Direct measurement of single-molecule dynamics and reaction kinetics in confinement using time-resolved transmission electron microscopy.

We report experimental methodologies utilising transmission electron microscopy (TEM) as an imaging tool for reaction kinetics at the single molecule level, in direct space and with spatiotemporal continuity. Using reactions of perchlorocoronene (PCC) in nanotubes of different diameters and at different temperatures, we found a period of molecular movement to precede the intermolecular addition of PCC, with a stronger dependence of the reaction rate on the nanotube diameter, controlling the local environments around molecules, than on the reaction temperature (-175, 23 or 400 °C). Once initiated, polymerisation of PCC follows zero-order reaction kinetics with the observed reaction cross section σobs of 1.13 × 10-9 nm2 (11.3 ± 0.6 barn), determined directly from time-resolved TEM image series acquired with a rate of 100 frames per second. Polymerisation was shown to proceed from a single point, with molecules reacting sequentially, as in a domino effect, due to the strict conformational requirement of the Diels-Alder cycloaddition creating the bottleneck for the reaction. The reaction mechanism was corroborated by correlating structures of reaction intermediates observed in TEM images, with molecular weights measured by using mass spectrometry (MS) when the same reaction was triggered by UV irradiation. The approaches developed in this study bring the imaging of chemical reactions at the single-molecule level closer to traditional concepts of chemistry.

Quantum Chemical Characterization and Design of Quantum Dots for Sensing Applications.

The ability to tune the optoelectronic properties of quantum dots (QDs) makes them ideally suited for the use as fluorescence sensing probes. The vast structural diversity in terms of the composition and size of QDs can make designing a QD for a specific sensing application a challenging process. Quantum chemical calculations have the potential to aid this process through the characterization of the properties of QDs, leading to their in silico design. This is explored in the context of QDs for the fluorescence sensing of dopamine based upon density functional theory and time-dependent density functional theory (TDDFT) calculations. The excited states of hydrogenated carbon, silicon, and germanium QDs are characterized through TDDFT calculations. Analysis of the molecular orbital diagrams for the isolated molecules and calculations of the excited states of the dopamine-functionalized quantum dots establish the possibility of a photoinduced electron-transfer process by determining the relative energies of the electronic states formed from a local excitation on the QD and the lowest QD → dopamine electron-transfer state. The results suggest that the Si165H100 and Ge84H64 QDs have the potential to act as fluorescent markers that could distinguish between the oxidized and reduced forms of dopamine, where the fluorescence would be quenched for the oxidized form. The work contributes to a better understanding of the optical and electronic behavior of QD-based sensors and illustrates how quantum chemical calculations can be used to inform the design of QDs for specific fluorescent sensing applications.

Density Functional Theory Calculations of Core-Electron Binding Energies at the K-Edge of Heavier Elements.

The capability to determine core-electron binding energies (CEBEs) is vital in the analysis of X-ray photoelectron spectroscopy, and the continued development of light sources has made inner shell spectroscopy of heavier elements increasingly accessible. Density functional theory is widely used to determine CEBEs of lighter elements (boron-fluorine). It is shown that good performance of exchange-correlation functionals for these elements does not necessarily translate to the calculation of CEBEs for the heavier elements from the next row of the periodic table, and in general, larger errors are observed. Two strategies are explored that improve the accuracy of the calculated CEBEs. The first is to apply element and functional dependent energy corrections, and the second is a reparametrization of a short-range corrected functional. This functional is able to reproduce experimental phosphorus and sulfur K-edge CEBEs with an average error of 0.15 eV demonstrating the importance of reducing the self-interaction error associated with the core electrons and represents progress toward a density functional theory calculation that performs equally well for ionization at the K-edge of all elements.

Observation of Double Excitations in the Resonant Inelastic X-ray Scattering of Nitric Oxide.

The nitrogen K-edge resonant inelastic X-ray scattering (RIXS) map of nitric oxide (NO) has been measured and simulated to provide a detailed analysis of the observed features. High-resolution experimental RIXS maps were collected using an in situ gas flow cell and a high-transmission soft X-ray spectrometer. Accurate descriptions of the ground, excited, and core-excited states are based upon restricted active space self-consistent-field calculations using second order multiconfigurational perturbation theory. The nitrogen K-edge RIXS map of NO shows a range of features that can be assigned to intermediate states arising from 1s → π* and 1s → Rydberg excitations; additional bands are attributed to doubly excited intermediate states comprising 1s → π* and π → π* excitations. These results provide a detailed picture of RIXS for an open-shell molecule and an extensive description of the core-excited electronic structure of NO, an important molecule in many chemical and biological processes.

Risuteganib Protects against Hydroquinone-induced Injury in Human RPE Cells.

Purpose: Cigarette smoking has been implicated in the pathogenesis of AMD. Integrin dysfunctions have been associated with AMD. Herein, we investigate the effect of risuteganib (RSG), an integrin regulator, on RPE cell injury induced by hydroquinone (HQ), an important oxidant in cigarette smoke. Methods: Cultured human RPE cells were treated with HQ in the presence or absence of RSG. Cell death, mitochondrial respiration, reactive oxygen species production, and mitochondrial membrane potential were measured by flow cytometry, XFe24 analyzer, and fluorescence plate reader, respectively. Whole transcriptome analysis and gene expression were analyzed by Illumina RNA sequencing and quantitative PCR, respectively. F-actin aggregation was visualized with phalloidin. Levels of heme oxygenase-1, P38, and heat shock protein 27 proteins were measured by Western blot. Results: HQ induced necrosis and apoptosis, decreased mitochondrial bioenergetics, increased reactive oxygen species levels, decreased mitochondrial membrane potential, increased F-actin aggregates, and induced phosphorylation of P38 and heat shock protein 27. HQ, but not RSG alone, induced substantial transcriptome changes that were regulated by RSG cotreatment. RSG cotreatment significantly protected against HQ-induced necrosis and apoptosis, prevented HQ-reduced mitochondrial bioenergetics, decreased HQ-induced reactive oxygen species production, improved HQ-disrupted mitochondrial membrane potential, reduced F-actin aggregates, decreased phosphorylation of P38 and heat shock protein 27, and further upregulated HQ-induced heme oxygenase-1 protein levels. Conclusions: RSG has no detectable adverse effects on healthy RPE cells, whereas RSG cotreatment protects against HQ-induced injury, mitochondrial dysfunction, and actin reorganization, suggesting a potential role for RSG therapy to treat retinal diseases such as AMD.

Density Functional Theory Based Methods for the Calculation of X-ray Spectroscopy.

The availability of new light sources combined with the realization of the unique capabilities of spectroscopy in the X-ray region has driven tremendous advances in the field of X-ray spectroscopy. Currently, these techniques are emerging as powerful analytical tools for the study of a wide range of problems encompassing liquids, materials, and biological systems. Time-resolved measurements add a further dimension to X-ray spectroscopy, opening up the potential to resolve ultrafast chemical processes at an atomic level. X-ray spectroscopy encompasses a range of techniques which provide complementary information, and these include X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS). In many studies, the interpretation of the experimental data relies upon calculations to enable the nature of the underlying molecular structure, electronic structure, and bonding to be revealed. Density functional theory (DFT) based methods are some of the most widely used methods for the simulation of X-ray spectra. In this Account, we focus on our recent contributions to the simulation of a range of X-ray spectroscopic techniques using DFT and linear-response time-dependent density functional theory (TDDFT) and show how these methods can provide a computational toolkit for the simulation of X-ray spectroscopy. The importance of the exchange-correlation functional for the calculation of XAS is discussed, and the introduction of short-range corrected functionals is described. The application of these calculations to study large systems through the use of efficient implementations of TDDFT will be highlighted, with the use of these methods illustrated through studies of ionic liquids and transition metal complexes. The extension of TDDFT to calculate XES through the use of a reference determinant for the core-ionized state will be described, and the factors that affect the accuracy of the computed spectra discussed. The application of these approaches will be illustrated through the study of a range of organic molecules and transition metal complexes, which also show how going beyond the dipole approximation in determining the transition intensities can be critical. The application of these approaches to the simulation of the RIXS spectrum of water will also be described, highlighting how ultrafast dynamics on the femtoscale time scale are evident in the measured spectra. In these calculations, the description of the core-ionized and core-excited states becomes increasingly important, and the role of the basis set in accurately describing these states will be explored.

Resveratrol Protects Against Hydroquinone-Induced Oxidative Threat in Retinal Pigment Epithelial Cells.

Purpose: Oxidative stress in retinal pigment epithelial (RPE) cells is associated with age-related macular degeneration (AMD). Resveratrol exerts a range of protective biologic effects, but its mechanism(s) are not well understood. The aim of this study was to investigate how resveratrol could affect biologic pathways in oxidatively stressed RPE cells. Methods: Cultured human RPE cells were treated with hydroquinone (HQ) in the presence or absence of resveratrol. Cell viability was determined with WST-1 reagent and trypan blue exclusion. Mitochondrial function was measured with the XFe24 Extracellular Flux Analyzer. Expression of heme oxygenase-1 (HO-1) and glutamate cysteine ligase catalytic subunit was evaluated by qPCR. Endoplasmic reticulum stress protein expression was measured by Western blot. Potential reactions between HQ and resveratrol were investigated using high-performance liquid chromatography mass spectrometry with resveratrol and additional oxidants for comparison. Results: RPE cells treated with the combination of resveratrol and HQ had significantly increased cell viability and improved mitochondrial function when compared with HQ-treated cells alone. Resveratrol in combination with HQ significantly upregulated HO-1 mRNA expression above that of HQ-treated cells alone. Resveratrol in combination with HQ upregulated C/EBP homologous protein and spliced X-box binding protein 1. Additionally, new compounds were formed from resveratrol and HQ coincubation. Conclusions: Resveratrol can ameliorate HQ-induced toxicity in RPE cells through improved mitochondrial bioenergetics, upregulated antioxidant genes, stimulated unfolded protein response, and direct oxidant interaction. This study provides insight into pathways through which resveratrol can protect RPE cells from oxidative damage, a factor thought to contribute to AMD pathogenesis.

Influence of molecular design on radical spin multiplicity: characterisation of BODIPY dyad and triad radical anions.

A strategy to create organic molecules with high degrees of radical spin multiplicity is reported in which molecular design is correlated with the behaviour of radical anions in a series of BODIPY dyads. Upon reduction of each BODIPY moiety radical anions are formed which are shown to have different spin multiplicities by electron paramagnetic resonance (EPR) spectroscopy and distinct profiles in their cyclic voltammograms and UV-visible spectra. The relationship between structure and multiplicity is demonstrated showing that the balance between singlet, biradical or triplet states in the dyads depends on relative orientation and connectivity of the BODIPY groups. The strategy is applied to the synthesis of a BODIPY triad which adopts an unusual quartet state upon reduction to its radical trianion.

Improving the predictive quality of time-dependent density functional theory calculations of the X-ray emission spectroscopy of organic molecules.

The simulation of X-ray emission spectra of organic molecules using time-dependent density functional theory (TDDFT) is explored. TDDFT calculations using standard hybrid exchange-correlation functionals in conjunction with large basis sets can predict accurate X-ray emission spectra provided an energy shift is applied to align the spectra with experiment. The relaxation of the orbitals in the intermediate state is an important factor, and neglect of this relaxation leads to considerably poorer predicted spectra. A short-range corrected functional is found to give emission energies that required a relatively small energy shift to align with experiment. However, increasing the amount of Hartree-Fock exchange in this functional to remove the need for any energy shift led to a deterioration in the quality of the calculated spectral profile. To predict accurate spectra without reference to experimental measurements, we use the CAM-B3LYP functional with the energy scale determined with reference to a Δself-consistent field calculation for the highest energy emission transition.

Probing the electronic structure of ether functionalised ionic liquids using X-ray photoelectron spectroscopy.

The charge distribution associated with individual components in functionalised ionic liquids (ILs) can be tuned by careful manipulation of the substituent groups incorporated into the ions. Here we use X-ray photoelectron spectroscopy to investigate the impact of substituent atoms on the electronic structure of similar imidazolium-based systems each paired with a common anion, [Tf2N]-. The experimental measurements revealed an unexpected variation in the charge density distribution within the IL cation when the oxygen atom in a poly-ether containing side chain is moved by just one atomic position. This surprising observation is supported by density functional theory calculations.

Infrared spectroscopy of a small ion solvated by helium: OH stretching region of HeN-HOCO.

Messenger spectroscopy is a well-established method for recording infrared (IR) spectra of molecular ions. It relies upon the tagging of weakly bound atoms or molecules, known as the "messenger," to the ion of interest. The ideal tag species is helium since it has the weakest possible interaction with any molecular ion and is consequently the least likely to alter the structure and function. However, the attachment of a helium tag is challenging because of the exceptionally cold conditions that are inherently required. In this work, electron ionization of doped liquid helium nanodroplets has been used to create cations tagged with a variable number (N) of helium atoms. Mass-selective ion detection has made it possible to record IR spectra as a function of N, thus revealing the effect on the structure and charge distribution within the ionic core as solvation becomes more extensive. We illustrate this capability for protonated carbon dioxide tagged with up to 14 helium atoms, HeN-HOCO+. The first atom preferentially binds to the proton and results in a substantial redshift of 44 cm-1 for the OH stretching vibration, while the stepwise attachment of additional atoms up to N = 7 causes small and progressive blueshifts, which are attributed to the gradual formation of a ring of helium around the carbon atom. The methodology described herein offers a new route to obtain IR spectra of He-tagged ions and provides an insight into ion-solvent interactions at the molecular level.

Zinc 1s Valence-to-Core X-ray Emission Spectroscopy of Halozincate Complexes.

The Zn 1s valence-to-core (VtC) X-ray emission spectra of seven ionic liquids have been measured experimentally and simulated on the basis of time-dependent density-functional theory (TDDFT) calculations. Six of the ionic liquids were made by mixing [C8C1Im]X and Zn(II)X2 at three different ZnX2 mole fractions (0.33, 0.50, or 0.67) for X = Cl or Br, and a further ionic liquid was made by mixing [P6,6,6,14]Cl and a mole fraction of ZnCl2 of 0.33. Calculations were performed for the [ZnX4]2-, [Zn2X6]2-, and [Zn4X10]2- ions to capture the expected metal complex speciation. The VtC emission spectra showed three bands arising from single-electron processes that can be assigned to emission from ligand p-type orbitals, zinc d-orbitals, and ligand s-type orbitals. For all seven ionic liquids, the highest occupied molecular orbital arises from the ligand p orbitals, and the spectra for the different size metal complexes for the same X were found to be very similar, in terms of both relative peak intensities and peak energies. For both experiments and TDDFT calculations, there was an energy difference of 0.5 eV between the Cl-based and Br-based metal complexes for the ligand s and p orbitals, while the Zn 3d orbital energies were relatively unaffected by the identity of the ligand. The TDDFT calculations find that for the ions with symmetrically equivalent zinc atoms ([Zn2X6]2- and [Zn4X10]2-), the most appropriate core-ionized reference state has a core-hole that is localized on a single zinc atom. In this framework, the spectra for the larger ions can be viewed as a sum of spectra for the tetrahedral complex with a single zinc atom with small variations in the structure of the coordinating ligands. Because the spectra are relatively insensitive to small changes in the geometry of the ligands, this is consistent with the small variation in the spectra measured in the experiment.

AIRBED: A Simplified Density Functional Theory Model for Physisorption on Surfaces.

Dispersion interactions are commonly included in density functional theory (DFT) calculations through the addition of an empirical correction. In this study, a modification is made to the damping function in DFT-D2 calculations to describe repulsion at small internuclear distances. The resulting Atomic Interactions Represented By Empirical Dispersion (AIRBED) approach is used to model the physisorption of molecules on surfaces such as graphene and hexagonal boron nitride, where the constituent atoms of the surface are no longer required to be included explicitly in the density functional theory calculation but are represented by a point charge to capture electrostatic effects. It is shown that this model can reproduce the structures predicted by full DFT-D2 calculations to a high degree of accuracy. The significant reduction in computational cost allows much larger systems to be studied, including molecular arrays on surfaces and sandwich complexes involving organic molecules between two surface layers.

A scaled CIS(D) based method for the calculation of valence and core electron ionization energies.

The calculation of electron ionization energies is a key component for the simulation of photoelectron spectroscopy. CIS(D) is a perturbative doubles correction for the single excitation configuration interaction (CIS) method which provides a new approach for computing excitation energies. It is shown that by introducing a virtual orbital subspace that consists of a single "ghost" orbital, valence electron ionization energies can be computed using a scaled CIS(D) approach with an accuracy comparable with considerably more computationally intensive methods, such as ionization-potential equation of motion coupled cluster theory, and simulated spectra show a significant improvement relative to spectra based upon Koopmans' theorem. When the model is applied to the ionization energies for core orbitals, there is an increase in the error, particularly for the heavier nuclei considered (silicon to chlorine), although the relative energy of the ionization energies are predicted accurately. In addition to its inherent computational efficiency relative to other wavefunction based approaches, a significant advantage of this approach is that the ionization energies for all electrons can be obtained in a single calculation, in contrast to Δself-consistent field based methods.

Monitoring the Formation and Reactivity of Organometallic Alkane and Fluoroalkane Complexes with Silanes and Xe Using Time-Resolved X-ray Absorption Fine Structure Spectroscopy.

Complexes with weakly coordinating ligands are often formed in chemical reactions and can play key roles in determining the reactivity, particularly in catalytic reactions. Using time-resolved X-ray absorption fine structure (XAFS) spectroscopy in combination with time-resolved IR (TRIR) spectroscopy and tungsten hexacarbonyl, W(CO)6, we are able to structurally characterize the formation of an organometallic alkane complex, determine the W-C distances, and monitor the reactivity with silane to form an organometallic silane complex. Experiments in perfluorosolvents doped with xenon afford initially the corresponding solvated complex, which is sufficiently reactive in the presence of Xe that we can then observe the coordination of Xe to the metal center, providing a unique insight into the metal-xenon bonding. These results offer a step toward elucidating the structure, bonding, and chemical reactivity of transient species by X-ray absorption spectroscopy, which has sensitivity to small structural changes. The XAFS results indicate that the bond lengths of metal-alkane (W-H-C) bond in W(CO)5(heptane) as 3.07 (±0.06) Å, which is longer than the calculated W-C (2.86 Å) for binding of the primary C-H, but shorter than the calculated W-C (3.12 Å) for the secondary C-H. A statistical average of the calculated W-C alkane bond lengths is 3.02 Å, and comparison of this value indicates that the value derived from the XAFS measurements is averaged over coordination of all C-H bonds consistent with alkane chain walking. Photolysis of W(CO)6 in the presence of HSiBu3 allows the conversion of W(CO)5(heptane) to W(CO)5(HSiBu3) with an estimated W-Si distance of 3.20 (±0.03) Å. Time-resolved TRIR and XAFS experiments following photolysis of W(CO)6 in perfluoromethylcyclohexane (PFMCH) allows the characterization of W(CO)5(PFMCH) with a W-F distance of 2.65 (±0.06) Å, and doping PFMCH with Xe allows the characterization of W(CO)5Xe with a W-Xe bond length of 3.10 (±0.02) Å.

Probing Elusive Cations: Infrared Spectroscopy of Protonated Acetic Acid.

Protonated carboxylic acids, (RCOOH)H+, are the initial intermediates in acid-catalyzed (Fischer) esterification reactions. However, the identity of the isomeric form has been debated. Surprisingly, no optical spectra have been reported for any isomer of the protonated carboxylic acid monomer, despite it being a fundamental organic cation. Here, we address these issues by using a new approach to prepare cold He-tagged cations of protonated acetic acid (AA), which entails electron ionization of helium nanodroplets containing metastable dimers of AA. The protonated species is subsequently probed using infrared photodissociation spectroscopy, and following a comparison with calculations, we identify the two isomers whose roles in Fischer esterification are debated. These are the carbonyl-protonated E, Z isomer and the metastable hydroxyl-protonated isomer. Our technique provides a novel approach that can be applied to other elusive ionic species.

Synthesis and characterisation of rylene diimide dimers using molecular handcuffs.

A strategy for positioning, and loosely connecting, molecules in close proximity using mechanically interlocked handcuffs is described. The strategy is demonstrated using rylene diimides, creating dimeric structures in which two components are linked through pillar[5]arene/imidazolium rotaxanes. Investigation of the resulting molecules demonstrates intriguing and new properties that arise from placing these redox active dye molecules together, allowing interactions, whilst allowing the molecules to separate as required. In particular we observe excimer emission from a perylene diimide dimer handcuff and the formation of an unusual radical anion π-dimer upon double reduction of the same molecule. The latter exhibits a unique visible absorption profile for a PDI-based molecule. We demonstrate the flexibility of our approach by making an unprecedented mixed perylene diimide/naphthalene diimide dimer which also reveals interactions between the two components. Our synthetic strategy facilitates the creation of unusual dimeric structures and allows the investigation of intermolecular interactions and the effects they have on electronic and magnetic properties.

Dimers of acetic acid in helium nanodroplets.

The structural arrangement of small carboxylic acid molecules in the liquid phase remains a controversial topic. Some studies indicate a dominance of the cyclic dimer that prevails in the gas phase, whilst other studies favor short fragments of the infinite catemer chains that are found in the crystalline phase. Furthermore, difficulties in preparing and probing size-selected catemer segments have resulted in a lack of benchmark data upon which theoretical models of the condensed phases can be built. To address these issues, we have combined infrared spectroscopy and quantum chemical calculations to explore regions of the intermolecular potential energy surface associated with the formation of metastable dimer isomers. The OH stretching region of the spectrum shows that aggregation of acetic acid molecules inside liquid helium nanodroplets yields two distinct metastable dimers, whilst negligible signal is observed from the cyclic dimer that typically overwhelms this spectral region. We deduce that the most abundant isomer in superfluid helium has one strong O-HO[double bond, length as m-dash]C and one weak C-HO[double bond, length as m-dash]C hydrogen bond. Since this bonding motif is common to the dimeric repeating unit of the catemer, it is of fundamental importance for understanding intermolecular interactions in the condensed phases of small carboxylic acids.

Simulation of the Raman spectroscopy of multi-layered carbon nanomaterials.

Multi-layered carbon nanomaterials can have an important role in modern nanotechnology. Raman spectroscopy is a widely used analytical technique that is used to characterise the structure of these materials. In this work, an approach based upon an empirical potential for the simulation of the Raman spectroscopy of carbon nanomaterials [P. M. Tailor, R. J. Wheatley and N. A. Besley, Carbon, 2017, 113, 299-308] is extended through the addition of a term to describe the van der Waals interaction between layers of sp2 hybridised carbons. The resulting model accurately describes the properties of the shearing modes of few layer graphene and is used to characterise the low frequency modes of multi-walled carbon nanotubes and carbon nanofibres.

Photoionization of the iodine 3d, 4s, and 4p orbitals in methyl iodide.

Ionization of the I 3d, 4s, and 4p orbitals in methyl iodide (CH3I) has been studied by using synchrotron radiation to measure the total ion yield and by recording photoelectron spectra with linearly polarized radiation in two polarization orientations. The complete photoelectron spectrum of CH3I has been recorded at several photon energies, and bands due to the C 1s, I 3d, 4s, 4p, and 4d atomic-like orbitals, as well as the molecular orbitals, have been observed and assigned. In the vicinity of the I 3d5/2 and 3d3/2 ionization thresholds at 626.8 and 638.3 eV, respectively, the ion yield displays weak structure in the pre-edge region due to transitions into valence or Rydberg states, and, at higher energies, a shoulder and a broad maximum attributed to the I 3d5/2 → εf and the I 3d3/2 → εf shape resonances, respectively. The absorption spectrum calculated using time-dependent density functional theory, within the Tamm-Dancoff approximation, has allowed assignments to be proposed for the valence and Rydberg states. The Stieltjes imaging technique has been used to simulate the absorption spectrum above the ionization threshold and indicates that transitions into the f(l = 3) continuum channel dominate. This conclusion has been corroborated by a Continuum Multiple Scattering-Xα (CMS-Xα) calculation. The asymmetric broadening of the photoelectron bands associated with the I 3d orbital, due to post collision interaction, is taken into account in our experimental analysis. Experimentally derived photoelectron anisotropy parameters for the I 3d orbital are in good agreement with the theoretical predictions obtained with the CMS-Xα approach. The I 3d shake-up/shake-off photoelectron spectrum has been recorded, and assignments have been proposed for several of the satellites. The M4N45N45 and M5N45N45 Auger electron yields have been measured, and that for the M5N45N45 decay exhibits a maximum due to interchannel coupling between the 3d5/2 and 3d3/2 continua. The photoelectron band associated with the I 4p orbital has an unusual appearance. Based upon previous theoretical work for the analogous Xe 4p orbital, it appears that the initial I 4p-1 hole state decays rapidly through Coster-Kronig and super-Coster-Kronig transitions. This leads to a redistribution of the spectral intensity associated with the I 4p orbital and results in a photoelectron spectrum containing a single structured band together with an extended continuum. Another continuum is observed on the high binding energy side of the peak due to the 4s orbital, and we assign this to super-Coster-Kronig transitions into the 4p-14d-1 continuum.