Transition manifolds of complex metastable systems: Theory and data-driven computation of effective dynamics

We consider complex dynamical systems showing metastable behavior but no local separation of fast and slow time scales. The article raises the question of whether such systems exhibit a low-dimensional manifold supporting its effective dynamics. For answering this question, we aim at finding nonlinear coordinates, called reaction coordinates, such that the projection of the dynamics onto these coordinates preserves the dominant time scales of the dynamics. We show that, based on a specific reducibility property, the existence of good low-dimensional reaction coordinates preserving the dominant time scales is guaranteed. Based on this theoretical framework, we develop and test a novel numerical approach for computing good reaction coordinates. The proposed algorithmic approach is fully local and thus not prone to the curse of dimension with respect to the state space of the dynamics. Hence, it is a promising method for data-based model reduction of complex dynamical systems such as molecular dynamics

Offshell effects in electromagnetic reactions on the deuteron

Offshell contributions to the electromagnetic nuclear current are evaluated within a nonrelativistic approach by incorporation one-pion loop contributions in time-ordered perturbation theory. By construction, the correct experimental onshell properties of the nucleon current are ensured so that only the genuine offshell effects appear as model dependent. For a qualitative assessment of such offshell effects, this model is applied to photodisintegration of the deuteron for photon energies up to 500 MeV. While at low energies offshell contributions are small, above 300 MeV they lead to sizeable effects in observables up to about 30 percent pointing to the necessity of incorporating such effects if one aims at theoretical predictions of high precision.Comment: 22 pages revtex including 17 figure

Cooperative mechanism for anchoring highly polar molecules at an ionic surface

Schütte J, Bechstein R, Rohlfing M, Reichling M, Kühnle A. Cooperative mechanism for anchoring highly polar molecules at an ionic surface. Physical Review B. 2009;80(20):205421.Structure formation of the highly polar molecule cytosine on the (111) cleavage plane of calcium fluoride is investigated in ultrahigh vacuum using noncontact atomic force microscopy at room temperature. Molecules form well-defined trimer structures, covering the surface as homogeneously distributed stable structures. Density-functional theory calculations yield a diffusion barrier of about 0.5 eV for individual molecules suggesting that they are mobile at room temperature. Furthermore, it is predicted that the molecules can form trimers in a configuration allowing all molecules to attain their optimum adsorption position on the substrate. As the trimer geometry facilitates hydrogen bonding between the molecules within the trimer, we conclude that the stabilization of individual diffusing molecules into stable trimers is due to a cooperative mechanism involving polar interactions between molecules and substrate as well as hydrogen bonding between molecules

Treating asthma: is there a place for leukotriene receptor antagonists?

SummaryAsthma is a chronic disorder, characterized by airway hyperresponsiveness (AHR), airway inflammation and airway remodelling. Evidence has been provided for a relationship between pathophysiology, airway inflammation and remodelling. Moreover, these asthma features have been shown to respond to anti-inflammatory therapy. According to current guidelines, monitoring of asthma is predominantly based on symptoms and lung function data. However, these parameters appeared as poor indices for asthma control. Alternatively, asthma control relates well to exacerbations and (anamnestic) surrogate biomarkers of airway inflammation. Hence, appropriate treatment of asthma should primarily target the airway inflammation.According to current guidelines for asthma management, anti-inflammatory therapy with inhaled corticosteroids (ICS) is the cornerstone in the treatment of persistent asthma. To further optimize asthma control, add-on therapy with long-acting β2-agonists (LABA) or leukotriene receptor antagonists (LTRA) should be combined with low to high doses of ICS. While the first combination focuses on optimal control of symptoms and lung function, the second provides a more complete suppression of the airway inflammation.In this paper we discuss treatment of asthma according to current guidelines versus new insights, addressing practical issues

Concept for support and cleavage of brittle crystals

Tröger L, Schütte J, Ostendorf F, Kühnle A, Reichling M. Concept for support and cleavage of brittle crystals. Review of Scientific Instruments. 2009;80(6):063703.We report on sample holders for crystals to be cleaved for the preparation of surfaces with large atomically flat terraces. The concept for mounting sample crystals is based on a vicelike clamping mechanism to securely hold the crystal in position while reducing the risk of fragmentation. Sample holders based on this concept and made of suitable materials allow preparation and cleavage of crystals in the ultrahigh vacuum at high or low temperatures. To cleave the crystal, we employ a scalpel blade mounted on a wobble stick to generate a highly localized stress field initiating the cleavage process. The sample holders are used for experiments of highest resolution scanning force microscopy, however, the concept can be transferred to any other system where cleavage faces of crystals are of interest. Exemplarily, scanning force microscopy results demonstrate that (111) cleavage faces of CaF(2) crystals can be prepared with steps only a few F-Ca-F triple-layers high and atomically flat terraces extending over areas of several mu m(2)

Field-Induced Tunneling Ionization and Terahertz-Driven Electron Dynamics in Liquid Water

Liquid water at ambient temperature displays ultrafast molecular motions and concomitant fluctuations of very strong electric fields originating from the dipolar H2O molecules. We show that such random intermolecular fields induce tunnel ionization of water molecules, which becomes irreversible if an external terahertz (THz) pulse imposes an additional directed electric field on the liquid. Time-resolved nonlinear THz spectroscopy maps charge separation, transport and localization of the released electrons on a few-picosecond time scale. The highly polarizable localized electrons modify the THz absorption spectrum and refractive index of water, a manifestation of a highly nonlinear response. Our results demonstrate how the interplay of local electric field fluctuations and external electric fields allows for steering charge dynamics and dielectric properties in aqueous systems

Quantitative description of C-60 diffusion on an insulating surface

Loske F, Lübbe J, Schütte J, Reichling M, Kühnle A. Quantitative description of C-60 diffusion on an insulating surface. Physical Review B. 2010;82(15): 155428.The diffusion of C-60 molecules on large, atomically flat terraces of the CaF2(111) surface is studied under ultrahigh vacuum conditions at various substrate temperatures below room temperature. The weak molecule-substrate interaction on this insulating surface makes a direct observation of hopping events difficult. Therefore, to determine a quantitative value of the diffusion barrier, we employ the so-called onset method. This method is based on the analysis of spatial properties of islands created by nucleation of diffusing C-60 molecules, as measured by noncontact atomic force microscopy. We first determine the critical cluster size to be i* = 1 from coverage-dependent island size distributions prepared at a fixed substrate temperature. The diffusion barrier of E-d=(214 +/- 16) meV and an attempt frequency of nu(0)=1.4 X 10(12 +/- 0.6) s(-1) are then obtained by analyzing the island densities at different substrate temperatures

Metastability and small eigenvalues in Markov chains

In this letter we announce rigorous results that elucidate the relation between metastable states and low-lying eigenvalues in Markov chains in a much more general setting and with considerable greater precision as was so far available. This includes a sharp uncertainty principle relating all low-lying eigenvalues to mean times of metastable transitions, a relation between the support of eigenfunctions and the attractor of a metastable state, and sharp estimates on the convergence of probability distribution of the metastable transition times to the exponential distribution.Comment: 5pp, AMSTe