Fritz-Haber-Institut der Max-Planck-Gesellschaft  Department of Physical Chemistry
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Recent Developments in the
Department of Physical Chemistry (2005)

Director until 11/ 2004: Gerhard Ertl
present Acting Director: Gerard Meijer

Since the last meeting of the Fachbeirat the following changes occurred among the staff scientists:

·  Gerhard Ertl retired from his position as Director in October 2004.  
·  Tobias Hertel became Associated Professor of Physics at the Vanderbilt University, Nashville, TN, USA, in January 2004. 
·  Josef F. Holzwarth will retire in November 2005. 
·  Karl Jacobi retired from a part-time position in October 2004. 
·  Wilfried Schulze retired in February 2005. 
·  Rolf Schuster became Professor of Physical Chemistry (C3) at the Technische Universität Darmstadt in August 2004. 
·  Mau-Scheng Zei retired at the end of 2004. 

As the Fachbeirat is well aware, the offer by the President of the MPG, Peter Gruss, to Lynn Gladden from Cambridge University, of a Directorship at the Fritz Haber Institute as Head of the Department of Physical Chemistry was not accepted, so that this position has not yet been filled. At present, Gerard Meijer is the Acting Director while Harm H. Rotermund takes care of current affairs.

 
 
Research:
 

Overview

 
Methods


Activity reports

2011 · 2009

2007 · 2005 · 2003

The work of the remaining groups concentrates on theoretical and experimental studies on aspects of nonlinear dynamics in various interface reactions, in part within the framework of DFG Sfb 555: “Komplexe nichtlineare Prozesse” as well as on the development of Raman spectroscopy of nanoscale objects.

1. Complex systems

Engineering of self-organizing chemical systems cannot be based on the same principles as traditional chemical technology. Application of rigid controls may destructively interfere with the fine interactions between the elements of a system responsible for its self-organization. Instead, spontaneous activity of a system could be steered in a desired direction by applying weak control impulses and imposing various feedbacks. In this manner, transitions between different organization states can be initiated and new forms of collective behavior can be achieved. The aim of this project is to explore, theoretically and experimentally, new directions in chemical engineering. In continuation of the previous research, pattern formation in the CO oxidation reaction on platinum, under global delayed feedback and periodic forcing through the gas phase, have been studied. These investigations have been extended to composite materials created by microlithography. The second research direction involves local manipulation and control of reaction patterns by focused laser beams on microlithographically modified catalytic surfaces.

The activities in the area of nonlinear dynamics and complex surface reactions have been broadened and moved towards important practical applications by starting experimental and theoretical research on critical nonequilibrium phenomena during the onset of pitting corrosion. In a way, this has been a move back to the roots of the institute, when during the nineteen twenties optical investigations of the passivity of iron and steel had been performed by L. Tronstad (Nature 124, 373, 1929) under the guidance of Fritz Haber and Herbert Freundlich. Recently front propagation in the regime of metastable pitting on stainless steel has been visualized, for the first time, by utilizing simultaneously contrast-enhanced optical microscopy and elliptical microscopy for surface imaging (EMSI). While the contrast-enhanced optical microscopy allows real time in situ observations of the creation of single pits at the diffraction limit (about 2 Ám), EMSI images at the same time show the depletion of the oxide layer, although, due to experimental restrictions, only at a spatial resolution of about 20 Ám.

Parallel theoretical investigations have led to a new view on the development of corrosion, in which diffusion-mediated interactions between metastable corrosion pits play a key role. The sudden onset of corrosion is rationalized as a critical phenomenon involving an autocatalytic explosion of corrosion pits. Theoretical analysis and numerical simulations of the proposed mathematical models reproduce the experimental observations well.

Further experimental studies with electrochemical systems concerned the propagation of potential excitations in the course of an electrochemical reaction (formic acid oxidation on platinum). On a thin Pt ring electrode small sections were insulated, which caused various novel effects such as period–doubled pulses, trapped oscillatory states and, in particular, pronounced velocity changes, which may serve as models for ‘saltatory’ conduction across the Ranvier nodes in nervous systems.

On the theoretical level, nanoscale pattern formation was studied for surface chemical reactions with promoters, and for Langmuir monolayers formed by organic lipid or amphiphilic molecules disposed on a water–air interface. The patterns representing stationary periodic structures or traveling waves are maintained and controlled in such systems by chemical reactions, illumination or transmembrane flows.

In another activity, research was focused on enzymes acting as cyclic protein machines. Statistical methods for the analysis of experimental data of single-molecules fluorescence correlation spectroscopy have been developed and applied to determine the operation mechanism of the enzyme cholesterol oxidase. Similarly, stochastic simulations of pattern formation and molecular cycle synchronization phenomena in enzymic arrays have served to shed light on these complex phenomena.

In an even more abstract sense, progress has been made in predicting dynamic instabilities from a chemical reaction mechanism by using concepts from algebraic topology as analytic tools. One can define certain algebraic structures (polynomial rings) as the kinetic terms of a reaction mechanism (rewritten in binomial form). After a change of basis these can be solved for their roots in the form of a deformed toric variety, allowing solutions for the multiplicity of the states and the location of bifurcations. In this way, a complete mechanistic classification of chemical oscillators including competitive autocatalyses and nonautocatalytic systems could be achieved. The methods were successfully applied to a number of real systems, such as oscillating Langmuir-Hinshelwood mechanisms (e.g. CO oxidation), the electrocatalytic oxidation of formic acid, the peroxidase oscillator and the calcium oscillations in cilia during olfactory response. The latter system is interesting because it includes refractoriness with respect to a stimulus without exhibiting excitability (Eiswirth, Mikhailov, Rotermund).

2. Raman spectroscopy

The sensitivity of Raman spectroscopy can be immensely enhanced by the excitation of surface plasmons by primary (visible) light, but this surface-enhanced Raman spectroscopy (SERS) is restricted to rough surfaces of silver or gold. Another approach, that of tip-enhanced Raman spectroscopy (TERS), is based on the optical excitation of localized surface plasmons between the Au (or Ag) tip of a scanning tunneling microscope and any (smooth) arbitrary surface, whereby a very strong local field enhancement is achieved in the cavity. Spectra from various adsorbed organic and inorganic molecules were recorded in this way. With the dye malachite green isothiocynate, as few as 200 molecules underneath the tip can be ‘seen’ by this technique. Using the ClO4  stretch vibration, the dependence of the TER band intensity on the distance to the substrate was recorded. For a tip radius of 20 nm the TERS signal was found to decrease by one order of magnitude for 10 nm vertical displacement of the tip. Thus, a powerful tool for vibrational spectroscopy with spatial resolution in the nanometer regime is being developed (Pettinger).

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