach recognizes that many industrial catalysts involve highly dispersed metals, and surface defects are often the active sites for chemistry. Although they will not account for the precise thermodynamics and kinetics operating in applied catalysis, gas-phase measurements provide an efficient means to investigate reactivity patterns, reaction mechanisms, bonding and structures, and structure-reactivity relationships of crucial intermediates. These outcomes of the gas-phase research on a wide range of cluster sizes are valuable to model surface chemistry, surface defects, and surface-reactivity relationships of catalytic materials. The growing interests in the transition metal oxides have motivated numerous research groups to study the size-dependent physical and chemical properties of their clusters in the gas phase, where complications of solvents and counterions are removed. Extensive studies have been reported on reactivity5 and photodissociation,6~8 photoelectron,9" and photoionization spectroscopy.4'8"2 In addition, matrix-isolation infrared spectroscopy has been used to study a number of small neutral clusters.'3 In spite of the extensive studies, the current knowledge about the electronic states and molecular structures of the transition metal oxide clusters is largely from theoretical predictions. However, the reliable prediction of the electronic and geometric structures is complicated by many low-energy structural isomers and high-dense, low-lying electronic states of each isomer. Inconsistent theoretical results have been reported on the ground electronic states and the minimum energy structures of both neutral and ionic clusters.14 Therefore, a reliable identification of the cluster structure generally requires the confirmation by spectroscopic measurements. Previous spectroscopic measurements have yielded valuable structural information for a variety of transition metal oxides. However, some of these measurements have limitations. For example, the multiphoton infrared ionization or dissociation spectroscopy requires a high-power infrared laser with wavelength tunable to far-infrared region, which is still not widely available. Therefore, most of these multiphoton infrared measurements have been carried out with intense free electron lasers. Moreover, the nonlinear nature of the multiphoton process may yield an infrared spectrum that is not necessarily the same as a regular linear absorption spectrum, complicating the interpretation of the measured spectra.7'14'15 On the other hand, the anion photoelectron spectra generally show unresolved vibrational structures for transition metal clusters, except for a few special cases.9" Pulsed-field ionization-zero electron kinetic energy (ZEKE) photoelectron spectroscopy provides exceptional spectral resolution, and we have successftilly applied this tecbnique to study metal-organic complexes.