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Van der Waals clusters composed of He and aromatic molecules provide fundamental information about intermolecular interactions in weakly bound systems. In this study, phenol–helium clusters (PhOH–Hen with n ≤ 2) are characterized for the first time by UV and IR spectroscopies. The S1 ← S0 origin and ionization energy both show small but additive shifts, suggesting π-bound structures of these clusters, a conclusion supported by rotational contour analyses of the S1 origin bands. The OH stretching vibrations of the PhOH moiety in the clusters match with those of bare PhOH in both the S0 and D0 states, illustrating the negligible perturbation of the He atoms on the molecular vibration. Matrix shifts induced by He attachment are discussed based on the observed band positions with the help of complementary quantum chemical calculations. For comparison, the UV and ionization spectra of PhOH–Ne are reported as well. [ABSTRACT FROM AUTHOR]

We present the observation of the N2–H2O van der Waals complex in the 2OH stretch overtone region. The high-resolution jet cooled spectra were measured using a sensitive continuous wave cavity ringdown spectrometer. Several bands were observed and vibrationally assigned in terms of ν1, ν2, and ν3, the vibrational quantum numbers of the isolated H2O molecule, as ( ν 1 ′ ν 2 ′ ν 3 ′ ) ← ( ν 1 ″ ν 2 ″ ν 3 ″ ) = (200) ← (000) and (101) ← (000). A combination band involving the excitation of the in-plane bending motion of N2 and the (101) vibration of water is also reported. The spectra were analyzed using a set of four asymmetric top rotors, each associated with a nuclear spin isomer. Several local perturbations of the (101) vibrational state were observed. These perturbations were assigned to the presence of the nearby (200) vibrational state and to the combination of (200) with intermolecular modes. [ABSTRACT FROM AUTHOR]

Measurements of the 0–0 hyperfine resonant frequencies of ground-state 85Rb atoms show a nonlinear dependence on the pressure of the buffer gases Ar, Kr, and Xe. The nonlinearities are similar to those previously observed with 87Rb and 133Cs and presumed to come from alkali-metal–noble-gas van der Waals molecules. However, the shape of the nonlinearity observed for Xe conflicts with previous theory, and the nonlinearities for Ar and Kr disagree with the expected isotopic scaling of previous 87Rb results. Improving the modeling alleviates most of these discrepancies by treating rotation quantum mechanically and considering additional spin interactions in the molecules. Including the dipolar-hyperfine interaction allows simultaneous fitting of the linear and nonlinear shifts of both 85Rb and 87Rb in either Ar, Kr, or Xe buffer gases with a minimal set of shared, isotope-independent parameters. To the limit of experimental accuracy, the shifts in He and N2 were linear with pressure. The results are of practical interest to vapor-cell atomic clocks and related devices. [ABSTRACT FROM AUTHOR]

We experimentally studied the three-body fragmentation dynamics of a noble gas cluster (ArKr2) upon its multiple ionization by an intense femtosecond laser pulse. The three-dimensional momentum vectors of correlated fragmental ions were measured in coincidence for each fragmentation event. A novel comet-like structure was observed in the Newton diagram of the quadruple-ionization-induced breakup channel of ArKr 2 4 + → Ar+ + Kr+ + Kr2+. The concentrated head part of the structure mainly originates from the direct Coulomb explosion process, while the broader tail part of the structure stems from a three-body fragmentation process involving electron transfer between the distant Kr+ and Kr2+ ion fragments. Due to the field-driven electron transfer, the Coulomb repulsive force of the Kr2+ and Kr+ ions with respect to the Ar+ ion undergoes exchange, leading to changes in the ion emission geometry in the Newton plot. An energy sharing among the separating Kr2+ and Kr+ entities was observed. Our study indicates a promising approach for investigating the strong-field-driven intersystem electron transfer dynamics by using the Coulomb explosion imaging of an isosceles triangle van der Waals cluster system. [ABSTRACT FROM AUTHOR]

The fine structure excitation of the interstellar CCS radical induced by collisions with He is investigated. The first potential energy surface (PES) for the CCS–He van der Waals complex is presented. It was obtained from a highly correlated spin unrestricted coupled cluster approach with single double and perturbative triple excitations. The PES presents two shallow minima of 31.85 and 37.12 cm−1 for the linear (He facing S) and the nearly T-shaped geometries, respectively. The dissociation energy of the complex was calculated and found to be D0 = 14.183 cm−1. Inelastic scattering calculations were performed using the close-coupling approach. Cross-sections for transitions between the 61 first fine structure levels of CCS were obtained for energy up to 600 cm−1 and rate coefficients for the 5–50 K temperature range were derived. This set of collisional data can be used to model CCS emission spectra in dark molecular interstellar clouds and circumstellar envelopes and enable an accurate determination of CCS abundance in these astrophysical media. [ABSTRACT FROM AUTHOR]

This paper presents the experimental and theoretical analyses of ArICl(IP,vIP,nIP) states' population and decay at energies lower than the ArICl(E,vE = 0,nE) dissociation limit (IP = E0+, D′2, β1), vIP = 0, 1, and nIP are the quantum numbers of the van der Waals (vdW) modes. We have measured the excitation spectra of the ArICl(E,vE = 0,1,nE → X,vX,nX) and ArICl(β,0,nβ → A and/or D ′ , v D ′ , n D ′ → A ′ luminescence as well as luminescence spectra themselves. To construct potential energy surfaces (PESs) for valence (A1, A′2) and ion-pair (E, β, and D′) electronic states of the complex, we utilized the intermolecular diatomic-in-molecule perturbation theory first order method. The experimental and calculated spectroscopic characteristics of the T-shaped ArICl valence and E, β states agree well. The ArICl(D′) state PES has no vdW levels in the T-shaped configuration, and collinear ArICl(D′) binding energy is larger than that of the T-shaped ArICl(β) state. We calculated vibrational state energies and the ArICl(IP → valence states) luminescence excitation spectra, as well as luminescence spectra themselves, by using the Heidelberg MCTDH code. The comparison of the experimental and calculated excitation spectra shows that the latter describe their principal features. The bound–bound ArICl(E,0,nE → X and β,0,nβ → A) parts of experimental luminescence spectra are described adequately by the calculated spectra, whereas bound-free ArICl(E,0,nE → X, D′, 0, nD′ → A′) parts are not described since the bound-free transitions occur in repulsive parts of the ArICl(X, A ′ PESs, which we cannot describe accurately. [ABSTRACT FROM AUTHOR]

Due to the lack of specific collisional data, the abundance of NS+ in cold dense interstellar clouds was determined using collisional rate coefficients of CS as a substitute. To better understand the chemistry of sulfur in the interstellar medium, further abundance modeling using the actual NS+ collisional rate coefficients is needed. For this purpose, we have computed the first full 4D potential energy surface of the NS+–H2 van der Waals complex using the explicitly correlated coupled cluster approach with single, double, and non-iterative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. The potential energy surface exhibits a global minimum of 848.24 cm−1 for a planar configuration of the complex. The long-range interaction energy, described using multipolar moments, is sensitive to the orientation of H2 up to radial distances of ∼50 a0. From this new interaction potential, we derived excitation cross sections, induced by collision with ortho- and para-H2, for the 15 low-lying rotational levels of NS+ using the quantum mechanical close-coupling approach. By thermally averaging these data, we determined downward rate coefficients for temperatures up to 50 K. By comparing them with the previous NS+–H2 data, we demonstrated that reduced dimensional approaches are not suited for this system. In addition, we found that the CS collisional data underestimate our results by up to an order of magnitude. The differences clearly indicate that the abundance of NS+, in cold dense clouds retrieved from observational spectra, must be reassessed using these new collisional rate coefficients. [ABSTRACT FROM AUTHOR]

The CO2 molecule is of great interest for astrophysical studies since it can be found in a large variety of astrophysical media where it interacts with the dominant neutral species, such as He, H2, or H2O. The CO2–He collisional system was intensively studied over the last two decades. However, collisional data appear to be very sensitive to the potential energy surface (PES) quality. Thus, we provide, in this study, a new PES of the CO2–He van der Waals complex calculated with the coupled-cluster method and a complete basis set extrapolation in order to provide rotational rate coefficients that are as accurate as possible. The PES accuracy was tested through the calculations of bound state transition frequencies and pressure broadening coefficients that were compared to experimental data. An excellent agreement was globally found. Then, revised collisional data were provided for the 10–300 K temperature range. Rate coefficients were compared to previously computed ones and are found to be up to 50% greater than previously provided ones. These differences can induce non-negligible consequences for the modeling of CO2 abundance in astrophysical media. [ABSTRACT FROM AUTHOR]

We present quantum five-dimensional bound-state calculations of the fully coupled intermolecular rovibrational states of H2O–CO2 and D2O–CO2 van der Waals (vdW) complexes in the rigid-monomer approximation for the total angular momentum J values of 0, 1, and 2. A rigid-monomer version of the recent ab initio full-dimensional (12D) potential energy surface of H2O–CO2 [Q. Wang and J. M. Bowman, J. Chem. Phys. 147, 161714 (2017)] is employed. This treatment provides for the first time a rigorous and comprehensive description of the intermolecular rovibrational level structure of the two isotopologues that includes the internal-rotation tunneling splittings and their considerable sensitivity to rotational and intermolecular vibrational excitations, as well as the rotational constants of the two vdW complexes. Two approaches are used in the calculations, which differ in the definition of the dimer-fixed (DF) frame and the coordinates associated with them. We demonstrate that with the approach introduced in this work, where the DF frame is fixed to the CO2 moiety, highly accurate results are obtained using significantly smaller basis sets in comparison to those for the alternative approach. In addition, the resulting wavefunctions tend to lend themselves better to physical interpretation and assignment. The H2O–CO2 ground-state internal-rotation tunneling splittings, the rotational transition frequencies, and the rotational constants of both vdW complexes are in excellent agreement with the experimental results. The calculated intermolecular vibrational fundamentals agree well with the scant terahertz spectroscopy data for these complexes in cryogenic neon matrices. [ABSTRACT FROM AUTHOR]

We consider the origin of nonlinear collision shifts for the 0–0 hyperfine transition in alkali/noble-gas systems due to van der Waals molecule formation. Developing a semi-empirical model, we describe the shift as arising from three fundamental interactions: (1) a fractional change in the alkali's valence electron density at the alkali nucleus, η, which affects the hyperfine contact term; (2) a mixing of p-wavefunction character into the alkali ground state (characterized by the probability for p-state character appearing in the perturbed wavefunction ξ12), which gives rise to an electric quadrupole term in the ground-state hyperfine splitting; and (3) an interaction of the alkali's valence electron with the magnetic field produced by molecular rotation, characterized by a magnetic field strength BvdW. In addition to these molecular parameters, the model also depends on the formation rate of van der Waals molecules, kfP2, and the breakup rate of the molecules, kbP, where P is the noble-gas pressure. Fitting the model to the 85Rb/Xe and 87Rb/Xe experimental data of McGuyer and co-workers (and taking previously measured values for kf and BvdW), we find that η = 9 × 10−3, ξ12 = 5 × 10−3, and kb = 2.9×107 s−1/Torr. [ABSTRACT FROM AUTHOR]

Collisional data for the excitation of NH by H2 are key to accurately derive the NH abundance in astrophysical media. We present a new four-dimensional potential energy surface (PES) for the NH–H2 van der Waals complex. The ab initio calculations of the PES were carried out using the explicitly correlated partially spin-restricted coupled cluster method with single, double, and perturbative triple excitations [RCCSD(T)-F12a] with the augmented correlation-consistent polarized valence triple zeta basis set. The PES was represented by an angular expansion in terms of coupled spherical harmonics. The global minimum corresponds to the linear structure with a well depth De = 149.10 cm−1. The calculated dissociation energy D0 is found to be 30.55 and 22.11 cm−1 for ortho-H2 and para-H2 complexes, respectively. These results are in agreement with the experimental values. Then, we perform quantum close-coupling calculations of the fine structure resolved excitation cross sections of NH induced by collisions with ortho-H2 and para-H2 for collisional energies up to 500 cm−1. We find strong differences between collisions induced by ortho-H2 and para-H2. Propensity rules are discussed. The cross sections are larger for fine structure conserving transitions than for fine structure changing ones, as predicted by theory. These new results should help in interpreting NH interstellar spectra and better constrain the abundance of NH in interstellar molecular clouds. [ABSTRACT FROM AUTHOR]

Photodissociation of the van der Waals complex Ar–I2 after excitation into the Rydberg states of I2 has been investigated with velocity map imaging of photofragments. Formation of the translationally hot ions of argon Ar+ with three modes in kinetic energy distribution has been revealed. The measured dependence of the kinetic energy of Ar+ on the pumping photon energy indicates the appearance of Ar+ from three channels of the photodissociation of the linear intermediate Ar+–I–I− containing chemically bound argon. These channels are (1) dissociation into Ar++ I2−; (2) three-body dissociation into (Ar+)* + I* + I−, with (Ar+)* and I* being the 2P1/2 states of the species; and (3) two-body electron photodetachment, giving rise to Ar+ + I2 + e. Three indicated channels are similar to those established for the photodissociation of trihalide anions. This similarity confirms the conclusion on the formation of the Ar+–I–I− intermediate, which is isoelectronic to the trihalide anion Cl–I–I−. The mechanism of the Ar+–I–I− formation involves two-photon excitation of the complex Ar–I2 into the Rydberg state of I2 converted into the ion-pair state and further electron transfer from Ar to I+ of the ion-pair state. The self-assembling of the structure making the formation of the Ar+–I–I− intermediate energetically accessible is confirmed by modeling the dynamics in the excited linear complex Ar–I2. Photoexcitation of the van der Waals complexes of noble gases with halogens into the ion-pair states of halogen is supposed to be a promising approach for generating the new chemical compounds of noble gas atoms. [ABSTRACT FROM AUTHOR]

Photodissociation of the van der Waals complex Ar–I2 after excitation into the Rydberg states of I2 has been investigated with velocity map imaging of photofragments. Formation of the translationally hot ions of argon Ar+ with three modes in kinetic energy distribution has been revealed. The measured dependence of the kinetic energy of Ar+ on the pumping photon energy indicates the appearance of Ar+ from three channels of the photodissociation of the linear intermediate Ar+–I–I− containing chemically bound argon. These channels are (1) dissociation into Ar++ I2−; (2) three-body dissociation into (Ar+)* + I* + I−, with (Ar+)* and I* being the 2P1/2 states of the species; and (3) two-body electron photodetachment, giving rise to Ar+ + I2 + e. Three indicated channels are similar to those established for the photodissociation of trihalide anions. This similarity confirms the conclusion on the formation of the Ar+–I–I− intermediate, which is isoelectronic to the trihalide anion Cl–I–I−. The mechanism of the Ar+–I–I− formation involves two-photon excitation of the complex Ar–I2 into the Rydberg state of I2 converted into the ion-pair state and further electron transfer from Ar to I+ of the ion-pair state. The self-assembling of the structure making the formation of the Ar+–I–I− intermediate energetically accessible is confirmed by modeling the dynamics in the excited linear complex Ar–I2. Photoexcitation of the van der Waals complexes of noble gases with halogens into the ion-pair states of halogen is supposed to be a promising approach for generating the new chemical compounds of noble gas atoms. [ABSTRACT FROM AUTHOR]

We study the role of pairwise long-range interactions in the formation of van der Waals molecules through direct three-body recombination processes A + B + B → AB + B, based on a classical trajectory method in hyperspherical coordinates developed in our earlier works [J. Pérez-Ríos et al., J. Chem. Phys. 140, 044307 (2014); M. Mirahmadi and J. Pérez-Ríos, J. Chem. Phys. 154, 034305 (2021)]. In particular, we find the effective long-range potential in hyperspherical coordinates with an exact expression in terms of dispersion coefficients of pairwise potentials. Exploiting this relation, we derive a classical threshold law for the total cross section and the three-body recombination rate yielding an analytical expression for the three-body recombination rate as a function of the pairwise long-range coefficients of the involved partners. [ABSTRACT FROM AUTHOR]

We present a full-dimensional ab initio intermolecular potential energy surface (IPES) for the OC–HF van der Waals complex. 3167 ab initio points were computed at the frozen-core (FC) explicitly correlated coupled cluster [FC-CCSD(T)-F12b] level, with the augmented correlation-consistent polarized valence quadruple-zeta basis set plus bond functions. Basis set superposition error correction was also considered by the full counterpoise procedure. Gaussian process regression (GPR) was used to map out the potential energy surface, while a multipole expansion method was employed to smooth the ab initio noise of intermolecular potential in the long range. The global minimum of −1248.364 cm−1 was located at the linear configuration with the C atom pointing toward the H atom of the HF molecule. In addition, a local minimum of −602.026 cm−1 was found at another linear configuration with the O atom pointing toward the H atom of the HF molecule. The eigenstates were calculated on the vibrational averaged four-dimensional IPESs with the mixed radial discrete variable representation/angular finite basis representation method and Lanczos propagation algorithm. The dissociation energy D0 was calculated to be 701.827 cm−1, well reproducing the experimental value of 732 ± 2 cm−1. The dipole moment surfaces were also fitted by GPR from 3132 ab initio points calculated using the coupled cluster method [CCSD(T)] with AVTZ basis set plus bond functions. The frequencies and relative line intensities of rovibrational transitions in the HF (DF) and CO stretching bands were further calculated and compared well with the experimental results. These results indicate the high fidelity of the new IPES. [ABSTRACT FROM AUTHOR]

Recent studies of the weakly bound anisole⋯CH4 complex found a dual mode of binding, featuring both C/H⋯π and C/H⋯O noncovalent interactions. In this work, we examine the dissociation energies of related aniline⋯(CH4)n (n = 1, 2) van der Waals clusters, where both C/H⋯π and C/H⋯N interactions are possible. Using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity-mapped ion imaging (VMI), we derive the dissociation energies of both complexes in the ground (S0), excited (S1), and cation radical (D0) states. As the amide group is non-planar in the ground state, the optimized ground state geometry of the aniline⋯CH4 1:1 complex shows two isomers, each with the methane positioned above the aniline ring. The observed redshift of the electronic origin from the aniline monomer is consistent with TDDFT calculations for the more stable isomer, where the methane sits on the same face as the amino hydrogens. The dissociation energies of the 1:1 complex, obtained from 2CAP measurements, are in good agreement with the calculated theoretical values from selected density functional theory methods. VMI data for the 1:1 complex gave a binding energy value overestimated by ∼179 cm−1 when compared to the 2CAP results, indicating that dissociative ionization selectively populates an excited vibrational level of the aniline cation radical. Given that the electron donating ability of aromatic substituents trends as –NH2 > –OCH3 > –CH3, it is noteworthy that the strength of methane binding also trends in this order, as found by experiment (dissociation energies in kJ/mol: 6.6 > 5.8 > 4.5) and predicted by theory (PBE0-D3/def2-QZVPPD, in kJ/mol: 6.9 > 6.0 > 5.0). For the 1:2 complex of aniline and methane, calculations predict that the more stable conformer is the one where the two methane molecules lie on opposite faces of the ring, consistent with the observed redshift of the electronic origin. Unlike the anisole–methane 1:2 complex, which shows an enhanced dissociation energy for the loss of one methane in comparison with the 1:1 complex, here, we find that the energy required to remove one methane from the ground state aniline–methane 1:2 complex is smaller than that of the 1:1 complex, consistent with theoretical expectations. [ABSTRACT FROM AUTHOR]

We derive the effective Flory-Huggins parameter in polarizable polymeric systems, within a recently introduced polarizable field theory framework. The incorporation of bead polarizabilities in the model self-consistently embeds dielectric response, as well as van der Waals interactions. The latter generate a χ parameter (denoted χ ̃ ) between any two species with polarizability contrast. Using one-loop perturbation theory, we compute corrections to the structure factor S k and the dielectric function ϵ ^ ( k ) for a polarizable binary homopolymer blend in the one-phase region of the phase diagram. The electrostatic corrections to S(k) can be entirely accounted for by a renormalization of the excluded volume parameter B into three van der Waals-corrected parameters BAA, BAB, and BBB, which then determine χ ̃ . The one-loop theory not only enables the quantitative prediction of χ ̃ but also provides useful insight into the dependence of χ ̃ on the electrostatic environment (for example, its sensitivity to electrostatic screening). The unapproximated polarizable field theory is amenable to direct simulation via complex Langevin sampling, which we employ here to test the validity of the one-loop results. From simulations of S(k) and ϵ ^ ( k ) for a system of polarizable homopolymers, we find that the one-loop theory is best suited to high concentrations, where it performs very well. Finally, we measure χ ̃ N in simulations of a polarizable diblock copolymer melt and obtain excellent agreement with the one-loop theory. These constitute the first fully fluctuating simulations conducted within the polarizable field theory framework. [ABSTRACT FROM AUTHOR]

Electron correlations in molecules can be divided in short range dynamical correlations, long range Van der Waals type interactions, and near degeneracy static correlations. In this work, we analyze for a one-dimensional model of a two-electron system how these three types of correlations can be incorporated in a simple wave function of restricted functional form consisting of an orbital product multiplied by a single correlation function f (r12) depending on the interelectronic distance r12. Since the three types of correlations mentioned lead to different signatures in terms of the natural orbital (NO) amplitudes in two-electron systems, we make an analysis of the wave function in terms of the NO amplitudes for a model system of a diatomic molecule. In our numerical implementation, we fully optimize the orbitals and the correlation function on a spatial grid without restrictions on their functional form. Due to this particular form of the wave function, we can prove that none of the amplitudes vanishes and moreover that it displays a distinct sign pattern and a series of avoided crossings as a function of the bond distance in agreement with the exact solution. This shows that the wave function ansatz correctly incorporates the long range Van der Waals interactions. We further show that the approximate wave function gives an excellent binding curve and is able to describe static correlations. We show that in order to do this the correlation function f (r12) needs to diverge for large r12 at large internuclear distances while for shorter bond distances it increases as a function of r12 to a maximum value after which it decays exponentially. We further give a physical interpretation of this behavior. [ABSTRACT FROM AUTHOR]

Standard density functionals without van der Waals interactions yield an unsatisfactory description of ice phases, specifically, high density phases occurring under pressure are too unstable compared to the common low density phase Ih observed at ambient conditions. Although the description is improved by using functionals that include van der Waals interactions, the errors in relative volumes remain sizable. Here we assess the random phase approximation (RPA) for the correlation energy and compare our results to experimental data as well as diffusion Monte Carlo data for ice. The RPA yields a very balanced description for all considered phases, approaching the accuracy of diffusion Monte Carlo in relative energies and volumes. This opens a route towards a concise description of molecular water phases on surfaces and in cavities. [ABSTRACT FROM AUTHOR]

Graphene pseudohalides are natural candidates for use in molecular sensing due to their greater chemical activity as compared to both graphene halides and pristine graphene. Though their study is still in its infancy, being hindered until recently by the unavailability of both selective and efficient procedures for their synthesis, they promise to considerably widen the application potential of chemically modified graphenes. Herein, we employ van der Waals density functional theory to study the structural and electronic properties of two selected graphene pseudohalides, namely, cyanographone and isocyanographone and investigate the potential use of the latter as a chemical sensor via electron transport calculations. [ABSTRACT FROM AUTHOR]

The fragmentation of doubly and triply charged mercury clusters is theoretically studied to analyze an experiment performed by Katakuse’s group at Osaka University [T. Satoh et al., J. Mass Spectrom. Soc. Jpn. 51, 391 (2003)]. The fission barrier is calculated using a liquid-drop model proposed by Echt et al. In the decay of doubly charged clusters, the barrier height is found to take the minimum value for nearly symmetric fission. On the other hand, in the decay of triply charged clusters, the barrier is the lowest for strongly asymmetric fission. These results well explain the product size distribution observed in the experiment. The appearance size for multiply charged clusters measured in the experiment is found to be the size where the fission barrier is equal to the monomer evaporation energy. These findings provide evidence that small mercury clusters behave like van der Waals clusters in the process of fragmentation. [ABSTRACT FROM AUTHOR]

A system-specific but very accurate density functional theory (DFT) correction scheme is proposed for precise calculations of adsorbent-adsorbate interactions by combining the non-empirical van der Waals density functional (vdW-DF) method and the empirical DFT/CC correction scheme to reach accuracy of the coupled clusters method with single, double and perturbative triple excitations (CCSD(T)). The new approach is applied to small molecules (CH4, CO2, H2, H2O, N2) interacting with silica surfaces and purely siliceous microporous solids. The vdW-DF/CC results for a perfectly reconstructed α-quartz surface are consistent with other dispersion-corrected DFT methods. Corrected for ZPVE, the vdW-DF/CC enthalpies of adsorption in pure-silica zeolite LTA (ΔHads(0 K)) of 3.6 and 5.2 kcal/mol for methane and carbon dioxide, respectively, are in excellent agreement with experimental values of 3.6 and 5.0 kcal/mol. The very high accuracy of the new scheme and its relatively easy use and numerical stability as compared to the earlier DFT/CC scheme offer a straightforward solution for obtaining reliable predictions of adsorption energies. [ABSTRACT FROM AUTHOR]

We report experimental results for the ionization and fragmentation of weakly bound van der Waals clusters of n C60 molecules following collisions with Ar2 +, He2 +, and Xe20 + at laboratory kinetic energies of 13 keV, 22.5 keV, and 300 keV, respectively. Intact singly charged C60 monomers are the dominant reaction products in all three cases and this is accounted for by means of Monte Carlo calculations of energy transfer processes and a simple Arrhenius-type [C60]n+ → C60++(n-1)C60 evaporation model. Excitation energies in the range of only ∼0.7 eV per C60 molecule in a [C60]13+ cluster are sufficient for complete evaporation and such low energies correspond to ion trajectories far outside the clusters. Still we observe singly and even doubly charged intact cluster ions which stem from even more distant collisions. For penetrating collisions the clusters become multiply charged and some of the individual molecules may be promptly fragmented in direct knock-out processes leading to efficient formations of new covalent systems. For Ar2 + and He2 + collisions, we observe very efficient C119+ and C118+ formation and molecular dynamics simulations suggest that they are covalent dumb-bell systems due to bonding between C59+ or C58+ and C60 during cluster fragmentation. In the Ar2 + case, it is possible to form even smaller C120-2m+ molecules (m = 2-7), while no molecular fusion reactions are observed for the present Xe20 + collisions. [ABSTRACT FROM AUTHOR]