S.H. Payne

First-principles theory of surface thermodynamics and kinetics

In this Letter, with the aim to improve upon this approach, we combine state-of-the-art procedures of (i) microscopic theories, i.e., DFT electronic structure calculations and (ii) macroscopic phenomenological approaches, i.e., lattice gas and rate equations, and Monte Carlo schemes. On doing this, we present a consistent first-principles-based approach for calculation of the thermodynamic and kinetic properties of an adsorbate, such as heats of adsorption, temperature programmed desorption (TPD) spectra, and the surface phase diagram. We have chosen the system of oxygen at Ru(0001) for which detailed structural [4 ‐9], thermodynamic [10], and kinetic data [11,12] exist. We will show that, with the present approach, a realistic description of these physical properties is indeed feasible.

Nonequilibrium thermodynamics of a two-phase adsorbate: Lattice gas and van der Waals models

Abstract The coupled rate equations governing the adsorption-desorption kinetics of a two-phase adsorbate have been analyzed for a lattice gas model with nearest neighbour interactions in the Bragg-Williams and quasichemical approximations and for the 2D van der Waals gas. Isothermal and temperature programmed desorption traces are calculated to illustrate the origin of zero and fractional order kinetics. Nonequilibrium effects are studied and a simplified quasi-equilibrium model is derived when they are unimportant.

Analysis of thermal desorption data

Abstract Using a nonequilibrium thermodynamic approach, we calculate temperature programmed desorption curves and analyse them with isosteric Arrhenius plots. For desorption from a two-phase adsorbate, we produce double-valued desorption energies and prefactors exhibiting a compensation effect. We introduce a differential desorption energy to understand temperature averaging and to describe nonequilibrium effects. Our results delineate, in part, the validity of an Arrhenius-type analysis.

Nonequilibrium thermodynamics of a two-phase adsorbate

Abstract Following the Onsager approach to nonequilibrium thermodynamics we formulate systematically a set of coupled phenomenological rate equations that control the time evolution of a two-phase adsorbate. The latter contains islands of a two-dimensional (2D) condensed phase (liquid or solid) and a dilute 2D gas phase on the bare substrate surface and on top of the condensed islands. As rate processes we include desorption from the 2D gas phase and from the condensed phase, as well as 2D condensation and evaporation. Numerical examples cover isothermal and temperature-programmed desorption. Features associated with zero and fractional order desorption and nonequilibrium effects are discussed.

Theory of dissociative and nondissociative adsorption and desorption

Based on nonequilibrium thermodynamics we formulate a general theory of the kinetics of adsorption, desorption, and dissociation of gases at surfaces. We begin with a concise formulation of dissociation equilibrium at surfaces and then derive the kinetic equations for adsorption, desorption, and dissociation. For the explicit calculations we employ a lattice gas model for homonuclear molecules with both atoms and molecules present on the surface. Lateral interactions between all species are accounted for. In a series of model calculations we discuss equilibrium properties, such as heats of adsorption, and examine the role of dissociation dis-equilibrium on the time evolution of an adsorbate during temperature programmed desorption. The further effect of (attractive or repulsive) lateral interactions on the kinetics is elucidated in further model calculations. As a realistic example we apply the theory to oxygen on Pt(111).

Stretching a macromolecule in an atomic force microscope: statistical mechanical analysis.

We formulate the proper statistical mechanics to describe the stretching of a macromolecule under a force provided by the cantilever of an Atomic Force Microscope. In the limit of a soft cantilever, the generalized ensemble of the coupled molecule-cantilever system reduces to the Gibbs ensemble for an isolated molecule subject to a constant force in which the extension is fluctuating. For a stiff cantilever, one obtains the Helmholtz ensemble for an isolated molecule held at a fixed extension with the force fluctuating. Numerical examples and predictions for experiments with cantilevers of differing stiffness are given for short and long chains of poly (ethylene glycol), based on parameter-free ab initio calculations.

Lattice gas with multiple interactions: isosteric heat and thermal desorption

Abstract Using the transfer matrix method we calculate the isosteric heat and thermal desorption kinetics for particles adsorbed on a triangular lattice with repulsive nearest neighbor, attractive and repulsive next-nearest neighbor and trio interactions. As an example we study the adsorption of CO on a Ru(0001) surface.

Desorption from a two-phase adsorbate: zero or fractional order

Abstract We discuss the desorption kinetics from a two-phase adsorbate in terms of a simplified rate equation derived from an approach based on the general methods of nonequilibrium thermodynamics. We point out that within the coexistence region one should only expect zero-order desorption if (i) the adsorbate remains in quasi-equilibrium during the desorption process and (ii) if all sticking coefficients are equal.

Thermal desorption kinetics of hydrogen on rhodium (110)

Abstract An anisotropic 2-site lattice gas model with nearest-neighbor, second- and third-nearest-neighbor and trio interactions is set up and solved using transfer matrix techniques to explain equilibrium structural data and temperature-programmed desorption consistently.

Adsorption and desorption of hydrogen on Rh(311) and comparison with other Rh surfaces

Abstract We present LEED and thermal desorption data for hydrogen adsorption on Rh(311). An anisotropic lattice gas model with first, second and third nearest neighbor and trio interactions is set up and solved using transfer matrix techniques to explain equilibrium structural data and temperature-programmed desorption consistently. A comparison with results for hydrogen on Rh(110) and Rh(111) is made.