Electron attenuation in free, neutral ethane clusters.

The electron effective attenuation length (EAL) in free, neutral ethane clusters has been determined at 40 eV kinetic energy by combining carbon 1s x-ray photoelectron spectroscopy and theoretical lineshape modeling. More specifically, theory is employed to form model spectra on a grid in cluster size (N) and EAL (λ), allowing N and λ to be determined by optimizing the goodness-of-fit χ(2)(N, λ) between model and observed spectra. Experimentally, the clusters were produced in an adiabatic-expansion setup using helium as the driving gas, spanning a range of 100-600 molecules in mean cluster size. The effective attenuation length was determined to be 8.4 ± 1.9 Å, in good agreement with an independent estimate of 10 Å formed on the basis of molecular electron-scattering data and Monte Carlo simulations. The aggregation state of the clusters as well as the cluster temperature and its importance to the derived EAL value are discussed in some depth.

Structure of self-assembled free methanol/tetrachloromethane clusters.

The structure of molecular clusters of diameters at or below a nanometer is important both in nucleation phenomena and potentially for the preparation and application of nanoparticles. Little is known about the relationship between the structure and composition of the cluster and about the interplay between cluster composition, size, and temperature. The present project explores how the structure of mixed CH3OH/CCl4 clusters vary with composition and size; implicitly by changing the amount of noncondensing backing gas and thus the capacity to remove heat during cluster condensation, and explicitly through theoretical models. Experimentally, molecular clusters were produced by coexpansion of helium and a vapor of azeotropic methanol/tetrachloromethane composition in a supersonic nozzle flow. The clusters were subsequently characterized by means of carbon 1s photoelectron spectroscopy. Additional information was obtained by molecular-dynamics simulations of clusters at 3 different sizes, 4 different compositions and several temperatures, and using polarizable force fields. Mixed clusters were indeed obtained in the coexpansion experiments. The clusters show an increasing degree of surface coverage by methanol as the backing pressure is lowered, and at the lowest helium pressure the cluster signal from tetrachloromethane has almost vanished. The MD simulations show a gradual change in cluster structure with increasing methanol contents, from that of isolated rings of methanol at the surface of a tetrachloromethane core, to a contiguous methanol cap covering more than half of the cluster surface, to that of subclusters of tetrachloromethane submerged in a methanol environment. Both experimental and computational results support a thermodynamical driving force for methanol to dominate the surface structure of the mixed clusters. At high helium pressure, the growing clusters may cool efficiently, possibly impeding the diffusion of methanol to the surface. At low helium pressure, methanol is completely dominating the outermost few layers of the clusters, possibly in parts caused by preferential loss of tetrachloromethane through evaporative cooling.

Nonstoichiometric intensities in core photoelectron spectroscopy.

X-ray photoemission spectroscopy is used in a great variety of research fields; one observable is the sample's stoichiometry. The stoichiometry can be deduced based on the expectation that the ionization cross sections for innershell orbitals are independent of the molecular composition. Here we used chlorine-substituted ethanes in the gas phase to investigate the apparent carbon stoichiometry. We observe a nonstoichiometric ratio for a wide range of photon energies, the ratio exhibits x-ray-absorption fine structure spectroscopy (EXAFS)-like oscillations and hundreds of eV above the C1s ionization approaches a value far from 1. These effects can be accounted for by considering the scattering of the outgoing photoelectron, which we model by multiple-scattering EXAFS calculations, and by considering the effects of losses due to monopole shakeup and shakeoff and to intramolecular inelastic scattering processes.

Structure of neutral nanosized clusters produced by coexpansion of CF4 and CH4.

Mixed CH(4)/CF(4) clusters as well as pure clusters of CF(4) were produced by adiabatic expansion and studied by carbon 1s (C1s) X-ray photoelectron spectroscopy. Evidence is presented that CH(4) and CF(4) do indeed form binary clusters in CH(4)/CF(4) coexpansion experiments and that these clusters exhibit radial structure; i.e., CF(4) is primarily found in the bulk. The interpretation of the photoelectron spectra is supported by calculations of C1s ionization energies based on theoretical clusters models.

Size of free neutral CO2 clusters from carbon 1s ionization energies.

Free neutral CO(2) clusters were produced by adiabatic expansion and characterized by carbon 1s (C1s) photoelectron spectroscopy using synchrotron radiation. The shift in C1s ionization energy (IE) between the cluster and the monomer, i.e., ΔIE = IE(cluster) - IE(monomer), was found to vary systematically with the experimental conditions. A functional relationship is established between the mean cluster size in the beam, , and ΔIE, in good agreement with theoretical calculations of shifts in ionization energy for model clusters. This makes it possible to use core-level photoelectron spectroscopy to monitor the mean cluster size and also to estimate from expansion conditions.

Surface relaxation in water clusters: evidence from theoretical analysis of the oxygen 1s photoelectron spectrum.

We present a theoretical interpretation of the oxygen 1s photoelectron spectrum published by Ohrwall et al. [J. Chem. Phys. 123, 054310 (2005)]. A water cluster that contains 200 molecules was simulated at 215 K using the polarizable AMOEBA force field. The force field predicts longer O...O distances at the cluster surface than in the bulk. Comparisons to ab initio molecular dynamics (MD) simulations indicate that the force field underestimates the degree of surface relaxation. By comparing cluster lineshape models, computed from MD simulations, to the experimental spectrum we find further evidence of surface relaxation.

What can C1s photoelectron spectroscopy tell about structure and bonding in clusters of methanol and methyl chloride?

Single-component clusters of methanol and methyl chloride have been produced by adiabatic expansion, and their carbon 1s photoelectron spectra were recorded using synchrotron radiation and a high-resolution electron analyzer. The experimental spectra are interpreted by means of theoretical models based on molecular dynamics simulations. The data are used to explore to what extent core-level photoelectron spectra may provide information on the bonding mechanism and the geometric structure of clusters of polar molecules. The results indicate that the cluster-to-monomer shift in ionization energy and also the width of the cluster peak may be used to distinguish between hydrogen bonding and weaker electrostatic interactions. Moreover, the larger width of the cluster peak in methanol clusters as compared to methyl chloride clusters is partly due to the structured surface of methanol clusters. Theoretical modeling greatly facilitates the analysis of core-level photoelectron spectra of molecular clusters.

Effects of molecular conformation on inner-shell ionization energies.

Experimental evidence for an effect of molecular conformation on inner-shell ionization energies has been observed for the first time. Examples are seen in the carbon 1s spectra of butyronitrile, 1-fluoropropane, and propanal, and other similar molecules. At room temperature these exist in two different conformations, with different distances and, hence, different Coulombic interactions between the negatively charged electronegative group and the methyl carbon. The experimental results are in accord with theoretical predictions with respect to both ionization energies and populations of the different conformers.

Two size regimes of methanol clusters produced by adiabatic expansion.

Free neutral methanol clusters produced by adiabatic expansion have been studied by photoelectron spectroscopy and line shape modeling. The results show that clusters belonging to two distinct size regimes can be produced by changing the expansion conditions. While the larger size regime can be well described by line shapes calculated for clusters consisting of hundreds of molecules, the smaller size regime corresponds to methanol oligomers, predominantly of cyclic structure. There is little contribution from dimers to the spectra.

Lineshapes in carbon 1s photoelectron spectra of methanol clusters.

A general protocol for theoretical modeling of inner-shell photoelectron spectra of molecular clusters is presented and applied to C1s spectra of oligomers and medium-sized clusters of methanol. The protocol employs molecular dynamics for obtaining cluster geometries and a polarizable force field for computing site-specific chemical shifts in ionization energy and linewidth. Comparisons to spectra computed from first-principle theories are used to establish the accuracy of the proposed force field approach. The model is used to analyze the C1s photoelectron spectrum of medium-sized clusters in terms of surface and bulk contributions. By treating the surface-to-bulk ratio as an adjustable parameter, satisfactory fits are obtained to experimental C1s spectra of a beam of methanol clusters.

Size of neutral argon clusters from core-level photoelectron spectroscopy.

Theoretical models of lineshapes in Ar2p photoionization spectra have been calculated for free, neutral argon clusters of different sizes. The lineshape models are fitted to experimental spectra and used to estimate the mean cluster size realized in the experiment. The results indicate that size estimators working from stagnation conditions [R. Karnbach, M. Joppien, J. Stapelfeldt, J. Wörmer and T.Möller, Rev. Sci. Instrum., 1993, 64, 2838] may underestimate the mean cluster size.

Conformational effects in inner-shell photoelectron spectroscopy of ethanol.

The carbon 1s photoelectron spectrum of ethanol shows two peaks, one for the methyl carbon and one for the functionalized carbon. While the peak shape for the functionalized carbon is readily understood, the shape for the methyl carbon requires that there be comparable contributions from both the anti and gauche conformers of ethanol and that the torsional motion in the HOCC dihedral angle be strongly excited upon core ionization. An accurate description of the peak shape requires a high level of electronic-structure theory together with consideration of anharmonicity and coupling of the torsional motion with other vibrational modes.

Chemical insights from high-resolution X-ray photoelectron spectroscopy and ab initio theory: propyne, trifluoropropyne, and ethynylsulfur pentafluoride.

High-resolution carbon 1s photoelectron spectroscopy of propyne (HC triple bond CCH3) shows a spectrum in which the contributions from the three chemically inequivalent carbons are clearly resolved and marked by distinct vibrational structure. This structure is well accounted for by ab initio theory. For 3,3,3-trifluoropropyne (HC triple bond CCF3) and ethynylsulfur pentafluoride (HC triple bond CSF5), the ethynyl carbons show only a broad structure and have energies that differ only slightly from one another. The core-ionization energies can be qualitatively understood in terms of conventional resonance structures; the vibrational broadening for the fluorinated compounds can be understood in terms of the effects of the electronegative fluorines on the charge distribution. Combining the experimental results with gas-phase acidities and with ab initio calculations provides insights into the effects of initial-state charge distribution and final-state charge redistribution on ionization energies and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF5 and CF3 have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies.