Influence of charge state on the mechanism of CO oxidation on gold clusters

We present results from our joint experimental and theoretical study of the reactivity of anionic and cationic gold oxide clusters toward CO, focusing on the role of atomic oxygen, different charge states, and mechanisms for oxidation. We show that anionic clusters react by an Eley-Rideal-like mechanism involving the preferential attack of CO on oxygen rather than gold. In contrast, the oxidation of CO on cationic gold oxide clusters can occur by both an Eley-Rideal-like and a Langmuir-Hinshelwood-like mechanism at multiple collision conditions as a result of the high adsorption energy of two CO molecules. This large energy of CO adsorption on cationic gold oxide clusters is the driving force for the CO oxidation. Therefore, in the presence of cationic gold species at high pressures of CO, the oxidation reaction is self-promoting (i.e., the oxidation of one CO molecule is promoted by the binding of a second CO). Our findings provide new insight into the role of charge state in gold-cluster-based nanocatalysis.     

A stochastic interrogation method for experimental measurements of global dynamics and basin evolution: Application to a two-well oscillator

An experimental study of local and global bifurcations in a driven two-well magneto-mechanical oscillator is presented. A detailed picture of the local bifurcation structure of the system is obtained using an automated bifurcation data acquisition system. Basins of attractions for the system are obtained using a new experimental technique: an ensemble of initial conditions is generated by switching between stochastic and deterministic excitation. Using this stochastic interrogation method, we observe the evolution of basins of attraction in the nonlinear oscillator as the forcing amplitude is increased, and find evidence for homoclinic bifurcation before the onset of chaos. Since the entire transient is collected for each initial condition, the same data can be used to obtain pictures of the flow of points in phase space. Using Liouville's Theorem, we obtain damping estimates by calculating the contraction of volumes under the action of the Poincaré map, and show that they are in good agreement with the results of more conventional damping estimation methods. Finally, the stochastic interrogation data is used to estimate transition probability matrices for finite partitions of the Poincaré section. Using these matrices, the evolution of probability densities can be studied.     

Full observation of single-atom dynamics in cavity QED

We report the use of broadband heterodyne spectroscopy to perform continuous measurement of the interaction energy E_int between one atom and a high-finesse optical cavity, during individual transit events of ≈250 μs duration. We achieve a fractional sensitivity ≈4×10^-4/ to variations in E_int/? within a measurement bandwidth that covers 2.5 decades of frequency (1–300 kHz). Our basic procedure is to drop cold cesium atoms into the cavity from a magnetooptic trap while monitoring the cavity’s complex optical susceptibility with a weak probe laser. The instantaneous value of the atom–cavity interaction energy, which in turn determines the coupled system’s optical susceptibility, depends on both the atomic position and (Zeeman) internal state. Measurements over a wide range of atom–cavity detunings reveal the transition from resonant to dispersive coupling, via the transfer of atom-induced signals from the amplitude to the phase of light transmitted through the cavity. By suppressing all sources of excess technical noise, we approach a measurement regime in which the broadband photocurrent may be interpreted as a classical record of conditional quantum evolution in the sense of recently developed quantum trajectory theories. Received: 2 June 1998 / Revised version: 4 December 1998 / Published online: 31 March 1999     

Pseudosimilar vertices in a graph

Dissimilar vertices whose removal leaves isomorphic subgraphs are called pseudosimilar. We construct infinite families of graphs having identity automorphism group, yet every vertex is pseudosimilar to some other vertex. Potential impact on the Reconstruction Conjecture is considered. We also construct, for each n, graphs containing a subset of n vertices which are mutually pseudosimilar. the analogous problem for mutually pseudosimilar edges is introduced.     

Transient response in absorptive bistability

Measurements of transient response are described for a system composed of multiple beams of atomic sodium passing through a high-finesse optical cavity. For switching from zero to incident laser intensities above the upper turning point of the steady-state hysteresis cycle, a delay in excess of the empty cavity filling time is observed for the time taken to reach steady state. For switching of the incident laser intensity from zero to above the upper turning point, this delay increases to up to 12 times the empty cavity filling time. The results are in qualitative agreement with earlier theoretical work based upon the mean-field theory for plane waves in absorptive bistability.