M. C. van Hemert

Measurement of the conductance of a hydrogen molecule

Recent years have shown steady progress towards molecular electronics, in which molecules form basic components such as switches, diodes and electronic mixers. Often, a scanning tunnelling microscope is used to address an individual molecule, although this arrangement does not provide long-term stability. Therefore, metal–molecule–metal links using break-junction devices have also been explored; however, it is difficult to establish unambiguously that a single molecule forms the contact. Here we show that a single hydrogen molecule can form a stable bridge between platinum electrodes. In contrast to results for organic molecules, the bridge has a nearly perfect conductance of one quantum unit, carried by a single channel. The hydrogen bridge represents a simple test system in which to understand fundamental transport properties of single-molecule devices.

Ab initio calculations of Raman intensities; analysis of the bond polarizability approach and the atom dipole interaction model

For a series of ten electron molecules (HF, H2O, NH3, CH4) the molecular polarizability tensor and the derivatives with respect to the symmetry coordinates have been calculated from ab initio SCF wavefunctions using the finite field method as well as perturbation theory approaches. Raman intensities and degrees of depolarization derived from the finite field results agree well with the available experimental data. The zeroth order bond polarizability model and the atom dipole interaction model have been analysed. Both models can be used to describe the computed static polarizabilities and the derivatives with respect to bond stretching, but fail for the derivatives with respect to the bending coordinates.

Dynamics of co in amorphous water-ice environments

The long-timescale behavior of adsorbed carbon monoxide on the surface of amorphous water ice is studied under dense cloud conditions by means of off-lattice, on-the-fly, kinetic Monte Carlo simulations. It is found that the CO mobility is strongly influenced by the morphology of the ice substrate. Nanopores on the surface provide strong binding sites, which can effectively immobilize the adsorbates at low coverage. As the coverage increases, these strong binding sites are gradually occupied leaving a number of admolecules with the ability to diffuse over the surface. Binding energies and the energy barrier for diffusion are extracted for various coverages. Additionally, the mobility of CO is determined from isothermal desorption experiments. Reasonable agreement on the diffusivity of CO is found with the simulations. Analysis of the 2152 cm^−1 polar CO band supports the computational findings that the pores in the water ice provide the strongest binding sites and dominate diffusion at low temperatures.

Photodissociation of small carbonaceous molecules of astrophysical interest

Astronomical observations have shown that small carbonaceous molecules can persist in interstellar clouds exposed to intense ultraviolet radiation. Current astrochemical models lack quantitative information on photodissociation rates in order to interpret these data. We here present ab initio multi-reference configurationinteraction calculations of the vertical excitation energies, transition dipole moments and oscillator strengths for a number of astrophysically relevant molecules: C3, C4, C2H, l and c C3H, l and c C3H2, HC3H, l C4H and l C5H. Highly excited states up to the 9’th root of each symmetry are computed, and several new states with large oscillator strengths are found below the ionization potentials. These data are used to calculate upper limits on photodissociation rates in the unattenuated interstellar radiation field by assuming that all absorptions above the dissociation limit lead to dissociation.

Comparison of electron gas and abinitio potentials for the N2–N2 interactions. Application in the second virial coefficient

A detailed potential for the interaction between two rigid N2 molecules is given in the form of a spherical expansion. The interaction energy is found as the sum of the so‐called Hartree–Fock part of the electron gas expression including the Rae correction and the ’’ab initio’’ dispersion energy in the multipole expansion. Potential surface cuts computed with this expansion agree to a large extent with a similar potential completely based on ab initio calculations. Comparison of the experimental second virial coefficient curve with the curves obtained from a four dimensional quadrature using both ab initio and electron gas potentials demonstrates the usefulness of these potentials, and underlines the importance of the anisotropic contributions.

Hydrogen dissociation on small aluminum clusters.

Transition states and reaction paths for a hydrogen molecule dissociating on small aluminum clusters have been calculated using density functional theory. The two lowest spin states have been taken into account for all the Al(n) clusters considered, with n=2-6. The aluminum dimer, which shows a (3)Π(u) electronic ground state, has also been studied at the coupled cluster and configuration interaction level for comparison and to check the accuracy of single determinant calculations in this special case, where two degenerate configurations should be taken into account. The calculated reaction barriers give an explanation of the experimentally observed reactivity of hydrogen on Al clusters of different size [Cox et al., J. Chem. Phys. 84, 4651 (1986)] and reproduce the high observed reactivity of the Al(6) cluster. The electronic structure of the Al(n)-H(2) systems was also systematically investigated in order to determine the role played by interactions of specific molecular orbitals for different nuclear arrangements. Singlet Al(n) clusters (with n even) exhibit the lowest barriers to H(2) dissociation because their highest doubly occupied molecular orbitals allow for a more favorable interaction with the antibonding σ(u) molecular orbital of H(2).

Conformational instability of the lowest triplet state of the benzene nucleus: I. The unsubstituted molecule

Experiments on benzene have established that its lowest triplet state (3B1u) is conformationally unstable owing to vibronic coupling with the next higher state (3E1u). This instability was found to be critically dependent on the influence of a crystal field. An analogous vibronic coupling is to be expected in the singlet manifold, but here no direct evidence is available for a conformational instability. The distortion behavior of benzene is of importance for the interpretation of its photophysical and photochemical properties. We have therefore determined the potential‐energy surfaces of the 1,3B1u and 1,3E1u states along the two‐dimensional distortion coordinate S8(ρ,φ) using ab initio multireference single and double excitation‐configuration‐interaction calculations. The results show that for both B1u states the hexagonal conformation is unstable and lies 800 cm−1 above a wide, virtually cylindrical trough. A calculation of the vibrational spacing in the 3B1u state yields good agreement with the experi...

Potential energy surface for the study of inelastic collisions between nonrigid CO and H2

The electron gas model is employed to compute the first order part of the potential for the interaction between CO and H2. The intra‐ and intermolecular distances were varied over the range considered important for vibrationally inelastic scattering. The potential consisting of 4158 points is represented analytically by a spherical expansion for the angular part. The distance dependence of the expansion coefficients is given by polynomials. The isotropic second order dispersion energy was estimated from dipole oscillator strength distributions and sum rule ratios for CO and H2. The isotropic part of the complete potential thus obtained agrees better with results derived from molecular beam scattering data than previous potentials.

Analysis of satellite and undulation structure in the spectrum of Na+Hg continuum emission

The authors compare the emission spectral of a Na+Hg high-pressure discharge with spectral calculations using NaHg potentials recently reported by Huwel et al. (1981-2). The spectral calculations are based on, respectively, classical, semiclassical and quantum mechanical theory. They focus on the red part of the emission spectrum (630-1000 nm) and identify a satellite near 671 nm associated with the NaHg B 2 Sigma to X2 Sigma transition; the satellite is accompanied by an undulation structure which is accounted for by the quantum calculation. Based on the comparison between measured and calculated spectra the authors suggest some corrections to the NaHg A and B potentials of Huwel et al.

Molecular dynamics simulations of CO2 formation in interstellar ices.

CO2 ice is one of the most abundant components in ice-coated interstellar ices besides H2O and CO, but the most favorable path to CO2 ice is still unclear. Molecular dynamics calculations on the ultraviolet photodissociation of different kinds of CO-H2O ice systems have been performed at 10 K in order to demonstrate that the reaction between CO and an OH molecule resulting from H2O photodissociation through the first excited state is a possible route to form CO2 ice. However, our calculations, which take into account different ice surface models, suggest that there is another product with a higher formation probability ((3.00 ± 0.07) × 10(-2)), which is the HOCO complex, whereas the formation of CO2 has a probability of only (3.6 ± 0.7) × 10(-4). The initial location of the CO is key to obtain reaction and form CO2: the CO needs to be located deep into the ice. The HOCO complex becomes trapped in the cold ice surface in the trans-HOCO minimum because it quickly loses its internal energy to the surrounding ice, preventing further reaction to H + CO2. Several laboratory experiments have been carried out recently, and they confirm that CO2 can also be formed through other, different routes. Here we compare our theoretical results with the data available from experiments studying the formation of CO2 through a similar pathway as ours, even though the initial conditions were not exactly the same. Our results also show that the HCO van der Waals complex can be formed through the interaction of CO with the H atom that is formed as a product of H2O photodissociation. Thus, the reaction of the H atom photofragment following H2O photodissociation with CO can be a possible route to form HCO ice.