Michael Menzinger

High Intensity, Low Energy Spread Ion Source for Chemical Accelerators

A simple plasma ion source which is capable of delivering beams of 1×10−6 A at 200 eV and 0.1–0.2 eV energy spread is described. Gas efficiencies of the order of percents are obtained. This source was found to be very successful in experiments using low energy beams in the electron volt region.

Clustering and the synchronization of oscillator networks.

By manipulating the clustering coefficient of a network without changing its degree distribution, we examine the effect of clustering on the synchronization of phase oscillators on networks with Poisson and scale-free degree distributions. For both types of networks, increased clustering hinders global synchronization as the network splits into dynamical clusters that oscillate at different frequencies. Surprisingly, in scale-free networks, clustering promotes the synchronization of the most connected nodes (hubs) even though it inhibits global synchronization. As a result, they show an additional, advanced transition instead of a single synchronization threshold. This cluster-enhanced synchronization of hubs may be relevant to the brain that is scale-free and highly clustered.

Topology and computational performance of attractor neural networks.

To explore the relation between network structure and function, we studied the computational performance of Hopfield-type attractor neural nets with regular lattice, random, small-world, and scale-free topologies. The random configuration is the most efficient for storage and retrieval of patterns by the network as a whole. However, in the scale-free case retrieval errors are not distributed uniformly among the nodes. The portion of a pattern encoded by the subset of highly connected nodes is more robust and efficiently recognized than the rest of the pattern. The scale-free network thus achieves a very strong partial recognition. The implications of these findings for brain function and social dynamics are suggestive.

Laplacian spectra as a diagnostic tool for network structure and dynamics.

We examine numerically the three-way relationships among structure, Laplacian spectra, and frequency synchronization dynamics on complex networks. We study the effects of clustering, degree distribution, and a particular type of coupling asymmetry (input normalization), all of which are known to have effects on the synchronizability of oscillator networks. We find that these topological factors produce marked signatures in the Laplacian eigenvalue distribution and in the localization properties of individual eigenvectors. Using a set of coordinates based on the Laplacian eigenvectors as a diagnostic tool for synchronization dynamics, we find that the process of frequency synchronization can be visualized as a series of quasi-independent transitions involving different normal modes. Particular features of the partially synchronized state can be understood in terms of the behavior of particular modes or groups of modes. For example, there are important partially synchronized states in which a set of low-lying modes remain unlocked while those in the main spectral peak are locked. We find therefore that spectra are correlated with dynamics in ways that go beyond results relating a single threshold to a single extremal eigenvalue.

Molecular beam chemiluminescence. Three-body processes in the micro-torr region: The Ba + X2(Cl2, Br2, I2) → BaX*2 reactions

Abstract The intensity of the chemiluminescence continua from the title reactions was measured in crossed effusive molecular beams as a function of halogen beam flux. The dominant quadratic pressure dependence of the Ba + Cl 2 , Br 2 , I 2 reactions at halogen densities as low as ≈ 10 11 molecules/cm 3 indicates a three-body process (rapid collissional stabilization of a very long-lived collision complex) as the major mode of MX * 2 formation, while a two-body process is discernible at the lowest X 2 gas densities. The mechanism is discussed in some detail.

Parameter space analysis, pattern sensitivity and model comparison for Turning and stationary flow-distributed waves (FDS)

A new type of instability in coupled reaction-diffusion-advection systems is analysed in a one-dimensional domain. This instability, arising due to the combined action of flow and diffusion, creates spatially periodic stationary waves termed flow and diffusion-distributed structures (FDS). Here we show, via linear stability analysis, that FDS are predicted in a considerably wider domain and are more robust (in the parameter domain) than the classical Turing instability patterns. FDS also represent a natural extension of the recently discovered flow-distributed oscillations (FDO). Nonlinear bifurcation analysis and numerical simulations in one-dimensional spatial domains show that FDS also have much richer solution behaviour than Turing structures. In the framework presented here Turing structures can be viewed as a particular instance of FDS. We conclude that FDS should be more easily obtainable in chemical systems than Turing (and FDO) structures and that they may play a potentially important role in biological pattern formation.

Convective instability induced by differential transport in the tubular packed-bed reactor

Abstract Convective (or spatial) instability, which manifests itself as the tuned amplification of perturbations in the course of their propagation along a non-isothermal packed-bed tubular reactor, is shown to occur in the exothermic standard reaction A → B + heat. The instability is caused by the interplay of the differential transport of heat and matter and of the activator-inhibitor kinetics inherent in non-isothermal, exothermic reactions (where heat plays the role of autocatalytic species or activator, and matter represents the inhibitor). The differential transport is caused by the inert reactor packing which acts as a thermal reservoir and slows down the diffusive and advective transport of heat relative to that of matter. This instability appears to be relevant to an earlier observation (Puszynski and Hlavacek, 1980, Chem. Engng Sci. 35 , 1769–1774) of sustained temperature oscillations in a packed-bed reactor at high Lewis number.

Molecular beam chemiluminescence. VIII: Pressure dependence and kinetics of Sm + (N2O, O3, F2, Cl2) and Yb + (O3, F2, Cl2) reaction. Dissociation energies of the diatomic reaction products

Abstract The CL spectra of the title reactions and their pressure dependences have been studied over the 5 × 10 −6 − 5 × 10 −3 torr range in a beam-gas experiment. In the Sm + N 2 O, O 3 and Yb + O 3 reactions simple bimolecular formation of the short lived (radiative lifetime τ R −6 s) MO* emitters dominates the entire pressure range. In the other systems Sm + (F 2 , Cl 2 ), Yb + (F 2 , Cl 2 ) the CL spectra are strongly pressure dependent, indicating extensive energy transfer from long-lived intermediates. Reaction mechanisms are suggested. The quantum yields Φ, obtained by calibrating relative quantum yields with Dickson and Zares absolute value for Sm + N 2 O [Chem. Phys. 7 (1975) 367], range from Φ = 2.3% (for Sm + F 2 , the most efficient reaction) down to Φ = 0.005% for Yb + Cl 2 . The following lower limit estimates were obtained for the product dissociation energies from the short wavelength CL cutoffs: D 0 0 (SmF) ⩾ 121.3 ± 2.4 kcal/mole, D 0 0 (SmCl) ⩾ ⩾ 100 ± 3 kcal/mole, D 0 0 (YbO) ⩾ 94.2 ± 1.5 kcal/moie, D 0 0 (YbF) ⩾ 123.7 ± 2.3 kcal/mole.

On the local stability of limit cycles.

Orbital stability of limit cycles is the result of the competing local tendencies of perturbations from the cycle to decay (during phases of local stability) and to grow (during phases of local instability), averaged over a cycle. We examine this coexistence of attractive and repulsive phases on limit cycles, including the local rates of expansion and contraction of phase space volumes. This is done in a frame of reference that moves along the orbit, to partially decouple motions tangential and perpendicular to the cycle. Dynamical systems used for illustration are the generalized Bonhoeffer-van-der-Pol and Rossler models, both far from and near to different types of bifurcations. Finally, it is shown that the nonuniformity of local stability in phase space affects the response of limit cycle oscillators to perturbations and gives rise to their phase-dependent response. (c) 1999 American Institute of Physics.

Dynamics of the electronically chemiluminescent Ca + X2(F2, Cl2, Br2)

Abstract The reactions between the alkaline earth metal atoms, M(Ca, Sr, Ba) and halogen molecules X 2 (F 2 , Cl 2 , Br 2 ) proceed via a number of electronic channels. The corresponding products are (a) predominantly metal monohalides MX in the electronic ground state and (b) a small fractions in several of the lowest excited states MX * . The latter give rise to characteristically banded chemiluminescence in the visible. (c) Electronically excited dihalides MX * 2 emitting a strong broad quasi-continuum (the F 2 + M reactions to a lesser degree). It has been shown elsewhere that the continuum emitter is predominantly formed by collisional stabilization of an extremely long lived (“immortal”: lifetime τ > 10 −8 s) MX * 2 collision complex. This paper presents a study of the Ca + X 2 (F 2 , Cl 2 , Br 2 ) chemiluminescence, with particular emphasis on the internal energy distributions of the MX * O products. It is found that: 1. the excited halides from the Ca + Cl 2 and Ca + Br 2 reactions [CaCl(A 2 Π), CaCl(B 2 Σ + ), CaBr(A 2 Π), CaBr(B 2 Σ + )] are formed with essentially statistical vibrational-rotational distributions, characterized (disregarding minor deviations) by vibrational and rotational temperatures T v [CaCl(A 2 Π)] = 2600 ± 100 K; T v [CaCl(B 2 Σ + )]= 2300 ± 100 K; and T v [CaBr(A 2 Σ)] = 1550 ± 100 K. No analysas was performed for CaBr(B 2 Σ) because of the complexity of the spectrum, but similar results are expected. The temperatures agree reasonably with values calculated from energy equipartition. 2. The Ca + F 2 reaction populates CaF(A 2 Π) CaF(B 2 Σ + ) and CaF(C 2 Π). Moderately inverted, non-Boltamann vibrational distributions have been obtained for CaF(B) an CaF(c), No analysis was performed for CaF(A), but all evidence points to a likewise inverted u distribution. the rotational distribution are best described by a rotational temperature For CaF(B), T R ( u )S decreases with increasing u . 3. In all three reactions the population of electronic MX * states decreases very rapidly with electronic excitation energy. These results indicate that the chemiluminescent exchange channels of Ca + Cl(in2), Br 2 proceed via an essentially equilibrated, long lived complex, while Ca + F 2 is more direct via a shorter lived (partially equilibrated-osculating) complex. Molecular beam angular distributions by other research groups indicate that direct dynamics is characteristic for the ground state channels. This agrees with estimates of the collisions complex lifetimes by RRK unimolecular theory. Strong complex binding forces are dominant in all reaction channels. An extended harpooning model is proposed that considers the interaction of several potentials energy surfaces. It accounts for the occurrence of all observed exchange channels and for the long unimolecular lifetime of the MX * 2 complex (continuum emitter).