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The hypothesis that the anomalous behavior of liquid water is related to the existence of a second critical point in deeply supercooled states has long been the subject of intense debate. Recent, sophisticated experiments designed to observe the transformation between the two subcritical liquids on nano- and microsecond time scales, along with demanding numerical simulations based on classical (rigid) models parameterized to reproduce thermodynamic properties of water, have provided support to this hypothesis. A stronger numerical proof requires demonstrating that the critical point, which occurs at temperatures and pressures far from those at which the models were optimized, is robust with respect to model parameterization, specifically with respect to incorporating additional physical effects. Here, we show that a liquid–liquid critical point can be rigorously located also in the WAIL model of water [Pinnick et al., J. Chem. Phys. 137, 014510 (2012)], a model parameterized using ab initio calculations only. The model incorporates two features not present in many previously studied water models: It is both flexible and polarizable, properties which can significantly influence the phase behavior of water. The observation of the critical point in a model in which the water–water interaction is estimated using only quantum ab initio calculations provides strong support to the viewpoint according to which the existence of two distinct liquids is a robust feature in the free energy landscape of supercooled water.

Motivated by the intriguing phase behavior of charged colloidal platelets, we investigate the structure and dynamics of charged repulsive disks by means of Monte-Carlo simulations. The electrostatic interactions are taken into account through an effective two-body potential, obtained within the non-linear Poisson-Boltzmann formalism, which has the form of anisotropic screened Coulomb potential. Recently, we showed that the original intrinsic anisotropy of the electrostatic potential in competition with excluded volume effects leads to a rich phase behavior that not only includes various liquid-crsytalline phases but also predicts the existence of novel structures composed of alternating nematic-antinematic sheets. Here, we examine the structural and dynamical signatures of each of the observed structures for both translational and rotational degrees of freedom. Finally, we discuss the influence of effective charge value and our results in relation to experimental findings on charged platelet suspensions., Comment: 22 pages, 17 figures

We present ion mobility measurements for small gold cluster cations, Aun+(n⩽10) in the temperature range between 77 and 300 K. While at room temperature all cluster sizes show only one sharp peak in the arrival time distribution, the Au9+ signal becomes bimodal at temperatures below 140 K. We interpret this unusual behavior in terms of two different isomeric forms of this ion which undergo temperature dependent interconversion reactions. From the reaction rate the activation barrier for this isomerization can be estimated to be 10–20 kJ/mol.

A combined experimental and theoretical study of small gold cluster anions is performed. The experimental effort consists of ion mobilitymeasurements that lead to the assignment of the collision cross sections for the different cluster sizes at room temperature. The theoretical study is based on ab initiomolecular dynamics calculations with the goal to find energetically favorable candidate structures. By comparison of the theoretical results with the measured collision cross sections as well as vertical detachment energies (VDEs) from the literature, we assign structures for the small Au n − ions (n

We have performed ion mobility measurements on gold cluster cations Aun+ generated by pulsed laser vaporization. For clusters with n

The existence of a ferroelectric fluid phase for systems of 1000–2000 dipolar hard or soft spheres is well established by numerical simulations. Theoretical approaches proposed to determine the stability of such a phase are either in qualitative agreement with the simulation results or disagree with them. Experimental results for systems of molecules or particles with large electric or magnetic dipole moments are also inconclusive. As a contribution to the question of existence and stability of a fluid ferroelectric phase this simulation work considers system sizes of the order of 10 000 particles, thus an order of magnitude larger than those used in previous studies. It shows that although ferroelectricity is not affected by an increase of system size, different spatial arrangements of the dipolar hard spheres in such a phase are possible whose free energies seem to differ only marginally.

Electron detachment from doubly charged anions PtClxBr4−x2− to form PtClxBr4−x− (x=0–4) was probed for in a Fourier-transform ion-cyclotron-resonance mass spectrometer at 288 K. The corresponding decay rates were measured as a function of collision gas pressure. In the zero-pressure limit, nonzero unimolecular decay rate constants were observed for x=4 and 3 (0.273±0.035 and 0.056±0.019 s−1 for 288 K, respectively). These show a strong temperature dependence suggesting the new phenomenon of blackbody radiation induced tunneling electron loss. Comparison to electron tunneling rates calculated by means of the Wentzel–Kramers–Brillouin approximation using Coulomb barrier properties and electron detachment energies derived from ab initio quantum chemical calculations provides a qualitative rationalization of the observed trends.

We describe a new type of solid effect for dynamic nuclear polarization ~DNP! that is based on simultaneous, near resonant microwave ~mw! and radio frequency ~rf! irradiation of a coupled electron nuclear spin system. The interaction of the electron spin with the mw field is treated as an electron spin dressed state. In contrast to the customary laboratory frame solid effect, it is possible to obtain nuclear polarization with the dressed state solid effect ~DSSE! even in the absence of nonsecular hyperfine coupling. Efficient, selective excitation of dressed state transitions generates nuclear polarization in the nuclear laboratory frame on a time scale of tens of ms, depending on the strength of the electron‐nuclear coupling, the mw and rf offset and field strength. The experiment employs both pulsed mw and rf irradiation at a repetition rate comparable to T 1e 21 , where T 1e is the electronic spin lattice relaxation time. The DSSE is demonstrated on a perdeuterated BDPA radical in a protonated matrix of polystyrene. © 2000 American Institute of Physics. @S0021-9606~00!01340-4#

A model system of electrolytes, the three-dimensional restricted primitive model (RPM), is investigated for the case where the charged hard spheres are constraint to move on a two-dimensional surface. The properties of the thermodynamic states corresponding to the gas and liquid phases have been determined by computer simulation for a wide range of densities and temperatures. The simulations provide results for the equation of state, internal energy, and correlation functions of the 3D RPM confined to 2D, and allow a characterization of the local order of the charged spheres as a function of density and temperature. The most striking aspect of this order is the organization, at low, although supercritical, temperature and low to moderate densities, of the spheres into “clusters” where linear arrangements or chains of alternatively positive and negative charges dominate. The domain of validity of the hypernetted chain integral equation is investigated. The critical temperature of the gas–liquid transition is located with precision by means of a finite size scaling analysis of the simulation results in the critical region. With regard to the critical behavior of the 2D RPM, the limitations on system sizes which could be studied do not allow to conclude more than possible compatibility with the 2D Ising universality class.

Monte Carlo simulations of the critical region of the restricted primitive model are reported. Using mixed-field finite size scaling analysis we show that the critical behavior is compatible with Ising like behavior although due to statistical error on the simulation data and large correction-to-scaling contributions mean-field behavior cannot be totally excluded. With the assumption of Ising criticality the critical temperature is estimated to be 0.0488±0.0002 and the critical density 0.080±0.005.

The mechanical stability of two different smectic arrangements formed by molecules having a rigid core and a semiflexible tail has been investigated by Monte Carlo simulation as a function of temperature, pressure, and chain length.

We report Monte Carlo (MC) simulations for the pair correlation functions of a hard sphere fluid in a hard sphere matrix. In particular, we evaluate the blocking function hb(r) which is that part of the fluid pair correlation function which accounts for the correlation between fluid particles mediated through at least one matrix particle. The results are compared with integral equation theory. A scheme for determining the bridge functions is presented which predicts excellent agreement of the pair structure with MC results.

The interaction of C+60 and C+70 ion beams with a surface of highly oriented pyrolitic graphite was investigated by probing the ionization and fragmentation rates of scattered species within a time window of 20 μs following impact. Neutralization/reionization and fragmentation behavior was observed and followed by a pulsed deflection field applied to the surface at variable delays after the collision event. An almost complete collisional neutralization of the incident projectile was found. For an impact energy of 140–180 eV, a significant part of the scattered species was found to reionize by delayed electron emission within the experimental time window. The associated reionization and fragmentation kinetics were modeled with a system of differential equations assuming a simple unimolecular reaction diagram. Rate constants for delayed ionization and fragmentation were calculated as functions of internal energy and respective activation energies with the ‘‘finite heat bath’’ model (Klots) and the Rice–Rams...

Fragmentation of various fullerenes was studied by surface impact on highly oriented pyrolytic graphite at collision energies Ecol of 150–1050 eV/molecule. The projectiles C+60, C+70, C+76, C+84, and C+94 were formed by laser desorption of chromatographically separated samples, while large carbon clusters C+94, C+110, C+164 were produced by laser‐induced coalescence reactions. Except at the highest impact energies, the fragment distributions consist of even numbered C+n species with abundance maxima similar to those observed in fullerene synthesis. With increasing Ecol, we observe a size evolution in the fragment distributions characteristic of a sequential fragmentation process. Simulated fragment distributions based on statistical rate theory and a sequential C2 loss mechanism reproduce the experimental data well up to a maximum Ecol. They are used to determine the mean energy transfer during surface impact as a function of collision energy as well as its dependence on several experimental parameters su...

This paper reports a Monte Carlo (MC) study of the ordering in a semiflexible liquid crystal model designed to give a crude representation of 4‐n‐octyl‐4′‐cyanobiphenyl (PCB). A mechanically stable nematic phase is obtained and the question of its thermodynamic stability is addressed by comparing the free energy to that of the isotropic phase. The free energies are calculated by thermodynamic integration using an efficient configuration biased MC scheme. Pair distribution functions, orientational correlation functions, and conformational properties of the nematic and isotropic phases are contrasted. In the isotropic phase the equation of state is compared with theoretical predictions for systems of convex molecules and chains made up of tangent hard spheres.

The method of thermodynamic integration is used to determine the free energy of the restricted primitive model of electrolytes along two subcritical isotherms. The densities of liquid‐gas coexistence derived from these data are found to be in good agreement with recent estimates from Gibbs ensemble Monte Carlo calculations. The difficulties of the free energy approach—which reside in the necessity of obtaining extremely accurate free energy values at near critical temperatures—is stressed. A detailed account is given of the clustering properties of the ions in the vapor phase, including cluster populations and energies, as well as intercluster contributions to the thermodynamic properties. The validity of several cluster theories for the free energy is investigated.

A monodispersed fullerene material comprising exclusively C(58) cages was doped with Cs to generate Cs(x)C(58) films of various compositions. The resulting modified properties have been studied using a variety of surface analysis methods with emphasis on thermal desorption and ultraviolet photoelectron spectroscopies. Cs doping raises the thermal stability of C(58) films which are characterized by quasi-covalent cage-cage bonds between annelated pentagon sites. Desorption mass spectra show emission of significant amounts of C(60) at elevated temperatures implying that Cs doping can activate C(58)→C(60) conversion in the condensed phase. In the case of saturated Cs(x)C(58) films, up to 4.5% of the initially deposited C(58) can be desorbed as C(60). From the spectroscopic data, we infer that Cs insertion and transport into the interstitial sites of the C(58) solid is accompanied by spontaneous electron transfer to the electronegative fullerene framework-leading to a weakening of intercage carbon-carbon bonds. At the same time, the overall cohesion of the solid film is enhanced by the formation of multiple ionic Cs(+) (β)C(58) (-) (δ) interactions. Near 800 K, Cs(+) activates∕catalyzes concerted disproportionation reactions resulting in the transfer of C(2) from C(58) (-) (δ) to neighbouring cages to yield C(60) (and C(56)). Heating Cs(x)C(58) films to beyond this temperature range yields a (high temperature) stable reaction product with a significantly modified UP spectrum and a finite density of states at the Fermi level.

The material formed by depositing C(2)(-) anions onto/into thin C(60) films (on graphite) at room temperature has been studied by means of thermal desorption mass spectroscopy, ultraviolet photoionization spectroscopy, atomic force microscopy (AFM), and surface enhanced Raman spectroscopy. As-prepared, C(2)/C(60) films manifest thermal desorption behaviour which differs significantly from pure C(60) films. Whereas the latter can be fully sublimed, we observe decomposition of C(2)/C(60) films to a high-temperature-stable material while predominantly C(60), C(62), and C(64) are desorbed in parallel. Deposition of C(2)(-) also leads to significantly modified electronic and vibrational properties. Based on DFT model calculations of the Raman spectra, we suggest that as-prepared C(2)/C(60) films contain appreciable amounts of polymeric networks comprising -C(2)-C(60)-C(2)-C(60)- chains. Detection of sublimed C(62) and C(64) upon heating implies that thermal decomposition of C(2)/C(60) films involves addition/uptake of C(2) units into individual fullerene cages. Correspondingly, annealing films up to various intermediate temperatures results in significant modifications to valence-band UP spectra as well as to surface topographies as imaged by AFM. The novel carbonaceous material obtained by heating to T950 K has a finite density of states at the Fermi level in contrast to as-prepared C(2)/C(60). It comprises fused fullerene cages.

Neutral and positively charged clusters Mx(C60)z, where M=K/Rb and z=1–3 were prepared in pulsed molecular beams by laser desorption (266 nm) from preformed alkali metal fulleride thin films. Experimental results encompassed mass spectral characterization of beam abundance and in selected cases, determination of rough ionization potential brackets and elucidation of fragmentation pathways upon (i) unimolecular dissociation in gas phase and (ii) surface collision induced dissociation from Si(111). Results are contrasted with electrostatic model calculations as well as ab initio self‐consistent field computations.

Positively and negatively charged coalescence products of C60 and C70 were characterized following pulsed laser desorption from fullerene thin films. Coalescence occurs by gas‐phase reactions. Positive ions are generated by thermoionization of hot neutrals, while negative ions likely form by electron attachment to neutral species in the outermost region of the desorption plume. Thermoionization rate and surface‐induced dissociation (SID) measurements are consistent with fullerene‐like structures for coalescence products detected as positive ions.

The instability of organic monolayer composed of polar molecules at the air-water interface has been a spotlight in interface science for many decades. However, the effect of electrostatic energy contribution to the free energy in the system is still not understood. Herein, we investigate the mechanical and electrical properties by studying the isobaric relaxation process of a dipalmitoyl phosphatidylcholine monolayer on water subphase with various concentrations of divalent ions to reveal the effect of electrostatic energy on thermodynamics and kinetics of the collapse mechanism. Our results demonstrate that electrical energy among the dipolar molecules plays an important role in the stability of monolayer and enhances the formation of micelles into subphase under high pressure. In addition, to confirm the electrostatic energy contribution, the well-known thermal effect on the stability of the film is compared. Hence, the general description of the monolayer free energy with contribution of electrostatic energy is suggested to describe the phase transition.

Three‐dimensional potential energy functions (PEFs) and rovibronic spectra for several low‐lying electronically excited singlet states of HCO+ have been investigated based on the ab initio complete active space self‐consistent field (CASSCF) method. The calculated electronic transition moments show that the ultraviolet (UV) absorption spectrum of the ion should be dominated by the transition C 1Π–X 1Σ+. Analytical representations of the PEFs for the two components of the degenerated C 1Π state have been used in beyond Born–Oppenheimer calculations of the rovibronic energy levels of HCO+ and DCO+ by a Renner–Teller variational approach which accounts for anharmonicity, rotation–vibration, and electronic angular momentum coupling effects. Since the excited states of HCO+ are experimentally unknown up to now, the calculated spectroscopic constants, the vertical and adiabatic excitation energies, and the absorption spectra should provide valuable information for future experimental characterizations of the ex...

Based on extensive ab initio multiconfiguration reference configuration interaction (MRCI) electronic structure calculations, three‐dimensional potential energy functions for the A 2A1(Πu)–X 2B1 linear/bent Renner–Teller systems of H2O+, HDO+, and D2O+ have been generated and used in beyond Born–Oppenheimer calculations of the spin–rovibronic energy levels by a variational approach. The effects of anharmonicity, rotation–vibration, electronic angular momenta, and spin coupling effects have been accounted for. For H2O+, HDO+, and D2O+ vibronic band origins for the bending levels to Ka=5 and for selected stretching and combination levels are given. Almost all experimentally known data have been reproduced with an accuracy better than 10 cm−1 after minor modifications of the ab initio potential energy functions. Our calculated values will facilitate further experimental assignments. A consistent interpretation of the photoelectron spectra for the different isotopes of water is given. Previous assignments of the bending modes in the A 2A1 state have been revised by two quantum numbers.

We employ a combination of ion mobility measurements and an unbiased systematic structure search with density functional theory methods to study structure and energetics of gas phase tin cluster cations, Sn(n)(+), in the range of n = 3-15. For Sn(13)(+) we also carry out trapped ion electron diffraction measurements to ascertain the results obtained by the other procedures. The structures for the smaller systems are most easily described by idealized point group symmetries, although they are all Jahn-Teller distorted: D(3h) (trigonal bipyramid), D(4h) (octahedron), D(5h) (pentagonal bipyramid) for n = 5, 6, and 7. For the larger systems we find capped D(5h) for Sn(8)(+) and Sn(9)(+), D(3h) (tricapped trigonal prism) and D(4d) (bicapped squared antiprism) plus adatoms for n = 10, 11, 14, and 15. A centered icosahedron with a peripheral atom removed is the dominant motif in Sn(12)(+). For Sn(13)(+) the calculations predict a family of virtually isoenergetic isomers, an icosahedron and slightly distorted icosahedra, which are about 0.25 eV below two C(1) structures. The experiments indicate the presence of two structures, one from the I(h) family and a prolate C(1) isomer based on fused deltahedral moieties.

We present comparative experimental and theoretical studies of the absorption and fluorescence spectra of the alkali-metal dimer molecules Cs(2) and RbCs immersed in a solid helium matrix, thereby extending our recent observations of Rb(2) in solid (4)He. The laser-excited molecular states are mostly quenched by the interaction with the He matrix. The quenching efficiently populates the second lowest excited state of the molecule, i.e., (1) (3)Π((u)) that is metastable in the homonuclear dimers. Molecular excitation and emission bands are modeled by calculating Franck-Condon factors that give a reasonable agreement with the experimental findings.

The geometries, energetics, and some first and second order molecular properties(dipole moments, force constants, polarizabilities) of small stoichiometric (NaCl) n and nonstoichiometric (Na n Cl n−1), n≤4 clusters were studied at the SCF and various correlated levels.

Monte Carlo simulations of a system of 2500 parallel cut spheres of thickness L/D=0.1 give evidence for a phase transition, at packing fraction η∼0.40, from the low density nematic phase to a smectic phase. No tendency to formation of columnar ordering is observed up to densities as high as η=0.60. This behavior contrasts with that of freely rotating cut spheres for which the sequence of stable phases has been identified as isotropic–nematic–columnar–solid [D. Frenkel, Liquid Cryst. 5, 929 (1989)]. For the present system of highly anisotropic molecules the slow convergence of the expansion of correlation functions on rotational invariants precludes quantitative comparison between integral equation theory and simulation results.

Monte Carlo simulations of a Stockmayer fluid confined between two parallel walls are performed to investigate self-organization of magnetic nanocrystals in a field parallel to the walls as a function of density, field strength, and wall separation. In order to study the formation of mesoscopic structures, a large number of up to 12 000 particles have to be used. The particles organize into periodically spaced cylindrical-like columns whose width typically varies between 5 and 9 particle diameters at low density. At small heights the columns are quenched due to the parallel walls, while larger wall separations can accommodate several layers of columns in good agreement with experiments. An increase in density entails a clear increase in column thickness, whereas an increase in field strength seems to have the opposite effect.

The structures and energetics of small tin cluster Sn(n)(-) anions up to n=15 were determined by a combination of density-functional theory and three different experimental methods: Ion mobility spectrometry, trapped ion electron diffraction, and collision induced dissociation. We find compact, quasispherical structures up to n=12. Sn(12)(-) is a slightly distorted hollow icosahedron while Sn(13)(-) to Sn(15)(-) have prolate structures, consisting of merged, hollow, in part incomplete, deltahedral subunits: Sn(13)(-) consists of a face-sharing pentagonal bipyramid and tricapped trigonal bipyramid, Sn(14)(-) comprises a face-sharing dicapped trigonal prism and capped square-antiprism, and Sn(15)(-) consists of two face-sharing tricapped trigonal prisms.

The effect of external electrostatic charge on the shapes of liquid condensed (LC) phase domains in monolayer at the air/water interface was investigated. For this reason the thermodynamic properties, domain size, and spontaneous polarization were analyzed by surface pressure-area isotherms, Brewster angle microscopy (BAM), and Maxwell displacement current technique. The analysis indicated magnesium ions preferred to bond with dipalmitoyl phosphatidylcholine negative head group in liquid expanded phase and/or at domain boundary at low ion concentration and got an access to binding with molecules inside of the LC domains for higher ion concentration. Domain size increase characterized by BAM was discussed in respect to the shape equation on the basis of electrostatic energy contribution. Although molecular repulsive force increased by adding of ions into subphase, the growth of domain size exceeded this tendency. Following shape equation analysis it was suggested that this effect corresponded to change in dipole moment orientation represented by increase in spontaneous polarization in normal projection. This demonstrated impact of local electrostatic field on molecular dipoles and free energy of LC domains.

We apply integral equation theory to the calculation of the thermodynamics and the distribution functions of a mixture of hydrogen atoms and molecules at high densities and temperatures. The integral equations are found to tackle very efficiently the problem of the softness and the nonadditivity of the potentials, as encountered under these conditions, and give better results than hard sphere perturbation theory. Comparison with Monte Carlo calculations show that the so‐called HMSA and modified hypernetted chain (MHNC) equations, give almost perfect results. This extends the range of validity of these schemes to domains of high pressures and temperatures.

Highly correlated CEPA electronic wave functions have been used to calculate the three‐dimensional potential energy and electric dipole moment functions of the X 1A’ and a 3A‘ states of HCF. The analytic expansions of these functions have been employed in variational and perturbational calculations of the vibrational band origins and spectroscopic constants. For the singlet ground state the vibrational band origins agree with available experiments to within 2 to 20 cm−1, the rotational constants to within 0.04 cm−1. Theoretical spectroscopic constants of similar accuracy are presented also for the triplet state. The singlet–triplet separation is calculated to be 13.9 kcal/mol, in agreement with the experimental upper bound of 14.7±0.2 kcal/mol. The dipole moments (μ0) have been calculated to be 1.394±0.05 D (X 1A’) or 1.049±0.05 D (a 3A‘), respectively, and vibrational radiative transition probabilities are also given. Anomalous variations of radiative lifetimes in some low‐lying vibrational levels have been found in the singlet state of HCF which are due to anharmonic coupling effects.

Extensive Monte Carlo simulation results for the pair and triplet direct correlation functions of the one‐component, hard‐sphere system near freezing are presented. These results are in good agreement with the analytic predictions of a recently derived free energy density functional for the inhomogeneous hard‐sphere fluid. This free energy model is based on the fundamental geometric measures of the particles. It provides the first unified derivation of the analytic description of the hard‐sphere thermodynamics and pair structure as given by the scaled‐particle and Percus–Yevick theories, and yields simple explicit expressions for the high order direct correlation functions of the uniform fluid.

The thermodynamic, dielectric, and structural properties of systems of dipolar hard spherocylinders and ellipsoids with perfect nematic order are investigated by means of the hypernetted chain (HNC) equation. The density, temperature domain for which the HNC equation admits a stable solution is determined. For spherocylinders a large increase of the main peak of the total structure factor in the vicinity of the stability limit of the HNC equation is, tentatively, associated with the onset of a nematic–smectic‐A transition. The situation seems less clear for dipolar ellipsoids, though the results are in many aspects qualitatively similar to the spherocylinder case. The pair structure near the transition is analyzed. It is not characteristic of any of the smectic‐A phase structures typical of strongly polar liquid crystals. This is probably a consequence of the highly symmetric nature of the model.

The reaction of solid C(58) films with atomic deuterium to yield deuterofullerenes, C(58)D(x), has been investigated by thermal desorption spectroscopy coupled with mass spectrometric detection, ultraviolet photoionization spectroscopy (21.2 eV), and atomic force microscopy (AFM). The average composition of the deuterofullerenes created depends on deuterium dose, beam flux, and surface temperature. Low deuterium exposures at room temperature yield predominantly C(58)D(6-8) cages. Saturation exposures at room temperature yield mass spectra peaked at C(58)D(26). After saturation exposures at elevated surface temperatures (approximately 500 K), the (subsequently) desorbed material reveals a comparatively narrow mass spectral distribution centered at C(58)D(30). Deuteration is associated with cleavage of covalent cage-cage bonds in the starting C(58) oligomer material, as evidenced by a considerable lowering of the sublimation energies of C(58)D(x) compared to desorption of C(58) desorbed from pure oligomer films. Correspondingly, AFM images reveal a D-induced, thermally activated transition from dendritic C(58) oligomer islands into smooth-rimmed islands composed of deuterated cages. Deuterated films exhibit a significantly lower work function than bare C(58) films. Progressing deuteration also gradually raises the surface ionization potential.

A tandem time-of-flight mass spectrometer with an intermediate surface was used to quantify electron transfer during glancing incidence scattering of hyperthermal C(60) (+) (E(coll)=250-500 eV, theta(in)=75 degrees ) from (i) self-assembled monolayers of n-alkylthiols on gold (of various chain lengths), (ii) partly fluorinated alkylthiols on gold, as well as (iii) clean gold surfaces. Self-assembled monolayers (SAMs) behave as insulating layers with their thicknesses determining the electron tunneling probability during collision. Correspondingly, a roughly exponential dependence of the neutralization probability on the chain length n was found. A pronounced dependence of the neutral yield on the primary beam kinetic energy indicates that dynamic SAM deformation and associated projectile penetration depth also play a role in determining electron transfer efficiency. Results are consistent with the molecular deformability of SAMs as determined with other experimental methods.

Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected Cn+ (50or =n60; even n) on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, Cn films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that Cn aggregation is governed by reactive sites comprising adjacent pentagons (or heptagons) on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C60. The apparent activation energies for intact Cn sublimation range from 2.2 eV for C58 to 2.6 eV for C50 as derived from thermal desorption spectra. All Cn films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital (HOMO)-derived band (EB approximately 2.5 eV). This feature has been assigned to Cn units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function (4.8 eV) was determined for thick films of all Cn studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C60. C58 exhibited the lowest HOMO (6.5 eV). Band gaps of Cn films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV (C52) and 1.8 eV (C50) were observed, compared to 1.5 eV for solid C60.

We present a theoretical and experimental study of the laser-induced formation process and of the emission spectra of Cs*He(n) exciplexes in the hcp and bcc phases of solid helium. Two different exciplex molecules are detected: a linear triatomic Cs*He2, which can exist in two electronic states: APi(1/2) and BPi(3/2), and a larger complex, where six or seven He atoms form a ring around a single cesium atom in the 6P(1/2) state. A theoretical model is presented, which allows the interpretation of the experimentally observed spectra.

We present an experimental study of the laser-induced luminescence spectra of Mn atoms in solid helium matrices. We observe transitions of the valence electron and of inner-shell electrons. We find that the Mn-He interaction perturbs the inner-shell transitions to a lesser extent than the valence-electron transitions. The observed lineshapes of the inner-shell transitions of Mn are similar to those of an inner-shell transition in Ba studied earlier. At the same time, they are more strongly perturbed than the corresponding transitions in Au and Cu under the same conditions. We suggest a qualitative explanation of these observations based on the atomic bubble model. Our results also suggest that the inner-shell transitions of Mn in solid He are more strongly perturbed than the same lines of Mn isolated in solid Ar or Kr matrices.

We present a systematic experimental study of absorption and fluorescence spectra of Au atoms in bulk liquid and solid helium matrices. The results are compared to the spectra of Cu atoms studied earlier. We investigate the dependence of the observed spectral lineshapes on the helium pressure. The observed splitting of the 5d(10)6p - 5d(9)6s(2) transitions of Au in hcp solid He gives evidence for an anisotropic trapping site structure formed either by a non-spherical atomic bubble or a four-vacancy center.