Quantum simulations are reported for the dynamics following the photoexcitation Ba(S-1)–>Ba(P-1) in Ba(Ar)(10) and Ba(Ar)(20) clusters. The evolution in time is studied in a framework that treats quantum-mechanically all the coupled degrees of freedom. The focus is on the role of nonadiabatic transitions between the three adiabatic surfaces corresponding to the P states of the Ba atom. The time scales of electronic relaxation and of electronic depolarization (orbital reorientation) are computed, and the competition between adiabatic and nonadiabatic effects is assessed. The calculations are carried out by a new scheme that extends the recent classically based separable potential method. Semiclassical surface-hopping simulations are used to produce effective single-mode potentials on which nuclear `'orbitals'' are then generated. The full wave packet is constructed from the electronic states involved, and from these nuclear wave functions. Among the main results we find that nonadiabatic transitions become appreciable around 1 ps after photoexcitation, and they are stronger in the smaller cluster. Comparing Tully's semiclassical method with the quantum simulations, good qualitative agreement is found. Quantitatively, the semiclassical predictions for the electronic states branching rations deviate from the quantum results roughly by a factor of 2 after 1 ps. In the smaller cluster direct dissociation of the Ba atom dominates over energy redistribution within the cluster, the opposite being true for the large system. This example demonstrates the feasibility of quantum simulations of nonadiabatic processes in large systems with the new method. (C) 1996 American Institute of Physics.
The quantum dynamics of unimolecular dissociation are discussed for a typical situation where the number of energy rich but bound (zero order) states far exceeds the number of states with enough energy along the reaction coordinate in order to dissociate. It is shown that one can introduce a new basis of states, the `trapping' states, which diagonalize the effective Hamiltonian that governs the dynamics in the bound subspace. The properties of these states are discussed with special reference to the two distinct time regimes (prompt and delayed) for the dissociation that are expected for such a congested level structure.
Five essential oils and nine of their components were compared to di-Et toluamide (DEET) for their repellent activity against the human body louse, Pediculus humanus humanus. The abs. or intrinsic repellency of the compds. was tested by applying the repellent to corduroy patches and comparing them with untreated patches. The most effective repellents were DEET and citronella, whose activity lasted at least 29 days. The activity of rosemary lasted at least 18 days and that of eucalyptus more than 8 days. The repellent activity of the oil components such as citronellal and geraniol lasted more than 15 and 8 days, resp. DEET remained effective at a diln. of 1:32, geraniol at 1:8, citronella at 1:4 and rosemary and citronellal at 1:1. The comparative or std. repellency of the candidate repellents was examd. with the aid of a new screening technique using hairs treated with ammonium bicarbonate which is attractive to lice. Using this technique it could be shown that the repellent activity of citronella and geraniol lasted 2 days and that of rosemary and citronellal for only one day. DEET was active for less than one day. Serial dilns. of these substances also revealed that citronella was the most potent repellent for lice, followed by citronellal, rosemary, geraniol and DEET. The differences however, were not significant. [on SciFinder(R)]
The dissociation dynamics of oxygen on silver surfaces is studied theoretically. The method is based on a quantum-mechanical time-dependent non-adiabatic picture. A universal functional form for the potential energy surfaces is employed. The diabatic potentials describing the sequence of events leading to dissociation begin from the physisorption potential crossing over to a charged molecular chemisorption potential and crossing over again to the dissociated atomic-surface potential. Within such a potential surface topology, two different surfaces leading to dissociation are studied: the empirical potential of Spruit and the ab-initio potential of Nakatsuji. It is found that the system is captured by the molecular chemisorption well for a considerable length of time, long enough for thermalization. Thus the calculation is split into two parts: the calculation of “direct” dissociation probability and the calculation of nonadiabatic dissociative tunneling rate from the thermalized chemisorbed molecular state. For the direct probabilities, the Fourier method with the Chebychev polynomial expansion of the evolution operator is used to solve the time-dependent Schrödinger equation. For the tunneling rate calculation, a similar expansion of Green's operator is used. The output of the direct-reaction calculation is the dissociation probability as a function of the initial energy content, while the tunneling calculation yields the dissociation rate. The dependence of the direct dissociation probability on the initial kinetic energy is found to be non-monotonic. A strong isotope effect has been found, favoring the dissociation of the light species.
Using exact matrix elements for the coupling, the effect of the anisotropy of the core on high molecular Rydberg states is studied by quantum dynamics. It is found that on the time scale which can be probed by zero kinetic energy spectroscopy there is extensive interseries mixing. In particular, the long decay times are due to the sojourn in Rydberg series which are not directly effectively coupled to the continuum. These are series built on higher rotationally excited states of the core and a dynamical bottleneck controls the coupling to the bound series directly coupled to the ionization continuum. The computations are carried out for realistic molecular parameters and in the presence of a weak external de field. The quadrupolar coupling is often more effective in interseries coupling than the dipolar anisotropy even though the latter has a far higher range. The external field exhibits the expected `'dilution'' or `'time stretching'' effect at short times (of the order of the Stark period) but enhances the interseries mixing at longer times. An incomplete I mixing is the origin of another dynamical bottleneck. The time evolution is described both by exact quantum propagation and by a reduced description where degenerate states (i.e., states which differ only in the magnetic quantum numbers) are taken to be equally populated, on the average. This grouping, valid at longer times, facilitates the quantal computations which include several series with the full complement of angular momentum states of the electron. Such computations are possible by taking advantage of the conservation of the (total projection) quantum number M. For higher values of IM the coupling to the continuum is very much hindered and the bound Rydberg series exhibit extreme stability. The paper concludes by an analysis of the three bottlenecks which can give rise to longer decays. (C) 1996 American Institute of Physics.
The adsorption of L-cysteine on Au(111) from a perchlorate soln. was studied by in-situ scanning tunneling microscopy. The potential-induced adsorption of the thiols lifts the reconstruction of Au(111) and at more pos. potentials L-cysteine adopts a definite adlayer structure. The potential-induced desorption of the adlayer does not cause the reconstruction to reappear but instead leaves a roughened surface with a large no. of monoat. high gold islands and monoat. deep holes. [on SciFinder(R)]
The progress and extent of nucleophilic substitution and ion exchange reactions of self-assembled chromophoric monolayers are studied by X-ray photoelectron (XPS) and second harmonic generation (SHG) spectroscopy. Self-assembled monolayers prepared from 2-[4-(chloromethyl)phenyl]ethyl trichlorosilane (1) on glass substrates are susceptible to nucleophilic substitution of ∼90% of the surface-confined benzylic chloride functionalities with the “hypernucleophile” 4-(dimethylamino)pyridine; however, only ∼60% of the densely packed benzyl chloride groups undergo reaction with the high-β chromophore precursor 4‘-[4-[N,N-bis(3-hydroxypropyl)amino]styryl]pyridine (2a). Quaternization of a benzylic monolayer with this molecule yields a monolayer having a bulk second-order NLO response (χ(2)) of 3 × 10-7 esu at λ0 = 1064 nm, corresponding to a near-maximum chromophore coverage of ∼2 × 1014molecules/cm2. The kinetics of this substitution reaction and associated structural modifications are studied in real time by in situ polarized SHG techniques, which reveal non-Langmuirian kinetics and a rapidly increasing chromophore tilt angle with increasing coverage. The quaternization kinetics can be fit to a phenomenological biexponential rate equation with k‘1 ≈ 2 × 10-2 L mol-1 s-1 and k‘2 ≈ 2 × 10-3 L mol-1 s-1 and to a coverage-dependent activation energy model (EA = E0 + Ebθ), yielding a perturbative energy Eb of 6−8 kJ mol-1. Both models are compatible with increasing repulsive interactions between chromophores at high coverages. The charge-compensating chloride counterions within monolayers having dense chromophore packing can be ion exchanged with iodide, up to a maximum of ∼40% of available chloride ions. The introduction of larger anions (sulfanilate, ethyl orange, eosin B) is observed in less densely packed films; however, the ion exchange process is completely inhibited in monolayers capped with a siloxane overlayer. In all cases, exchange of the chloride leads to significant increases in the second-harmonic generation efficiency, up to 45% on exchange with eosin B. In the case of iodide and sulfanilate substitution for chloride, the increase in the second-order response upon ion exchange is attributable to the incoming anion assuming a position within the monolayer microstructure different from that of the displaced anion.
The formation and organization of self-assembled monolayers of alkanethiols and ω-mercaptocarboxylic acids on Hg surfaces were studied. The organization process was examd. in-situ and ex-situ by cyclic voltammetry. Charging current measurements as well as electron transfer expts. with Ru(NH3)63+/2+ indicate that the adsorption of homologous alkanethiols and ω-mercaptocarboxylic acids on Hg is followed by the formation of a densely packed array. [on SciFinder(R)]
Semiclassical molecular dynamics simulations are developed as a tool for studying anharmonic clusters and solids at energies near the zero point. The method employs the time-dependent self-consistent-held approximation, that describes each mode as moving in the mean dynamical field of all other modes. The method further describes each mode by a semiclassical Gaussian wave packet; The scheme is carried out in normal modes. The method is restricted to systems of moderate anharmonicity at low temperatures. It is, however, computationally efficient and practically applicable to large systems. It can be used for the dynamics of nonstationary states as well as for stationary ones. Structural, dynamical and a variety of spectroscopic properties can easily be evaluated. The method is tested for thermal equilibrium states of (Ne)(13), (Ar)(13) against `'numerically exact'' quantum Feynman path integral simulations. Excellent quantitative agreement is found for the atom-atom pair distribution functions. The method is also applied to (H2O)(n) clusters. Good agreement is found with experimentally available fundamental stretch-mode frequencies. (C) 1996 American Institute of Physics.
Micelle-like clusters of antibody mols. were prepd. by covalent attachment of various hydrophobic groups to the protein mols. These colloidal clusters were capable of solubilizing two hydrophobic probes, while the solubilizate:solubilizer molar ratio was dependent on the chain length of the hydrophobic groups, the degree of modification, and have, on the size of the colloidal clusters. By using a fluorescent solubilizate, it was demonstrated that the immunoclusters may have a specific recognition ability. [on SciFinder(R)]
Diffusion quantum Monte Carlo (DQMC) simulations are used to study the vibrational ground state of the clusters Hg(H-2)(12) and Mg(H-2)(12). In these clusters, the metal atom is located inside, surrounded by a highly delocalized, liquid-like and, for Hg(H-2)(12), also nearly spherical envelope of hydrogens. However, the wavefunctions of the clusters exhibit traces of icosahedral behavior within the delocalized structure, related to the geometry of the corresponding classical systems. A simple model wavefunction is fitted, with excellent agreement to the numerical DQMC data. It provides a convenient interpretation of the vibrational ground state of these clusters.
Communication in Repeated Games with Partial Monitoring
with M. Kahneman
The paper considers repeated games where each player can be observed by only a subset of the other players, and where players can make public announcements about the behavior of the players they observed. We address the following question: What is the minimal level of observability that is required to obtain efficient outcomes? The main result is that the limit set of sequential equilibrium payoffs, when the discount factor tends to one, contains the set of individual rational payoffs whenever each player is observed by at least two other players.
Molecular Dynamics simulations using a surface-hopping method for transitions between different electronic states are employed to study the dynamics following photoexcitation of the Ba(Ar)(125) cluster. The results are used to interpret spectroscopic experiments on large, size-distributed Ba(Ar)(n) clusters. The dynamics of the coupled electronic-nuclear motions in the cluster involves transitions between three potential energy surfaces, corresponding to the nearly-degenerate p-states of the excited Ba atom. Ejection of excited Ba atoms, adsorbed on the surface of the cluster, can take place. The focus in comparing theory and experiment is on the emission spectrum from the excited clusters, on the polarization of this radiation, and on the polarization of light emitted by excited Ba atoms ejected from the cluster. Based on the good agreement found between theory and experiment, a comprehensive picture of the excited state dynamics is given. It is found that upon excitation, energy is rapidly redistributed in the cluster and no direct ejection of Ba occurs. Electronic relaxation to the lowest P-state occurs, and the latter dominates the cluster emission spectrum and polarization. The electronic state relaxation is mostly complete within t less than or similar to 10 ps. Ejection of Ba atoms occurs as a rare and delayed event when a dynamical fluctuation creates a `'hot spot'' at the Ba site, with a non-adiabatic excitation to the highest electronic level. The results show the feasibility of near-quantitative understanding of non-adiabatic processes in large clusters. (C) 1996 American Institute of Physics.
A statistical thermodynamic approach is used to analyze the various contributions to the free energy change associated with the insertion of proteins and protein fragments into lipid bilayers. The partition coefficient that determines the equilibrium distribution of proteins between the membrane and the solution is expressed as the ratio between the partition functions of the protein in the two phases. It is shown that when all of the relevant degrees of freedom (i.e., those that change their character upon insertion into the membrane) can be treated classically, the partition coefficient is fully determined by the ratio of the configurational integrals and thus does not involve any mass-dependent factors, a conclusion that is also valid for related processes such as protein adsorption on a membrane surface or substrate binding to proteins. The partition coefficient, and hence the transfer free energy, depend only on the potential energy of the protein in the membrane. Expressing this potential as a sum of a `'static'' term, corresponding to the equilibrium (minimal free energy) configuration of the protein in the membrane, and a `'dynamical'' term representing fluctuations around the equilibrium configuration, we show that the static term contains the `'solvation'' and `'lipid perturbation'' contributions to the transfer free energy. The dynamical term is responsible for the `'immobilization'' Free energy, reflecting the loss of translational and rotational entropy of the protein upon incorporation into the membrane. Based on a recent molecular theory of lipid-protein interactions, the lipid perturbation and immobilization contributions are then expressed in terms of the elastic deformation free energy resulting from the perturbation of the lipid environment by the foreign (protein) inclusion; The model is formulated for cylindrically shaped proteins, and numerical estimates are given for the insertion of an a-helical peptide into a lipid bilayer. The immobilization free energy is shown to be considerably smaller than in previous estimates of this quantity, and the origin of the difference is discussed in detail.
