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.

Circumstellar disks are exposed to intense ultraviolet (UV) radiation from the young star. In the inner disks, the UV radiation can be enhanced by more than seven orders of magnitude compared with the average interstellar radiation field, resulting in a physical and chemical structure that resembles that of a dense photon-dominated region (PDR). This intense UV field affects the chemistry, the vertical structure of the disk, and the gas temperature, especially in the surface layers. The parameters which make disks different from more traditional PDRs are discussed, including the shape of the UV radiation field, grain growth, the absence of PAHs, the gas/dust ratio and the presence of inner holes. Illustrative infrared spectra from the Spitzer Space Telescope are shown. New photodissociation cross sections for selected species, including simple ions, are presented. Also, a summary of cross sections at the Lyman α 1216Å line, known to be strong for some T Tauri stars, is made. Photodissociation and ionization rates are computed for different radiation fields with color temperatures ranging from 30000 to 4000 K and grain sizes up to a few µm. The importance of a proper treatment of the photoprocesses is illustrated for the transitional disk toward HD 141569A which includes grain growth.

ABSTRACT:The dissociative chemisorption of methane on metal surfaces is of fundamental and practical interest, being a rate-limiting step in the steam reforming process. The reaction is best modeled with quantum dynamics calculations, but these are currently not guaranteed to produce accurate results because they rely on potential energy surfaces based on untested density functionals and on untested dynamical approximations. To help overcome these limitations, here we present for the first time statistically accurate reaction probabilities obtained with ab initio molecular dynamics (AIMD) for a polyatomic gas-phase molecule reacting with a metal surface. Using a general purpose density functional, the AIMD reaction probabilities are in semiquantitative agreement with new quantum-state-resolved experiments on CHD 3 + Pt(111). The comparison suggests the use of the sudden approximation for treating the rotations even though CHD 3 has large rotational constants and yields an estimated reaction barrier of 0.9 eV for CH 4 + Pt(111). SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis T he steam reforming process, in which methane and water react over a Ni catalyst, is the main commercial source of molecular hydrogen. The dissociation (or dissociative chemisorption) of CH 4 on the catalyst into CH 3 (ad) + H(ad) is a rate-determining step of the full process. 1 Moreover, dissociation of methane on metal surfaces is of fundamental interest. 2−13 Already from early molecular beam experiments, it is known that vibration is very effective in promoting reactivity. 3,4,14 More recently, it has been shown that the reaction is mode-specific, that is, the degree to which energizing the molecule promotes reaction depends on whether the energy is put in translation or vibration and even on which vibration it is put in (vibrational mode specificity). 5−8 These observations, which have been explained qualitatively on the basis of different models, 9,15 rule out the application of fully statistical models. For some vibrational modes, the vibrational efficacy, which measures how effective putting energy into vibration is at promoting reaction relative to increasing the incidence energy (E i ), is even larger than one. 7,10 In addition, the dissociation of partially deuterated molecules shows bond selectivity; for instance, in CHD 3 , the CH bond can be selectively broken upon excitation to an appropriate initial vibrational state. 11,12 Finally, dissociative chemisorption of methane on metal surfaces represents a current frontier in the theoretical description of the dynamics of reactions of gas-phase molecules on metal surfaces, 15−24 with much current efforts now being aimed at achieving an accurate description of this reaction through high-dimensional quantum dynamics calculations. 16,23,24 A wealth of experiments exist for the methane + Pt(111) system. 3,8,12,17,25−29 There has been considerable debate 2,25 concerning the importance of tunneling in this and similar systems. Recent calculations 17,23,30 suggest only a...

Results of ab initio calculations for the four lowest excited states of both A′ and A″ have been discussed. In the multireference configuration interaction calculations, a large Rydberg basis set has been used. Three-dimensional potential energy surfaces, and matrix elements of the transition dipole moment between the excited states and the ground X̃ state, and the electronic angular momentum operator between the à state and the B̃ and X̃ states have been presented. The calculations show that above about 124 nm the photodissociation can be well described by the three lowest electronic states, X̃, Ã, and B̃. The ab initio results of matrix elements of the electronic angular momentum operator allow a realistic nonadiabatic treatment of the photodissociation in the B̃ band. At wavelengths smaller than about 124 nm, the dynamics will be more complicated because of the coupling between various electronic states.

A complete three-dimensional quantum mechanical description of the photodissociation of water in the B̃ band, starting from its rotational ground state, is presented. In order to include B̃-X̃ vibronic coupling and the B̃-Ã Renner–Teller coupling, diabatic electronic states have been constructed from adiabatic electronic states and matrix elements of the electronic angular momentum operators, following the procedure developed by A. J. Dobbyn and P. J. Knowles [Mol. Phys. 91, 1107 (1997)], using the ab initio results discussed in the preceding paper. The dynamics is studied using wave packet methods, and the evolution of the time-dependent wave function is discussed in detail. Results for the H2O and D2O absorption spectra, OH(A)/OH(X) and OD(A)/OD(X) branching ratios, and rovibrational distributions of the OH and OD fragments are presented and compared with available experimental data. The present theoretical results agree at least qualitatively with the experiments. The calculations show that the absorption spectrum and the product state distributions are strongly influenced by long-lived resonances on the adiabatic B̃ state. It is also shown that molecular rotation plays an important role in the photofragmentation process, due to both the Renner–Teller B̃-X̃ mixing, and the strong effect of out-of-plane molecular rotations (K>0) on the dynamics at near linear HOH and HHO geometries.