R. Stephen Berry

Correlation of Rates of Intramolecular Tunneling Processes, with Application to Some Group V Compounds

A method is presented whereby the WKB expression for tunneling frequencies permits correlation of these frequencies for members of homologous series of molecules. The necessary data required are the tunneling rate for one member of the series, and structural parameters and vibrational frequencies for all members of interest. The method is applied to ND3, NF3, PH3, and AsH3, using NH3 as the molecule whose frequency is known. The method also is used to support the hypothesis that pseudorotation occurs in the trigonal bipyramids PF5 and PCl5.

Solid–liquid phase changes in simulated isoenergetic Ar13

Simulations by molecular dynamics of 13‐particle clusters of argon display distinct nonrigid, liquid‐like and near‐rigid, solid‐like ‘‘phases.’’ The simulations, conducted at constant total energy, display a low‐energy region in which only the solid‐like form appears, a high‐energy region in which only the liquid‐like form appears, and an intermediate band of energy—a ‘‘coexistence region’’— in which clusters exhibit both forms. The intervals of time spent in each phase in the two‐form coexistence region are long compared with the intervals required to establish equilibrium‐like properties distinctive of each form, such as mean square displacement and power spectrum, so that well‐defined phases can be said to exist. The fraction of time spent in each phase is a function of the energy. When a long simulation is separated into regions of solid‐like and liquid‐like behavior, the curve of the derived caloric equation of state is double valued in the two‐phase range of energy, forming two well‐defined, smooth ...

The effect of the range of the potential on the structures of clusters

We investigate the structures of clusters bound by the Morse potential by mapping the structure of the global minimum as a function of both cluster size and the range of the pair potential. We consider values of the range parameter appropriate to a loosely bound diatomic molecule (longest), two C60 molecules (shortest), and at regular intervals between these two limits. We have studied all cluster sizes with 25 atoms or less and a selection of sizes containing between 35 and 80 atoms. The effect of decreasing the range of the potential is to destabilize strained structures. For the larger clusters the structure of the global minimum changes from icosahedral to decahedral to face‐centered cubic as the range is decreased. We have also investigated the effects of temperature on the equilibrium structure by performing a model calculation for a 75‐atom cluster.

Absorption Spectrum of Gaseous F‐ and Electron Affinities of the Halogen Atoms

The absorption spectrum of gaseous fluoride ion has been observed in shock‐heated vapors of CsF, RbF, and KF. The spectrum is a continuum with two sharp thresholds at 3595 and 3542 A, similar to those of the other halide ions. The electron affinity determined from the low‐energy threshold is 3.448±0.005 eV. The cross‐section for photodetachment at 3565 A is about 2.5±2×10−18 cm2 and at 3525, about 3.3±2×10−18 cm2. The electron affinities of chlorine, bromine and iodine are revised, respectively, to 3.613, 3.363, and 3.063 eV, all with uncertainties of 0.003 eV.

Thermodynamics in finite time

Until the 19th century, technology was essentially the domain of skilled artisans and constructors who relied on practical experience to design and build their machines. One of the first efforts to use physical theory to study the functioning of machines was undertaken by the French engineer Sadi Carnot. Motivated by the concern of the French about the superiority of British steam engines, he undertook a systematic study of the physical processes governing steam engines, resulting in his remarkable paper Reflexions sur la puissance motrice du feu (On the Motive Power of Heat) published in 1826. Among the earliest successes of this new science, thermodynamics, was the formulation of criteria describing how well real processes perform in comparison with an ideal model. Carnot showed that any engine, using heat from a hot reservoir at temperature Th to do work, has to transfer some heat to a reservoir at lower temperature T1, and that no engine could convert into work more of the heat taken in at Th than the...

Ionization of Molecules at Low Energies

A general formal theory is developed for the autoionization of molecules at energies within a few electron volts or less of threshold, specifically to include effects of the interaction of an excited Rydberg electron with the rotating, vibrating molecule—ion to which it is bound. The dominant coupling mechanism involves conversion of vibrational energy to electronic energy and is most efficient when the core can undergo a single quantum jump. A number of simplifications and approximations are discussed. Some of these are used in an application of the theory to the autoionization of H2. Autoionization rates are computed for a number of vibronic Rydberg states of this molecule, and are consistent with the available data. Model calculations for heteronuclear diatomics are presented to show how the presence of a vibrating dipole in the molecular core enhances the rates of autoionization, in comparison with the rates in homonuclear diatomics where vibrating quadrupoles stimulate the process. Several experiments are suggested, particularly the measurement of the angular distributions of electrons detached from heteronuclear diatomics.

Rare gas clusters: Solids, liquids, slush, and magic numbers

Simulations by constant energy molecular dynamics have been performed for numerous clusters in the size range N=7–33. Detailed investigations have been conducted on the portions of the caloric curves in which the transition between rigid and nonrigid behavior occurs, to study the N dependence of the solid–liquid phase change. Clusters of several sizes display a coexistence of forms, each with a characteristic mean temperature, over a well‐defined energy range in the transition region, as had been observed for the Ar13 cluster. Within the coexistence region, the high temperature form is solid‐like and the low temperature form behaves in a liquid‐like fashion. The caloric curves of state for these clusters take on two‐valued forms when averages are calculated for each of the two ‘‘phases’’ separately; the two branches are smooth extensions of the curves from the single phase regions. Clusters of other sizes do not display this clear coexistence of phases, but appear to pass through a ‘‘slush‐like’’ state du...

The onset of nonrigid dynamics and the melting transition in Ar7

We have carried out a molecular dynamics (MD) simulation study of melting of Ar7. By periodically quenching trajectories, we are also able to follow the path of the cluster through configuration space. This procedure yields information about isomerization rates, isomerization dynamics, and the connectivity of the phase space as a function of energy. New criteria for melting and the coexistence of phases in small clusters are compared with the traditional T(E) curves and rms bond fluctuations available from time averages in MD simulations.

Optimal paths for thermodynamic systems: The ideal diesel cycle

Optimal control theory is used to determine the piston trajectory which yields maximum power output for a model which incorporates the diesel engine’s major irreversibilities. Optimal trajectories were obtained for the cases of unconstrained and constrained piston acceleration. Optimizing the path for our standard engine increased both the net work output per cycle and the net efficiency by about 10%.

Melting and freezing of small argon clusters

An investigation of melting, freezing, and coexistence phenomena is presented for small clusters using Ar7, Ar8, Ar13, and Ar14 as specific representative examples. Combining the results of molecular dynamics simulations, especially short‐time kinetic energy averages and quenching, with accurate calculations of the local minima and transition states illuminates the relationship between the potential energy surface and dynamical processes. The results are consistent with a recent general defect theory of melting.