George R. Darling

The dissociation of diatomic molecules at surfaces

We present an exposition of the various theoretical models currently in use for describing the dynamics of molecular dissociation at surfaces. We begin by outlining the representations of the nuclear and electronic dynamics and how these define the potential energy surfaces for the interactions. Strategies for solving the nuclear motion follow with particular emphasis being paid to a quantum description on the electronic ground state which is in line with experiments employing hyperthermal molecular beams. These can be performed in either a time-dependent or time-independent fashion and both approaches are considered. Following this, the methods that have been developed for treating the dissipative motion as the molecule nears the surface are presented. This is divided into energy loss to the electronic subsystem and to the substrate atomic vibrations. The final part of the review shows how the results of theoretical simulations have been usefully applied to rationalize data obtained from molecular beam scattering experiments.

An Adaptable Peptide-Based Porous Material

Swelling Pores Porosity is a key parameter when selecting materials for catalysts, chemical separations, gas storage, host-guest interactions, and related chemical processes. In most cases the porosity of a material is fixed. Rabone et al. (p. 1053; see the Perspective by Wright) have described a molecular material in which the size of the pores changed during the sorption process. The porosity increased because a dipeptide linker between metal centers reoriented during uptake of some gases, thus improving the capacity of the material to adsorb. Conformational changes in a porous material during the sorption of small molecules lead to a dynamic increase in porosity. Porous materials find widespread application in storage, separation, and catalytic technologies. We report a crystalline porous solid with adaptable porosity, in which a simple dipeptide linker is arranged in a regular array by coordination to metal centers. Experiments reinforced by molecular dynamics simulations showed that low-energy torsions and displacements of the peptides enabled the available pore volume to evolve smoothly from zero as the guest loading increased. The observed cooperative feedback in sorption isotherms resembled the response of proteins undergoing conformational selection, suggesting an energy landscape similar to that required for protein folding. The flexible peptide linker was shown to play the pivotal role in changing the pore conformation.

ROTATIONAL MOTION AND THE DISSOCIATION OF H2 ON CU(111)

The influence of rotational state on the dissociation probability of H2 on Cu(111) has been investigated with 3‐ and 4‐dimensional close‐coupling wave packet calculations. Recent experimental results have shown that the energetic threshold for dissociative adsorption increases and then decreases as the J state is continuously increased. This trend can be faithfully reproduced by modeling the H2 as a planar (cartwheel) rotor scattering from a flat surface. The agreement disappears when the model is extended to a 3‐dimensional rotor. Further, the degenerate mJ states have a spread of dissociation probabilities which results in a broad smearing of the dissociation threshold. This effect, which is absent from experiment, increases with Ji. These shortcomings can be partially corrected by corrugating the potential in the azimuthal coordinate in accord with recent ab initio results. The dynamical calculations also exhibit strong rotational inelasticity for the scattered fraction, during dissociation. Since this...

Polymorphism control of superconductivity and magnetism in Cs3C60 close to the Mott transition

The crystal structure of a solid controls the interactions between the electronically active units and thus its electronic properties. In the high-temperature superconducting copper oxides, only one spatial arrangement of the electronically active Cu2+ units—a two-dimensional square lattice—is available to study the competition between the cooperative electronic states of magnetic order and superconductivity. Crystals of the spherical molecular C603- anion support both superconductivity and magnetism but can consist of fundamentally distinct three-dimensional arrangements of the anions. Superconductivity in the A3C60 (A = alkali metal) fullerides has been exclusively associated with face-centred cubic (f.c.c.) packing of C603- (refs 2, 3), but recently the most expanded (and thus having the highest superconducting transition temperature, Tc; ref. 4) composition Cs3C60 has been isolated as a body-centred cubic (b.c.c.) packing, which supports both superconductivity and magnetic order. Here we isolate the f.c.c. polymorph of Cs3C60 to show how the spatial arrangement of the electronically active units controls the competing superconducting and magnetic electronic ground states. Unlike all the other f.c.c. A3C60 fullerides, f.c.c. Cs3C60 is not a superconductor but a magnetic insulator at ambient pressure, and becomes superconducting under pressure. The magnetic ordering occurs at an order of magnitude lower temperature in the geometrically frustrated f.c.c. polymorph (Néel temperature TN = 2.2 K) than in the b.c.c.-based packing (TN = 46 K). The different lattice packings of C603- change Tc from 38 K in b.c.c. Cs3C60 to 35 K in f.c.c. Cs3C60 (the highest found in the f.c.c. A3C60 family). The existence of two superconducting packings of the same electronically active unit reveals that Tc scales universally in a structure-independent dome-like relationship with proximity to the Mott metal–insulator transition, which is governed by the role of electron correlations characteristic of high-temperature superconducting materials other than fullerides.

Steering effects in non-activated adsorption

Abstract Classical and quantum calculations of the dissociation dynamics of H 2 on W(100) have been performed on an ab initio PES. The results show conclusively that at low translational energies the dissociation is dominated by strong steering in an essentially direct process. Starting from a value near unity, the dissociation falls with increasing energy because the steering has less time to operate and is therefore less effective. No precursor state is involved in the process. By examining the quantum flux we see that molecules oriented with their axes perpendicular to the surface reflect at high translational energy, but not at low translational energy. Some molecular trapping occurs as a result of rotational excitation which gives rise to sharp peaks in the quantum dissociation probability.

The role of parallel momentum in the dissociative adsorption of H2 at highly corrugated surfaces

Molecular beam experiments of H2 adsorption on Pt, Fe and Ni(100) show an inhibition of dissociation with increasing parallel momentum, which has been ascribed to surface corrugation. Here, we present the results of quantum dynamical calculations of the dissociation on a three-dimensional potential energy surface with strong corrugation across the unit cell. We show that there are two types of corrugation; energetic, where the barrier height varies across the unit cell, and geometric, where the barrier location above the surface is modulated. In the first case, dissociation is inhibited by momentum parallel to the surface, while in the second, off-normal incidence enhances dissociation. We demonstrate that a combination of the two is capable of giving approximate normal energy scaling in dissociation, thus explaining how the H2/Cu PES can appear “flat” in sticking experiments, while total energy calculations show it to be highly corrugated.

Translation‐to‐vibrational excitation in the dissociative adsorption of D2

In this work we have studied the dissociation dynamics of deuterium on two potential energy surfaces. In each case, there is significant translational‐to‐vibrational coupling that results in molecules that emerge vibrationally excited. In each case, the translational energy dependence of the onset of inelastic scattering is similar. The surfaces also result in dissociative adsorption but the thresholds in this case are well separated. The particular topological differences between the two surfaces that result in these findings are discussed.In this work we have studied the dissociation dynamics of deuterium on two potential energy surfaces. In each case, there is significant translational‐to‐vibrational coupling that results in molecules that emerge vibrationally excited. In each case, the translational energy dependence of the onset of inelastic scattering is similar. The surfaces also result in dissociative adsorption but the thresholds in this case are well separated. The particular topological differences between the two surfaces that result in these findings are discussed.

HOT ELECTRON MEDIATED PHOTODESORPTION : A TIME-DEPENDENT APPROACH APPLIED TO NO/PT(111)

Time‐dependent quantum wave packets have been used in a model calculation to investigate the substrate‐mediated photodesorption of a molecule from a metal surface. A ‘‘hot’’ electron, generated in the substrate by an absorbed photon, temporarily resonates in an unoccupied molecular orbital. This results in a new set of forces, and if the electron spends sufficient time in the resonance, then on returning to the electronic ground state the molecule will have acquired sufficient energy to desorb. Rather than modeling the excitation and relaxation steps independently, we treat the motion of the molecule and the hot electron on an equal footing. We have studied the dynamics on potential energy surfaces (PESs) explicitly including both the electronic and nuclear coordinates. PES parameters were chosen to model NO desorption from Pt where it has been suggested that the excited state is attractive. The desorption probability has been calculated as a function of hot electron energy and photon energy for different...

Theory of laser‐induced desorption of ammonia from Cu(111): State‐resolved dynamics, isotope effects, and selective surface photochemistry

A two‐dimensional, two‐state model is used to model the UV‐laser‐induced photodesorption dynamics of NH3 and ND3 from Cu(111) by solving the nuclear time‐dependent Schrodinger equation. By projecting the asymptotic wave functions on the asymptotic (‘‘umbrella’’) eigenstates of NH3/ND3, we find that the molecules leave the surface vibrationally hot, in agreement with experimental data. Within individual asymptotic tunneling doublets, however, the desorbates are clearly non‐Boltzmann with molecules of ‘‘gerade’’ symmetry desorbing with increased probability. Our study correlates this parity selection with details of the electronic ground state potential energy surface. An experimentally observed strong isotope effect in the desorption yields for the different isotopomers is traced back mainly to differences between the vibrational frequencies of the ‘‘umbrella’’ mode, in accord with earlier, classical models. Additionally, small tunneling and moderate zero‐point contributions are observed. Finally, the poss...

Dissociation thresholds and the vibrational excitation process in the scattering of H2

Abstract We have used low (two- and three-) dimensional models to examine the interplay between vibrational effects in the dissociative adsorption of H2 on Cu(111) and the vibrational excitation of the reflected molecules. It is found that the vibrational non-adiabaticity required for the latter causes the dissociation probabilities for vibrationally excited states to shift to lower energies, giving a vibrational efficacy of 1. To reduce this to the experimental value of ~ 0.5, it is necessary to force the vibrational excitation to high energy, in contradiction with the experimental findings. Introducing a rotational coordinate, to give the possibility of rotational-vibrational coupling, does not produce better agreement. These results suggest that the processes occur at different surface sites.