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Access to accurate reference data is a prerequisite in order to translate chemical shifts to an absolute scale for inner-shell ionization energies. Calibration standards for oxygen 1s (O 1s) ionization energies are less well established than, for instance, for carbon 1s. To improve upon this situation, adiabatic and vertical O 1s ionization energies for gaseous carbon dioxide (CO2) are critically reviewed and used to establish the most accurate values currently available: 541.085(17) and 541.253(17) eV, respectively. Combining these with new precise measurements of shifts in O 1s ionization energies for H2O, CO, and O2 allows us to establish equally accurate absolute ionization energies for these molecules as for CO2. The resulting adiabatic and vertical energies are 539.728(17) and 539.827(17) eV for H2O, 542.439(17) and 542.495(17) eV for CO, 543.285(17) and 543.294(17) eV for O2 (4Σ final state), and 544.338(17) and 544.423(17) eV for O2 (2Σ final state). It is proposed that O 1s in CO2 be adopted as a standard of higher precedence, and that H2O, CO, and O2 be used also. The O 1s ionization energies in these molecules occur in the range 540–543 eV. These proposed standards should provide optimal internal calibration for a wide range of oxygen-containing compounds. publishedVersion

Electron attenuation due to inelastic and elastic scattering in condensed media can often be described in terms of the effective attenuation length (EAL) of the electron. The EAL is thus an important parameter for describing electron transport processes as exemplified by dissipation of energy following radiolysis. Focusing on electrons at low electron kinetic energies (10-40 eV) in condensed argon, we determine EAL from x-ray photoelectron spectra of argon nanoparticles and compare to values obtained from valence ionization in thin argon films as well as from gas-phase electron-scattering data. EAL determined from argon clusters shows variation with cluster size. Moreover, the values are significantly lower than those obtained in valence-ionization studies and from scattering data. Our results corroborate recent x-ray photoelectron spectroscopy-based determination of EALs of water showing large differences to the EALs determined by other methods in amorphous ice at low kinetic energies of the photoelectron [Y.-I. Suzuki, K. Nishizawa, N. Kurahashi, and T. Suzuki, Phys. Rev. E 90, 010302 (2014)PLEEE81539-375510.1103/PhysRevE.90.010302], underlining that care must be taken when using EAL values from other sources for core-level electrons.

Molecular LiCl and Li2Cl2 have been studied in the vapor phase with valence photoelectron and photoelectron-photoion coincidence spectroscopies. These two techniques determine the binding energies in fundamentally different ways. Binding energies obtained from photoelectron spectra are usually taken as the vertical ionization energies of the corresponding electronic states. In cases with several overlapping bands, corresponding to different electronic states, the coincidence measurement can separate the bands if the respective final states fragment differently. This applies well to the monomer case. To facilitate the determination of state-specific ionization energies in the dimeric molecule, a theoretical Franck-Condon analysis has been carried out. Moreover, ab initio coupled-cluster and density-functional-theory calculations have been used to analyze the fragmentation pattern based on asymptotic dissociation energies. The fragmentation pattern is largely common to all the accessible valence-ionized states of the chiller, consistent with rapid conversion to the ionic ground state before fragmentation. However, the highest-lying state of Li2Cl2+, (2)A(g). shows enhanced propensity for Li+ as dissociation product. (C) 2012 Elsevier B.V. All rights reserved. (Less)

A database of 77 adiabatic carbon 1s ionization energies has been prepared, covering linear and cyclic alkanes and alkenes, linear alkynes, and methyl- or fluoro-substituted benzenes. Individual entries are believed to carry uncertainties of less than 30 meV in ionization energies and less than 20 meV for shifts in ionization energies. The database provides an unprecedented opportunity for assessing the accuracy of theoretical schemes for computing inner-shell ionization energies and their corresponding chemical shifts. Chemical shifts in carbon 1s ionization energies have been computed for all molecules in the database using Hartree-Fock, Møller-Plesset (MP) many-body perturbation theory of order 2 and 3 as well as various approximations to full MP4, and the coupled-cluster approximation with single- and double-excitation operators (CCSD) and also including a perturbational estimate of the energy effect of triple-excitation operators (CCSD(T)). Moreover, a wide range of contemporary density functional theory (DFT) methods are also evaluated with respect to computing experimental shifts in C1s ionization energies. Whereas the top ab initio methods reproduce the observed shifts almost to within the experimental uncertainty, even the best-performing DFT approaches meet with twice the root-mean-squared error and thrice the maximum error compared to CCSD(T). However, a number of different density energy functionals still afford sufficient accuracy to become tools in the analysis of complex C1s photoelectron spectra.

Free neutral CO2 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, ⟨N⟩, 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 ⟨N⟩ from expansion conditions.

Third-generation synchrotrons and high-resolution electron spectrometers coupled with modern theoretical techniques for molecular structure calculations have opened opportunities for measuring carbon 1s ionization energies in a large variety of complex molecules. Recent studies have provided new insights into the chemistry of substituent effects and on the effect of molecular conformation on inner-shell ionization energies.

In N2O a detailed study of the vibrational distribution of the (X) over tilde state reached after decay of core-to-pi* excitation of N terminal, N central and 0 1s core levels is reported. We obser ...

Single-component clusters of methyl chloride and methyl bromide have been produced by adiabatic expansion and their C 1s, Cl 2p and Br 3d photoelectron spectra recorded using synchrotron radiation and a high-resolution electron analyzer. The experimentally observed cluster-to-monomer shifts in core-level ionization energies are interpreted in terms of theoretical models based on molecular dynamics (MD) in conjunction with polarizable force fields developed and validated in the course of this work. MD simulations have also been used to explore the global and local structure of the clusters, providing evidence for a predominance of anti-parallel, head-to-tail arrangement of neighboring molecules. Whereas the cluster-to-monomer shifts are strongly dominated by polarization effects, the polarization contribution is very similar for ionization of carbon and the halogen, respectively. The difference in cluster shifts between the two ionization sites within the same molecule, C vs. Cl or C vs. Br, is thus determined by permanent electrostatic moments, i.e. the magnitude and direction of the permanent dipole moment and the local alignment of dipoles.

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.

Carbon 1s photoelectron asymmetry parameters beta for the chlorinated and the methyl carbon atom of CH3CH2Cl, CH3CHCl2, and CH3CCl3 have been measured using synchrotron radiation in the 340-600 eV energy range. We provide experimental evidence that the intramolecular scattering strongly affects beta values, even far from the ionization threshold. The results are in agreement with B-spline density functional theory calculations, making it possible to single out the behavior of the various continuum partial waves. We conclude that the intramolecular scattering makes electron angular distributions sensitive to the chemical environment, even in isolated gas phase molecules.

Core-level photoelectron spectra of free neutral methane clusters have been recorded. These spectra exhibit well-resolved surface and bulk features as well as vibrational fine structure. The vibrational structure in the cluster signal is well reproduced by a theoretical model that assumes independent contributions from inter- and intramolecular modes. The intramolecular contribution to the vibrational line-shape is taken to be equal to that of the monomer in the gas phase, while the intermolecular part is simplified to line broadening. An estimate of the cluster size has been made on the basis of the observed surface-to-bulk intensity ratio.

Kinetic effects associated with a neutralized plasma beam injected into a neutral background are investigated by numerical methods. We assume that the dominant interactions between ions and neutrals are charge exchange collisions. It is readily noted that by these processes, a fast ion is replaced by a slow one, implying that the injected fast-ion population will generate a new slow one. For a wide parameter range, we find that the combination of these two populations will become unstable with respect to kinetic ion-sound instabilities. The resulting enhanced level of electrostatic fluctuations gives rise to an enhanced scattering of the injected ions, with a resulting very wide velocity distribution. These observations have relevance for instance for chemical processes in the plasma state, where details in the velocity distribution of the constituents can be important. The basic features of the processes outlined here are illustrated by results from a particle-in-cell simulation in one spatial dimension. The results have a wider range of applicability than those outlined here, i.e. similar observations can be made for elastic ion neutral collisions as well.

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.

Carbon 1s ionization energies have been measured for 12 halomethanes. These together with earlier measurements provide 27 compounds for investigating the relationship between core-ionization energies and the electronegativity and hardness of the halogens. The ionization energies correlate nearly linearly with the sum of the electronegativities of the halogens attached to the central carbon. Both electronegativity and hardness play important roles in determining the ionization energy, and it is found that the linear relationship between ionization energy and electronegativity arises from an interplay of the electronegativity and hardness of the halogens and the length and ionicity of the carbon−halogen bond.

Formation and growth of neutral SO2 clusters is investigated in an adiabatic-expansion setup by means of sulfur 2p (S2p) photoelectron spectroscopy and theoretical modeling. The shift in S2p ionization energy between the cluster and a single molecule, i.e., IE(cluster)-IE(monomer), is recorded and used to monitor the mean cluster size over a wide range of expansion conditions. The produced clusters were shown to fall into two different size regimes. Comparison between theoretical simulations and experimental observations suggests that while the smallest clusters belong to the ultrafine particle mode and have a liquid-like structure, the larger clusters belong to the accumulation mode of fine particles and possibly have a frozen cluster core. The transition between the two size/structure regimes occurs over a narrow interval in expansion conditions and may possibly reflect a change in growth mechanism from monomer addition to growth by cluster-cluster collisions.(c) Jarle Harnes, Mahmoud Abu-Samha, Mathias Winkler, and Knut J. Børve

Carbon 1s photoelectron spectra for CF4 and CO have been measured at several photon energies near the carbon 1s threshold. The spectra have been analyzed in terms of the vibrational structure and the natural linewidth. For CO the vibrational structure shows evidence for anharmonicity in both the energy spacing and the intensity. Analysis of the results using an anharmonic model gives an equilibrium bond length for core-ionized CO that is 4.85 pm shorter than that of neutral CO. For CF4, the vibrational structure is very weak, and the analysis shows that the change in equilibrium CF bond length upon ionization is no more than 0.54 pm. Ab initio theoretical calculations give results in accord with these bond-length changes. The unusually small bond-length contraction in CF4 can be understood in terms of nonbonded fluorine–fluorine repulsion. The natural linewidth for core-ionized CO, 95±5 meV, is essentially the same as that of CH4. This result is in contrast with expectations based on the one-center model ...

Vibrational structures of the C1s and N1s photoelectron spectra of gas-phase HCN have been investigated using monochromated third-generation synchrotron radiation. Both spectra exhibit resolved fine structure associated with several vibrationally excited states. In the C1s spectrum a single vibrational progression is observed, while the N1s spectrum is more complex. High-level ab initio calculations were performed to simulate the spectra and the agreement with the experimental results is good. Based on the calculations, the C1s ionisation is found to induce vibrations solely in the CN stretching mode with an energy of 280 meV, while the N1s ionisation generates vibrations also in the CH stretching mode with an energy of about 387 meV, as well as combinations of these two modes.

Vibrationally resolved C1s photoelectron spectra of benzene and d6-benzene have been recorded using monochromated synchrotron radiation at photon energies of 330 eV. The spectrum of normal benzene displays considerable vibrational structure. Particularly noteworthy is the strong excitation of a combined CCH-bending and CC-stretching mode which splits the main peak into two well-defined maxima. In d6-benzene, the vibrational energy levels are less well separated and the vibrational structure is reduced to strong asymmetry of the main peak and a broad tail extending toward higher ionization energy. The recorded spectra are analyzed using first-principle and curve-fitting procedures. A theoretical model that allows for incomplete localization of the core hole, results in very good fits to the experimental spectra of both benzene and d6-benzene.

Adiabatic expansion of molecular vapors is a celebrated method for producing pure and mixed clusters of relevance in both applied and fundamental studies. The present understanding of the relationship between experimental conditions and the structure of the clusters formed is incomplete. We explore the role of the backing/carrier gas during adiabatic expansion of ethanol vapors with regard to cluster production and composition. Single-component clusters of ethanol were produced over a wide size-range by varying the rare gas (He, Ar) backing pressure, with Ar being more efficient than He in promoting the formation of pure ethanol clusters. However, at stagnation pressures Ps>1.34(4) bar and temperature 49(2) °C, synchrotron-based valence and inner-shell photoelectron spectroscopy reveals condensation of Ar carrier gas on the clusters. Theoretical calculations of cluster geometries as well as chemical shifts in carbon 1s ionization energies confirm that the experimental observations are consistent with an ethanol core covered by an outer shell of argon. Experiments on the 1-propanol/Ar system display a similar pattern as described for ethanol/Ar, indicating a broader range of validity of the results.

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.

High-resolution molecular X-ray photoelectron spectra of second-row atoms may reveal broadening or even splitting of the 2p 3/2 peak as compared to the 2p 1/2 component of the spectrum. This splitting reflects the lifted degeneracy of atomic 2p orbitals at sites of less than cubic symmetry and is referred to as molecular-field splitting (MFS). The prospect of using second-order Moller–Plesset theory (MP2) for ab initio calculation of the MFS for sulfur 2p 3/2 levels is examined. This method is subsequently applied to compute the MFS in ethynyl, ethenyl, and ethyl sulfur pentafluoride, resulting in values of 80, 116 and 121 meV, respectively. The initial-state contribution to the splitting is analyzed in terms of asymmetry in the electron density at sulfur, on the one hand, and in terms of the sulfur-ligand overlap density and electron density at the groups bonded to sulfur, on the other. At the Koopmans' theorem level of theory, the main source of splitting in the title compounds is found to be s–d hybridization of sulfur. At post-SCF levels of theory, core-valence electron correlation contributes substantially to the MFS, in proportion to the occupational asymmetry of the S3p shell. The realization that mixing of sulfur atomic orbitals of the same parity may add important contributions to the electric-field gradient at the core, resolves the observed diversity in the relative importance of core-valence electron correlation between different classes of sulfur compounds.

The formation and propagation of ion phase-space vortices are observed in a numerical particle-in-cell simulation in two spatial dimensions and with three velocity components. The code allows for an externally applied magnetic field. The electrons are assumed to be isothermally Boltzmann-distributed at all times, implying that Poisson's equation becomes nonlinear for the present problem. Ion phase-space vortices are formed by the nonlinear saturation of the ion-ion two-stream instability, excited by injecting an ion beam at the plasma boundary. We consider the effect of a finite beam diameter and a magnetic field, in particular. A vortex instability is observed, appearing as a transverse modulation, which slowly increases with time and ultimately breaks up the vortex. When many vortices are present at the same time, we find that it is their interaction that eventually leads to a gradual filling-up of the phase-space structures. The ion phase-space vortices have a finite lifetime, which is noticeably shorter than that found in one-dimensional simulations. An externally imposed magnetic field can increase this lifetime considerably. For high injected beam velocities in magnetized plasmas, we observe the excitation of electrostatic ion-cyclotron instabilities, but see no associated formation of ion phase-space vortices. The results are relevant, for instance, for the interpretation of observations by instrumented spacecraft in the Earth's ionosphere and magnetosphere.

In order to obtain chemical insight from shifts in core-ionization energies, Δ I , it is often desirable to separate the initial-state contribution, Δ V , from that caused by relaxation in the final state, Δ R . These quantities are related through Δ I =Δ V −Δ R . Whereas the chemical shift itself, Δ I , may be measured very accurately, the scope of the present contribution is to provide a tool for accurate quantification of the initial-state contribution Δ V to the measured shift. Common procedures of estimating Δ V either from Hartree–Fock orbital energies or from electrostatic potentials at nuclear positions are examined. Whereas orbital energies suffer from the neglect of valence-electron correlation, the use of electrostatic potentials does not take proper account of the finite extension of core orbitals. In order to circumvent both of these problems, a reformulation valid for any valence-correlated wave function is presented for V , the energy needed to remove a core electron without relaxation of spectator electrons. The resulting expression may be seen as an extension of Koopmans’ theorem, and reduces to the former in the case of a Hartree–Fock wave function. This extended Koopmans’ theorem is used to compare initial-state effects in X-ray photoelectron spectra for a set of simple hydrocarbons.

CrOH(g) is studied for the first time by theory, using accurate configuration interaction (CI) methods in conjunction with large basis sets. The ground and lowest excited state are established for the neutral and singly ionized molecule. The ionization potential is computed to 7.54 ± 0.05 eV, which, when applied to experimental results for CrOH+ [Magnera, T. F.; David, D. E.; Michl, J. J. Am. Chem. Soc. 1989, 111, 4100. Kang, H.; Beauchamp, J. L. J. Am. Chem. Soc. 1986, 108, 7502] opens access to experimental data on the bond dissociation energy in Cr−OH. Accurate quantum chemical methods have been applied to the calculation of bond dissociation energies of gaseous CrOH, CrF, and CrO. For the singly bound molecules the values obtained, D0(Cr−OH) = 3.74 ± 0.10 eV and D0(CrF) = 5.04 ± 0.10 eV, constitute the most accurate thermodynamical data available for these compounds. The high accuracy has been realized through the use of a dissociation process which is analogous to electron-attachment induced dissocia...

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.

Carbon 1s photoelectron spectra for 2-butyne (CH3C CCH3) measured in the photon energy range from threshold to 150 eV above threshold show oscillations in the intensity ratio C2,3/C1,4. Similar oscillations have been seen in chloroethanes, where the effect has been attributed to EXAFS-type scattering from the substituent chlorine atoms. In 2-butyne, however, there is no high-Z atom to provide a scattering center and, hence, oscillations of the magnitude observed are surprising. The results have been analyzed in terms of two different theoretical models: a density-functional model with B-spline atom-centered functions to represent the continuum electrons and a multiple-scattering model using muffin-tin potentials to represent the scattering centers. Both methods give a reasonable description of the energy dependence of the intensity ratios.

A multi-electron time-dependent model is applied in the calculation of single-electron capture cross sections to Ar6+ ions from the light noble-gas target atoms He, Ne and Ar. The model allows for a nonperturbative solution of the time-dependent Schrodinger equation, and contains a single parameter: the number of electrons susceptible to capture. Good agreement with experimental total cross sections is achieved for all systems. For partial cross sections at low energies, good agreement with experiments is obtained for He, whereas only the dominant final channels are in agreement with measurements for Ne and Ar.

The dependence of the electron capture cross section on cluster size in proton-cluster collisions is investigated within two different theoretical models. At low collision velocities a strong increase in the cross section with cluster size is found, whereas the size-dependency of the cross section becomes less pronounced at higher velocities.

Results on the formation and propagation of electron phase space vortices from laboratory experiments are summarized. The electron phase space vortices were excited in a strongly magnetized Q-machine plasma by applying a pulse to a segment of a waveguide surrounding the plasma. Depending on the temporal variation of the applied pulse, one or more phase space vortices can be excited, and their interaction can be followed in space and time. We were able to demonstrate, for instance, an irreversible coalescence of two such vortices. These results are extended by numerical simulations, showing how electron phase space vortices can also be formed by beam instabilities. Furthermore, a study of ion phase space vortices is performed by numerical simulations. Both codes allow for an externally applied magnetic field in three spatial dimensions. Ion phase space vortices are formed by the nonlinear saturation of the ion-ion two-stream instability, excited by injecting an ion beam at the plasma boundary. By following the evolution of the ion distribution of the velocity perpendicular to the direction of propagation of the injected ion beam, we find a significant ion heating in the direction perpendicular to the magnetic field associated with the ion phase space vortices being formed. The results are relevant, for instance, for the interpretation of observations by instrumented spacecraft in the Earth's ionosphere and magnetosphere.

International audience; The formation and propagation of isolated ion phase space vortices are observed in a 3-dimensional numerical simulation. The code allows for an externally applied constant magnetic field. The electrons are assumed to be isothermal and Boltzmann distributed at all times, implying that Poisson's equation becomes nonlinear for the present problem. Ion phase space vortices are formed by the nonlinear saturation of the ion-ion two-stream instability, excited by injecting an ion beam or short ion-bursts at the boundary. We consider the effects of finite beam diameters and the intensity of an externally imposed magnetic field.

XAFS and X-ray Photoelectron Spectroscopy (XPS) are element specific techniques used in a great variety of research fields. The near edge regime of XAFS provides information on the unoccupied electronic states of a system. For the detailed interpretation of the XAFS results, input from XPS is crucial. The combination of the two techniques is also the basis for the so called core-hole clock technique. One of the important aspects of photoelectron spectroscopy is its chemical sensitivity and that one can obtain detailed information about the composition of a sample. We have for a series of carbon based model molecules carefully investigated the relationship between core level photoelectron intensities and stoichiometry. We find strong EXAFS-like modulations of the core ionization cross sections as function of photon energy and that the intensities at high photon energies converge towards values that do not correspond to the stoichiometric ratios. The photoelectron intensities are dependent on the local molecular structure around the ionized atoms. These effects are well described by molecular calculations using multiple scattering theory and by considering the effects due to monopole shake-up and shake-off as well as to intramolecular inelastic scattering processes.

The effective attenuation length (λ) is determined to 5.7 ± 1.1 A at an electronic kinetic energy of 30 eV for carbon disulfide, CS2, from sulphur 2p photoelectron spectroscopy and lineshape modelling of CS2 clusters at a temperature of 150–170 K. The present approach is based on exploring the relation between λ and the cluster–monomer chemical shift and width of the cluster peak. A priori knowledge of the cluster temperature is required.

The claim by Shojaei et al (2011 J. Phys. B: At. Mol. Opt. Phys. 44 235101) that the entropy difference between the cis and gauche conformers of 1-butene changes dramatically upon inclusion of hindered-rotor corrections is shown to be incorrect.

We report on an O 1s photoelectron-spectroscopy study of small neutral water clusters produced by adiabatic expansion. The photoelectron spectra were acquired under two different experimental conditions. At intermediate resolution, the cluster signal was characterized by a very broad O 1s peak with a flat top. In the second set of measurements, resolution was significantly increased at the cost of lower count rates. The cluster signal was now partly resolved into a bimodal structure. Extensive theoretical calculations were undertaken to facilitate an interpretation of the spectrum. These results suggest that the bimodal feature may be ascribed to ionization of water molecules in different hydrogen-bonding configurations, more specifically, molecules characterized by donation of either one or both hydrogen atoms in H-bonding.

High-resolution photoelectron spectra of the inner-shell levels of molecular oxygen have been measured using synchrotron radiation. The vibrational structure of the two magnetically-split core-shell components is analyzed based upon ab initio calculations. The ratio between the intensities of the two components was analyzed at several different ionization energies up to about 600 eV, and the same is discussed and compared to high-energy ionization intensities. A theoretical calculation shows very good agreement with the measured spectra. The calculation implements a model where two parity components make up the 4Σ peak profile. The gerade–ungerade energy split for this state is found to be 50 meV.

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.