Infrared spectra and fragmentation dynamics of isotopologue-selective mixed-ligand complexes †

Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase Au(12/13CO)n(N2O)m+ complexes. The rich infrared action spectra, which are reproduced well using simulations of calculated lowest energy structures, clarify previous ambiguities in the assignment of vibrational bands, especially accidental coincidence of CO and N2O bands. The fragmentation dynamics exhibit the same unexpected behaviour as reported previously in which, once CO loss channels are energetically accessible, these dominate the fragmentation branching ratios, despite the much lower binding energy of N2O. We have investigated the dynamics computationally by considering anharmonic couplings between a relevant subset of normal modes involving both ligand stretch and intermolecular modes. Discrepancies between correlated and uncorrelated model fit to the ab initio potential energy curves are quantified using a Boltzmann sampled root mean squared deviation providing insight into efficiency of vibrational energy transfer between high frequency ligand stretches and the softer intermolecular modes which break during fragmentation

Gas-phase infrared spectroscopy of metal-containing clusters and complexes

Metal-containing clusters and complexes may represent active sites in heterogeneous catalysts, where catalytic transformations take place, thus offering an elegant approach in studying the fundamental interactions in a simplified way. In this thesis, infrared-(multiple) photodissociation spectroscopy is used to study metal-containing clusters and complexes with atmospherically relevant adsorbates to learn about their reactivity, structural binding motifs, and the degree of activation of adsorbed moieties. Complementary density functional theory calculations help in the assignment of experimentally observed phenomena. Each chapter describes a different system that presents unique and sometimes unexpected results. N2O binding to Pt+n proves to be size-dependent; small clusters form molecular N2O adsorption products whereas larger ones react to produce PtO+, and similar chemistry is observed upon infrared excitation. Examining N2O binding to Au+n and Co+n reveals inertness of gold clusters whereas cobalt species prove to be similar in their reactivity to platinum. Size-selective cooperative binding effects are discovered for AunO2CO− species both from mass spectrometry and spectroscopy. Infrared action spectra of Co(NO)+n and Rh(NO)+n complexes disclose spectral differences in the number and position of the bands as well as similarities in the NO binding motifs to these metal centres. Rh(CO)n(N2O)+m system proves to be different than Au(CO)n(N2O)+m studied by the Mackenzie group previously, and these observations are explained in terms of different electronic structures, relative binding energies, and coordination numbers. Isotopic study of Au(CO)n(N2O)+m using 13CO reveals a band in the infrared spectrum of the Au(12CO)2(N2O)+ complex that is an overlap between individual 12C≡O and N2O N=N vibrational bands

An infrared study of gas-phase metal nitrosyl ion-molecule complexes

We present a combined experimental and quantum chemical study of gas-phase group 9 metal nitrosyl complexes, M(NO)n + (M = Co, Rh, Ir). Experimental infrared photodissociation spectra of mass-selected ion-molecule complexes are presented in the region 1600 cm-1 – 2000 cm-1 which includes the NO stretch. These are interpreted by comparison with the simulated spectra of energetically low-lying structures calculated using density functional theory. A mix of linear and non-linear ligand binding is observed, often within the same complex and clear evidence of coordination shell closing is observed at n = 4 for Co(NO)n + and Ir(NO)n + . Calculations of Rh(NO)n + complexes suggest additional low-lying five-coordinate structures. In all cases, once a second coordination shell is occupied, new spectral features appear which are assigned to (NO)2 dimer moieties. Further evidence of such motifs comes from differences in the spectra recorded in the dissociation channels corresponding to single and double ligand loss

Infrared spectra and fragmentation dynamics of isotopologue-selective mixed-ligand complexes

Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase Au(12/13CO)n(N 2O)m+ complexes. The rich infrared action spectra, which are reproduced well using simulations of calculated lowest energy structures, clarify previous ambiguities in the assignment of vibrational bands, especially accidental coincidence of CO and N2O bands. The fragmentation dynamics exhibit the same unexpected behaviour as reported previously in which, once CO loss channels are energetically accessible, these dominate the fragmentation branching ratios, despite the much lower binding energy of N2O. We have investigated the dynamics computationally by considering anharmonic couplings between a relevant subset of normal modes involving both ligand stretch and intermolecular modes. Discrepancies between correlated and uncorrelated model fit to the ab initio potential energy curves are quantified using a Boltzmann sampled root mean squared deviation providing insight into efficiency of vibrational energy transfer between high frequency ligand stretches and the softer intermolecular modes which break during fragmentation.</p

Imaging the Reduction of Electron Trap States in Shelled Copper Indium Gallium Selenide Nanocrystals Using Ultrafast Electron Microscopy

Insertion of alkali metal ions, especially Na, is a well-established method to significantly increase the power conversion efficiency of copper indium gallium selenide (CIGSe)-based photovoltaic devices. However, although it is known that Na ions mostly reside on the surface of CIGSe layer following diffusion, the exact mechanism of how Na affects the carrier dynamics of CIGSe still remains ambiguous. This is mainly due to the unavailability of suitable surface-sensitive techniques. Herein, we employ four-dimensional scanning ultrafast electron microscopy (4D S-UEM), which has the unique capability of mapping the charge carrier dynamics in real time and space selectively on the materials surfaces, to directly observe the effect of Na insertion on the carrier dynamics of shelled CIGSe film. It is found that an additional layer of NaF to the thin film of ZnS-shelled CIGSe nanocrystals not only increases the grain size and improves the texture of the film but, more importantly, reduces fast electron trap channels on the surface of the material, as observed from the secondary electron dynamics in 4D S-UEM. Our density functional theory calculations further confirm that Na ions can occupy Cu vacancies and reduce the interfacial charge carrier-defect scatterings. Removal of such undesirable electron trapping channels results in increased photoconductivity of the material, thereby serving as one of the critical parameters that lead to enhancement of the efficiency of CIGSe for light harvesting purposes