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The formation process of binary clusters has been studied using synchrotron based core level photoelectron spectroscopy. Free neutral krypton clusters have been produced by adiabatic expansion and doped with chloromethane molecules using the pickup technique. The comparison between the integrated intensities, linewidths, and level shifts of the cluster features of pure krypton and of chloromethane-krypton clusters has been used to obtain information about the cluster geometry. We have shown that most of the chloromethane molecules remain on the surface of the clusters.

We present the study of free nanoscale lead clusters using photoelectron spectroscopy and synchrotron radiation. Pb 5d core-level spectra reveal the presence of different initial charge states of the clusters created by the magnetron-based source. We suggest a method for determining the cluster size from the charge-dependent core level binding energies. Both the core-level and the valence spectra demonstrate that we have created free metallic clusters with essentially the same electronic structure as the solid.

Free sodium metal clusters have been studied by probing the Na2p core level using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The development of electronic structu ...

The lifetime broadening and molecular field splitting of the sodium dimer 2p core level is studied by x-ray photoelectron spectroscopy and by Auger electron spectroscopy. The lifetime of the Na 2p core hole has been determined to be 15 +/- 8 fs, much shorter than what has previously been reported for solid sodium. The molecular field splitting of this core level has been determined to be 42 +/- 10 meV. The Auger measurements have experimentally confirmed that the sodium dimer is a good example of when the Coulomb explosion mechanism determines the spectral shape.

We present Auger spectroscopy studies of large krypton clusters excited by soft x-ray photons with energies on and just above the 3d(5/2) ionization threshold. The deexcitation spectra contain new features as compared to the spectra measured both below and far above threshold. Possible origins of these extra features, which stay at constant kinetic energies, are discussed: (1) normal Auger process with a postcollision interaction induced energy shift, (2) recapture of photoelectrons into high Rydberg orbitals after Auger decay, and (3) excitation into the conduction band (or 'internal' ionization) followed by Auger decay. The first two schemes are ruled out, hence internal ionization remains the most probable explanation. (C) 2007 American Institute of Physics.

International audience; We have studied the ultrafast dissociation of the H2S molecule upon S 2p3/26a1 inner-shell excitation by combining high-resolution resonant Auger spectroscopy and energy-selected Auger electron–ion coincidence measurements. Auger final states have been correlated to the different fragmentation pathways (S+, HS+, and H2S+ ions). As an original result, we evidence a three-step mechanism to describe the resonant production of S+: the Auger recombination in the HS* fragment is followed for the A 3 and c 1 states by the S++H fragmentation mechanism.

Free heterogeneous argon-krypton clusters have been produced by coexpansion and investigated by means of x-ray photoelectron spectroscopy. By examining cluster surface and bulk binding energy shifts, relative intensities, and peak widths, we show that in the mixed argon-krypton clusters the krypton atoms favor the bulk and argon atoms are pushed to the surface. Furthermore, we show that krypton atoms in the surface layer occupy high-coordination sites and that heterogeneous argon-krypton clusters produced by coexpansion show the same surface structure as argon host clusters doped with krypton. These observations are supported by site-dependent calculations of chemical shifts.

Argon clusters have been doped with krypton atoms in a pick-up setup and investigated by means of ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS). The width of the krypton surface feature in the XPS spectra from mixed krypton/argon clusters has been studied and found to be narrower than in the case of homogeneous krypton clusters. By considering known spectral broadening mechanisms of the cluster features and the electron binding energy shift of the cluster surface feature relative to the atomic signal, we conclude that krypton ad-atoms preferentially occupy high-coordination surface sites on the argon host-cluster.

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.

Heterogeneous clusters created by doping Ar host clusters with Kr or Xe are shown to have radically different structures from the mixed clusters of the same type created by co-expansion of Ar-Kr or Ar-Xe gas mixtures. In contrast to the co-expansion case, the doped mixed clusters can be produced with Kr or Xe on the surface and Ar in the bulk. With the doping technique it is thus possible to control the surface composition of a specific cluster. A study of the cluster properties as a function of the doping pressure is also reported for the case of Ar clusters doped with Xe. The clusters have been studied by means of synchrotron radiation based X-ray photoelectron spectroscopy.

Photofragmentation of small argon clusters with size below ten atoms is reported. In this size range significant modifications from the electronic properties and geometry take place. When tuning the photon energy through the argon 2p edge, the fragmentation pattern is changed. Specifically, cation dimer production is enhanced at the 2p(32)-->4s resonance, while above the 2p edge almost complete atomization is observed. In both cases, the widths of the peaks in the mass spectra indicate that a large amount of kinetic energy is imparted to the fragment due to the formation of multiply charged clusters. A model based on "Coulomb explosion"-charge separation, simply resulting in a complete atomization of the cluster with no dependence on the photon energy-is insufficient to explain the observed photofragmentation of small clusters.

The development of electronic states in krypton clusters is investigated by high-resolution core-level electron spectroscopy. The energy ordering of bulk versus surface 3d(-1)np(n > 5) core-excited states in neutral clusters is demonstrated to be reversed to the 3d(-1)5p level situation. The cluster 3d(-1)6p,7p states are proven to be at a lower energy than the corresponding atomic levels. These findings reveal the ioniclike energy structure of the neutral cluster core-excited levels. The phenomenon is explained by a spatial spread of the excited orbitals over the cluster lattice.

In an experiment combining a state-of-the-art visible laser system with the vacuum ultraviolet beamline I411 at the storage ring MAX II, shakedown satellite lines in the 2p photoelectron emission of excited Na atoms have been observed. A close investigation of these lines shows a strong dependency of the line intensities from the linear polarization angle of the laser light. The shakedown electrons are preferably emitted into the direction of the laser electric field vector.

The below-threshold region in core-excited O2 is very complex, consisting of a multitude of exchange-split states with mixed molecular orbital-Rydberg character. We have investigated the nature of these intermediate states by resonant Auger spectroscopy. In particular, we have obtained constant-atomic-final-state yield curves for several atomic peaks in the electron decay spectra which are stemming from ultrafast dissociation. The relative intensity of Auger decay leading to atomic final states is considered a signature of the relative weight of the sigma* character. This method allows one to "filter out" intermediate states with dissociative character. Extensive calculations have been performed by multi-reference configuration interaction at different interatomic distances in order to evaluate the potential curves of the core-excited states and propose a qualitative description of the dissociative molecular dynamics. The calculations show that the core-excited states have a relevant admixture of excitations to orbitals with Rydberg character and excitations to the sigma* orbital with different spin couplings. A diabatization of the adiabatic potential curves shows that the coupling between Rydberg and sigma* diabatic states is very different at the different crossing points and ultrafast dissociation occurs more easily on the lowest sigma* diabatic potential curve. As a consequence, the observation of atomic peaks only in the lower-energy region of the absorption curve is well justified.

Resonant Auger spectra of large argon clusters have been measured at the Ar 2p edge. The decay spectra can give some information on the states reached in photon absorption. For example, the observation of the 3p(4)4p final states indicates the occurrence of the 2p(3/2)-> 4p excitation in clusters, whereas in atoms this excitation is dipole forbidden. Also the 3p(4)3d final states are identified in the resonant Auger spectra. A peculiar finding is that the normal Auger transitions in bulk cluster atoms are observed below the 2p ionization thresholds. This is explained by the excitation of the 2p electrons into the conduction band, followed by a rapid delocalization of the electron.

A new approach for obtaining an estimate of the effective size of the free neutral clusters is proposed. The approach relies on an experimental measure of the surface and interior or "bulk" cluster atoms provided by the x-ray photoelectron spectroscopy and on a model for the attenuation of photoelectrons ejected from the bulk of the cluster as the result of the ionizing irradiation. The experimental part gives the ratio of the electron signal from the bulk cluster atoms to that from the cluster surface atoms for a wide range of cluster sizes and electron kinetic energies. The attenuated response of the bulk atoms is modeled using an exponential law with the cluster size and kinetic-energy-dependent electron escape depth as parameters. For the experimental size range, model-based calculations for Ar, Kr, and Xe clusters are presented. The cluster size estimates obtained from comparison of the model calculations and experimental results agree well with those determined from the parameters of the cluster creation process. The combination of experiment and modeling also makes it possible to estimate the effective escape depth for electron propagation in free clusters. For Ar, Kr, and Xe clusters of varying mean size, absolute determination of the surface and bulk electron binding energies of the core levels used in the experiments has also been made.

A quantitative determination of 2s vacancy lifetimes in surface and bulk atoms of free Ne clusters has been made. While for free atoms the 2s inner-valence hole has a ps lifetime, it reduces to 6+/-1 fs for cluster bulk atoms. For surface atoms, the lifetime is on average longer than 30 fs. The lifetime estimate was obtained from fits of high-resolution photoelectron spectra of Ne clusters. The shortening of the lifetime is attributed to the coordination dependent interatomic Coulombic decay, which is extremely sensitive to internuclear distances.

Using photoelectron spectroscopy, we demonstrate how the self-assembling process of cluster formation in an adiabatic expansion leads to radial segregation and layering as well as to variable surface composition for binary Ar/Xe clusters. The radial structuring can be qualitatively understood from the different interatomic bonding strengths of the two components.

The extreme ultraviolet beamline BW3 at Hasylab is a state of the art beamline for the energy range 15–2000 eV consisting of a triple undulator equipped with a modified high‐flux SX‐700 plane grating monochromator. The first three optical elements of the beamline are made of graphite coated with SiC to withstand the high heat load at the 4.5 GeV storage ring Doris III. Excellent spectral resolution of the order of 104 at the nitrogen K edge at 400 eV is obtained by replacing the elliptical focusing mirror of the original SX‐700 design by a spherical mirror with very small tangent errors and with a large focal length in order to suppress spherical aberrations. In the energy range 50–1700 eV a photon flux of 1011–1013/s is obtained in a bandpass of 0.1%. Photoionization and photoemission measurements on atoms, molecules, and clusters making use of time‐of‐flight techniques demonstrate the excellent performance of the beamline.

Angle-resolved resonant Auger-electron spectroscopy has been carried out on the nitrogen molecule at selected photon energies around 419 eV, where a 1s core electron and two valence electrons are promoted into the lowest unoccupied molecular orbital 1pi(g). Significant enhancement of a specific band, which cannot be disentangled in direct photoionization, is observed at a binding energy of 37.6 eV, with a value of the anisotropy parameter beta much smaller than 2. We assign this new band to the transition to a doubly excited cationic state of N-2, in which two of the excited valence electrons remain in the 1pi(g) orbital, proposing a "double spectator" type decay mechanism. This observation shows how to preferentially probe multiply excited configurations of cations using multiple resonant excitation. (Less)

Different lifetime broadenings in molecular-field-split 2p core levels in H2S are predicted theoretically and are identified in an experimental investigation of the S 2p Auger electron spectrum. The measurements were performed for the transition to the vibrationally resolved X1A1 ground state of H2S2+. The lifetimes of the 3e1/2 and 5e1/2 levels of the 2p ionized molecule are found to be 64 and 74 meV, respectively. This unambiguous determination of the lifetime difference of 10±1 meV is only possible as the 4e1/2X1A1(2b) decay channel that overlaps the 5e1/2X1A1(2b) channel is practically suppressed in Auger decay in H2S. The lifetime difference is confirmed by ab initio calculations. A theoretical analysis shows that it results from the mutual orientation of the core hole in the intermediate states and the valence electron density in the sulfur 3p orbitals. Both are strongly influenced by the chemical bond. Thus the observed effect is the direct result of a fundamental property of molecular electronic structure.

An interference quenching of the m = 1 final state vibrational line in the resonant Auger decay of N 1s* core-excited N2 is observed and analyzed. The intensity ratio between the m = 1 and m = 0 vibrational levels of the X 2 final state shows a surprising nonmonotonic variation as a function of frequency detuning, going through a minimum with a complete suppression of m = 1. We have developed a simple model which indicates a linear relation between the value of the detuning frequency for this minimum and the equilibrium bond distance of the core-excited state. This implies the possibility of determining the equilibrium bond distances for core-excited states to a high degree of accuracy. Simultaneously with the simple model we present a strict theory of the studied effect. This strict theory allows us to explore the accuracy of determining the bond length of the core-excited state from resonant Auger spectra. We obtain a weak influence of the core-hole lifetime on the determined bond length, whereas the number of intermediate vibrational states accounted for in the numerical simulations seems to be quite important.

International audience; High-resolution, angle-resolved resonant Auger electron spectra of the NO molecule in the regions of both N and O 1s→2π core electron excitations are presented. A large number of vibrational final states are resolved due to high energy resolution. Calculations based on lifetime vibrational interference (LVI) theory neglecting interference between different electronic intermediate states and between direct and resonant channels have been performed. A comparison between theoretical and experimental spectra shows that LVI theory describes the major spectroscopic features quite well. The same holds for the evolution of the angular averaged partial cross sections with the change of excitation energy. The angular distribution of particular vibrational final states are, however, not described successfully with LVI calculations at the present level of sophistication. A theoretical analysis supports that one reason for this deviation is electronic state interference.

International audience; We study the dynamics of highly excited states in acetylene initiated by an ultrashort vacuum ultraviolet laser pulse. Electronic states lying in the 4s-3d Rydberg region are excited with one femtosecond pulse, and the dynamic development of the states is monitored by a second short pulse which ionizes the system. We show that even for femtosecond pulses where the bandwidth of the exciting pulse covers several electronic states, it is possible to extract short decay lifetimes through time-resolved photoelectron spectroscopy by using a frequency-modulated (chirped) excitation pulse. We report decay lifetimes for the F 402 and E 4-502 states in acetylene, and for the E 402 and E 502 states in d-acetylene. The time evolution measured in the electron spectra is compared to decay spectra measured using ion yield and the differences in these results are discussed.

International audience; Resonant Auger electron spectra of core excited O3 (1s−12b11) are presented for the first time. The photoionization valence spectrum with sample purity and resolution superior to those published is presented. The first direct experimental evidence for the 2 B1 state of O3+ is found. Ab initio calculations of the resonant Auger electron (RAE) spectrum have been performed supporting the assignment of the -state.

Vibrationally resolved C1s and S2p photoelectron spectra of the thiophene molecule have been recorded using monochromated synchrotron radiation at photon energies of 330 eV and 210 eV, respectively. The photoelectron bands contain complex vibrational structures which are analyzed using ab initio and curve-fitting procedures. The analysis is in good agreement with the experimental spectrum which enables identification of two chemically shifted carbon 1s core hole states. We were also able to determine the molecular-field splitting of the S2p3/2 ionic state to about 99 meV. The molecular-field splitting was moreover calculated using second-order Møller–Plesset perturbation theory, confirming the result from the fitting procedure.

International audience; High-resolution electron spectroscopy applied under resonant Auger-Raman conditions is shown to be a powerful tool for characterizing complex potential energy surfaces of core-excited states in polyatomic systems. BF3 is used as a prototype molecule: excitation of the B1s →2a2″ transition leads to an exceptional out-of-plane vibrational excitation resolved over a wide energy range (3.6 eV). The photon energy dependence of the spectra through the resonance profile is rationalized in the frame of the relative slopes of the potential energy curves along the boron out-of-plane displacement coordinate.

"Hidden" vibrations in CO: Reinvestigation of resonant Auger decay for the C 1s ->pi* excitation

We report the experimental observation of a strongly selective population of the spin-orbit components in the HCl1 X˜ 2PV states after excitation into the dissociative 2p216s* core excited state. A progression of highly excited vibrational states with either the V53/2 or V51/2 component of the final state is populated, respectively, when the excitation is tuned in the 2p3/2 216s* or 2p1/2 216s* part of the resonance. This effect is explained theoretically to be due to the orientational selectivity of the excitation process and the preference of the L2,3VV Auger process to produce valence holes with the same orientation as the core holes.

Possible ion pair formation in highly concentrated aqueous potassium halide solutions (fluoride, chloride, and bromide) has been investigated using Auger electron spectroscopy. The potassium Auger spectra exhibit features at high kinetic energy which are related to final states with vacancies on the potassium ion and a neighbouring water molecule. The potassium ion could possibly associate with a halide ion as well, giving rise to additional final states with a vacancy on the halide instead of a water molecule which should give rise to separable spectral features. The necessity of close proximity of the ions for such decays to occur would be strong evidence that contact ions pairs are present in the solution, but no features related to final states involving the anion were observed.

The resonant Auger decay spectra associated with doubly excited core-hole states near the N 1s threshold in the nitrogen molecule was measured. Electron-yield spectrum was obtained by setting a kinetic-energy window at 384 eV and scanning across the threshold region. At ground 410, 414, and 416 eV, photon energy, three dominant structures were observed. The increased signal-to-background ratio, intensity and resolution at third generation facilities were found to cast a different light on the dynamics of core photoionization in molecules.

High-resolution Auger spectroscopy applied under resonant Auger Raman conditions is shown to be a powerful tool for characterizing complex potential energy surfaces in core-excited systems. Using the example of Nt 1s–1*2 resonant Auger transition in nitrous oxide we emphasize the interplay between the nuclear motion and the electronic decay. We show how the choice of excitation energy allows selection of core-excited species of different geometries. The nuclear dynamics of these species are mapped by measuring the resonant Auger decay spectra. In addition to the changes in vibrational structure observed for the resonant Auger decay spectra, a strong influence of nuclear motion on the electronic decay is revealed, inducing the so-called "dynamical Auger emission." The experimental results are supported by ab initio quantum chemical calculations restricted to a linear geometry of the core-excited state.

The photoelectron spectra of argon was demonstrated during resonant photoemissions. The interference effects of the photoelectron spectra of argon was monitored by following the intensity of final states through resonant and direct channels. The need for a unified one-description of the ionization through a direct and core-excitation mediated channel was described.

We present direct evidence for ultra-fast dissociation of molecular water in connection photo-excitation of the O1s→4a1 resonance. The core-excited H2O molecules are shown to dissociate into core-excited O*H and neutral H on a time scale comparable to the core hole lifetime, i.e. a few femtoseconds. This conclusion is based on a resonant Auger study, qualitative arguments and ab initio calculations. ‘Hot bands' connected to Auger transitions from vibrationally excited OH fragments are identified, and these exhibit a dynamical behavior which qualitatively follows the change in excess energy.

International audience; High-resolution electron spectroscopy studies of the NO molecule in the regions of the N 1s→2π core excitations have been performed. By selecting electrons within certain binding energy ranges, either the Auger electron yield - a good approximation for the x-ray absorption spectrum - or the electrons emitted after decay to a particular ionic final state (constant ionic state (CIS)) were detected. By selecting the X 1Σ+ (2π0) final state, the superposition of several intermediate states can be disentangled by exploiting a selection rule which permits only two of the three dipole-allowed intermediate states to decay to this specific final state. This makes it possible to obtain more detailed information on the potential energy curves of the intermediate states than is available from regular absorption measurements. We have also obtained CIS spectra for individual vibrational sublevels within this state. The role of lifetime vibrational interference on the appearance of these spectra is discussed.

The role of avoided crossings for resonant x-ray Raman scattering is explored. The strong dependence of the electronic transition amplitudes on internuclear distance at avoided crossings causes new ...

The femtosecond dissociation of HCl after core excitation has been studied through the resonant Auger decay. The spectra contain contributions from decay occurring at both "molecular" and "atomic" internuclear distances. We have observed a new interference mechanism in these spectra: An atomic spectral line develops into a negative spectral contribution, a "hole," when detuning the excitation energy from the maximum of the Cl2p(-1)sigma(*) resonance. Resonant x-ray scattering theory quantitatively explains this behavior as due to a novel destructive continuum-continuum interference between molecular and atomic contributions to the Auger decay.

We present a review of recent research in Lund on applications of high-order harmonic generation. First, we have used high-order harmonics in spectroscopic studies of atoms and molecules. The experimental technique is based on a pump-probe scheme, where the pump pulse is a high-order harmonic synchronized with a laser pulse (probe). Short dissociation lifetimes in acetylene, as well as photoionization cross-sections from excited states of helium have been measured. In a second application, using the coherence of the high-order harmonic radiation, we have measured the electron density of a dense plasma, by an interferometry technique in the extreme ultraviolet region.

When an intense laser field interacts with an atomic gas, odd harmonics of the fundamental frequency are generated.

Physical review letters 78(9), 1675 - 1678 (1997). doi:10.1103/PhysRevLett.78.1675, Extended x-ray absorption fine structure (EXAFS) spectroscopy is used to study the size-dependent structural evolution of free clusters. K-edge EXAFS data are obtained for Ar clusters in beams having mean cluster size ranging from 12 to 2900 atoms per cluster. EXAFS analysis yields size-dependent features which indicate the presence of an fcc structure at ⟨N⟩≥200. It has been proposed in the literature that small rare-gas clusters have an icosahedral structure. The transition of this structure to the experimentally observed fcc lattice of rare-gas solids has been of long-standing interest and debate. The implications of our results are discussed in this light., Published by APS, College Park, Md.

Core-shell segregation of copper and silver in self-assembled, free nanoparticles is established by means of photoelectron spectroscopy in a wide range of relative Cu-Ag concentrations. These conclusions are based on the analysis of the photon-energy-dependent changes of the Cu 3d and Ag 4d photoelectron spectra. The nanoparticles are formed from mixed Cu-Ag atomic vapor created by magnetron sputtering of a bimetallic sample in a gas-aggregation cluster source. Even at similar Cu and Ag fractions in the primary vapor the surface of the nanoparticles is dominated by silver. Only at low Ag concentration copper appears on the surface of nanoparticles. For the latter case, a threefold decrease in the Ag 4d spin-orbit splitting has been detected. The specific component distribution and electronic structure changes are discussed in connection with the earlier results on Cu-Ag macroscopic and surface alloys.

The evolution of electronic properties in free antimony (Sb) nanoclusters as a function of the cluster size has been studied experimentally using synchrotron radiation. Antimony 4d core-level and valence-band regions have been probed, and the 4d binding energies and valence ionization potentials of clusters of various mean sizes have been determined. The binding-energy shifts with respect to polycrystalline solid have been used for deriving the electronic properties of the clusters. The observed results suggest that even large Sb clusters are not metallic.

Interaction between slow multiply charged ions and small to medium-size neutral water clusters is investigated. Strong projectile charge dependence is found for all of the fragmentation patterns. A variation of the projectile charge dramatically changes the proportion of multi-electron removal, the fragment kinetic energies as well as the mean size of the emitted fragments. First results for highly charged ion - water cluster ion collisions will also be presented.

This presentation is intended to illustrate various types of collisional processes relevant for free clusters probed by x-ray photoelectron spectroscopy using our own examples ranging from free-electron-metal clusters, through half-metal and semiconductor clusters to dielectrics, the latter from van-der-Waals to ionic clusters.

The photo-fragmentation of ammonia molecular clusters ionized with soft x-rays is studied for photon energies near the N-1s threshold. The fragmentation is studied with a 3D momentum mass spectrometer to access the energy and angular correlations between fragments. By choosing the cluster sizes below or above the critical size of stable dication, we investigate the coulomb explosion dynamics and the interplay between charge delocalization and mobility of molecules in the clusters.

Alkali-halide molecular clusters were formed from the vaporized molecules in collisions with an intense beam of argon clusters. Photoionization of the alkali-halide clusters as the result of interaction with x-ray radiation has been utilized in a photoelectron spectroscopy study of cluster bonding mechanisms. The changes in the energy structure due to the transformation from molecules to the solid salt were observed both in the inner and outer shells of constituent elements (metal and halogen) and were used to deduce the geometric structure of the clusters in question.

Silver clusters in the size range of ∼102 constituent atoms have been studied using photoelectron spec-troscopy. The 5s and 4d valence bands have been probed with 40 and 60.5 eV photon energies. Di ...

Physical review letters 74(15), 3017 - 3020 (1995). doi:10.1103/PhysRevLett.74.3017, Core level photoelectron and x-ray absorption spectra for free argon clusters from 〈N〉≈5 to 〈N〉≈4000 are presented. Spectral features originating from surface and bulk sites of the clusters are identified. These are seen to shift with cluster size. From the development of the spectra from the isolated atom to the largest clusters, information about both the size-dependent cluster-specific electronic structure and that of the “infinite” solid is obtained. Using a simple model for the core level binding energy shifts, effective surface coordination numbers are derived. These range from 5.3 for 〈N〉≈10 to 8.5 for the solid., Published by APS, College Park, Md.

Physical review letters 73(11), 1549 - 1552 (1994). doi:10.1103/PhysRevLett.73.1549, Published by APS, College Park, Md.

Mixed neutral molecular clusters have been produced by co-expansion of chloroform and methanol and characterized by carbon 1s photoelectron spectroscopy and theoretical modeling. The produced clusters are in the range 50–150 molecules and the clusters are not homogeneously mixed: chloroform is enriched in the bulk and methanol is found on or close to the surface. This is based on evidence from photoelectron depth profiling and molecular dynamics simulations of mixed clusters. The simulations suggest that methanol forms cyclic or linear oligomers in the surface layers of the mixed clusters. From thermodynamic models, the enrichment of methanol on the cluster surface can be rationalized from the difference in surface tension between the two pure components, which is connected to the qualitative differences in the respective bonding patterns.