Professor of physical chemistry

Honors:

1965    Alfred P. Sloan Fellow

1984     Elected Fellow of the American Physical Society

1985     John Wilfred Linnett visiting professor of chemistry, Cambridge University

1987     Woodward Lecturer, Yale University

1988    Doctor of Philosophy Honoris Causa, Hebrew University, Jerusalem

1988     Fritz Haber Lecturer, Hebrew University of Jerusalem

1990     Ames Lecturer, University of Edinburgh

1993     Gold Honorary J. Heyrovsky Medal, Academy of Sciences of the Czech Republic, Prague

1994     Invited Lecturer, The Welch Foundation Conference

             "Chemical Dynamics of Transient Species" 

1995     John Wilfred Linnett Visiting Professor of Chemistry University at Cambridge, England

1995     Invited Lecturer, The Solvay Conference

"Photochemistry: Chemical Reactions and their Control on the Femtosecond Time Scale"

1996     Charles M. Knight Lecturer, The University of Akron

1997     Bonhoeffer-Eucken-Scheibe lecturer

1998     James-Franck lecturer, Israel Academy of Sciences, Jerusalem

1999     Edward K.C. Lee Distinguished Lecturer in honor of Professor E.K.C. Lee , University of California, Irvine, USA

1999     CRC Lecturer California Institute of Technology

2001     Heisenberg Medal of the Alexander von Humboldt Foundation

2009    The 25th  anniversary of ZEKE Spectroscopy was honored in a Symposium at the National Meeting of the American Chemical Society in August 2009 in Washington DC: managed by D. Neumark, Berkeley and Masaaki Fujii  Japan

2016     Emeritus of Excellence, TUM

Scientific Academies:

1978      Election to Membership in the Bayerische Akademie der Wissenschaften

Current Interests:

ZEKE-Spectroscopy,

Charge migration in protein structures - molecular wires

ZEKE

ZEKE spectroscopy is based on the discovery that Rydberg states with high quantum number, very close to ionization that display extremely long lifetimes. This is due to slight perturbations from external fields. This was first observed in a doctoral thesis of Peatman at Northwestern. The high resolution laser result was then confirmed in the doctoral thesis of Sander in Munich.

ZEKE- a new spectroscopy for molecular ions, ie. molecules in which one electron is removed so that they are considered ionized. Many chemical reactions occur in the ionized state. Hence it is important to know their structure. The structure of such molecular ions will be different from the structure of the neutral molecules. The spectroscopy of ions has so far been done by analyzing the electron emitted from the neutral molecule. Such Photoelectron Spectroscopy is inherently fraught with difficulties since it is very difficult to measure the electron velocities. This is an inherent problem related to the fact that all electron measuring devices require surfaces to define a potential. However, a surface has a structure, and as such an inherent and uneven potential that cannot be cancelled. Hence the resolution of all such devices is extremely limited.

Our method is based on the measurement of the last neutral Rydberg state prior to ionization. When an atom is excited with ever higher energy it will go into ever higher orbits, until it eventually ionizes. This is called a Rydberg series.The same is true for molecules. But here the final convergence of the excitation energy can lead to a vibrational or even rotational state of the molecule. Hence a molecule has thousands of ionized states, each with it own Rydberg series.

We discovered that the many ionized states of a molecule are not so very short lived, as it was described in the literature, but rather the very highest states, just before ionization could be quite long lived. This has to do with the coupling of the surrounding fields, created by other ions. This coupling changes the quantum states of the molecule into an orbit that avoids the core, and as such forms a long lived state. A molecule can only become short lived if its electron goes near the core. Hence we discovered that very high Rydberg states bear the signature of being very long lived. Hence our ZEKEspectroscopy ionizes the system with a high energy photon, but delays the extraction until only the long lived molecules survive. Then by just measuring the remaining current I obtain an extremely high resolution spectrum of the state just below ionization. This state is nor subject to the surface potential problems of photoelectron spectroscopy-

In this way thousands of molecular ions have been measured by this new speoctroscopy with an accuracy thousands of times better than existing methods. This enables us to ask questions on the nature of aromatic molecules and chemical intermediates in chemical reactions.

History of ZEKE

We wanted to measure the spectral energy at which no energy was left in the electron i.e. zero energy electrons. For this we built a 127° Rojinski electron analyzer. This was part of the thesis of William Peatman at Northwestern University in 1969. The slits in the analyzer could not be made too small due to a loss of signal. A group in Chicago chose to solve the problem by accelerating the threshold electrons, which, of course, negated the desire to get at the detector. We discovered that opening the slits increased the signal but it led to a strange and quite puzzling discovery, namely that the resolution was independent of the slit width. We then discovered that the analyzer was sensitive to the electrons positioned at the focus and not those of zero energy. We then constructed a parallel plate analyzer which produced the same resolution. We discovered that we now had a steradian analyzer that selected threshold electrons as a new principle. This was the birth of ZEKE spectroscopy. All this was part of Peatman thesis. It was done with a CW source from an Argon discharge and a McPherson monochromator outfitted with a Vacuum UV Al coated Schmid-Cassegrain confocal mirror system constructed for us at the observatory in Wisconsin. This work was done at Northwestern in a group with Tomas Baer, Paul Marie-Guyon and Tommy Borne, early work on steradiency was also done at Argonne by J.Berkowitz, W. Chupka and R. Spohr.

1971 I moved to the Technische Universität Munich and started work with dye lasers which had just become available. Using multiphoton excitation as developed together with the Neusser group the ionization of the ZEKE states was pursued in a new group with Müller-Dethlefs. We then hit on the idea of separating the ions not in space but now in time. Martin Sander was a new graduate student in 1984 assigned to attempt pulse delay studies. This was not possible in the Peatman CW work. The success was dramatic and it has become the method of preference. We later developed the theory started by Leonid Baranov, a student with Raphy Levine. We further tested the results to study the effect of magnetic fields with A. Held.

In short ZEKE spectroscopy employs optical Rydberg spectra to study electron spectra at thresholds and thus avoids the inevitable field effects from electrostatic analyzers. The improvement in resolution is thus many orders of magnitude over any method requiring electrostatic plates. Thus even though CW sources correctly identified ZEKE states by steradial analysis the advent of pulsed laser delay in 1984 enabled Sander et al. to greatly simplify the method and improve the accuracy of electron ionization by many orders of magnitude.
(E.W.Schlag ZEKE Spectroscopy Cambridge University Press 1998)

Gäste

Other fields:

Multiphoton ionization mass spectrometry

High resolution sub-Doppler molecular spectroscopy and dynamics

Spectroscopy and kinetics of molecular ions

Dynamics of photoexcited states and van der Waal's molecules

Synchrotron radiation experiments on molecular ions, inner shell excitation

Scientific Synopsis click here

AREAS OF RESEARCH

d) Photoelectron spectroscopy with a new analyzer

ZEKE Spectroscopy by E.W. Schlag, Cambridge Univeristy Press 1998

2. Molecular clusters and van der Waals molecules