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Prof. Dr. Peter Gilch
Ehemaliger

LMU München
Fakultät für Physik


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Impressum
(c) 2002 BMO

Tracing Molecular Changes on the
"Genuine" Chemical Time scale -
From Electron Transfers to Complex Re-arrangements

Simply put chemistry is about molecules which change their shape. It therefore fascinates chemical physicists and physical chemists (including myself) to trace such changes in real time. As the "speed limit" of chemical processes is given by vibrational frequencies of molecules, "real time" can be as short as a couple of femtoseconds (1 fs = 10-15 s). Thanks to modern laser technology real time experiments on that time scale are nowadays close to routine. We apply elaborated femtosecond techniques to look at diverse chemical process ranging from electron transfer reactions ("the simplest chemical process") over isomerisations to complex photo rearrangements. Some of these reaction occur in biological systems or have relevance for such systems.

To take ever sharper pictures of such reactions we not only apply femtosecond techniques but also seek to improve these techniques. We have developed a femtosecond fluorescence spectrograph with truly unique capabilities and have strongly contributed to the emerging technique of femtosecond stimulated Raman spectroscopy (FSRS).

The sections below highlight some of our former and present research interests. More detailed descriptions can be found under "Projects".

Electron Transfer and Spin Dynamics

The spin angular momentum of electrons can influence the kinetics and outcome of charge transfer reactions. Such dynamic spin effects were commonly believed to be restricted to rather slow processes (nanoseconds or slower). Femtosecond experiments performed during my PhD time (with Prof. Dr. M.E. Michel-Beyerle, TU München) proved that such spin effects can also influence much faster processes occurring on the picosecond time scale.

In these experiments the impact of high magnetic fields up to 9 Tesla on the reaction kinetics served as a diagnostic tool for the spin effects. The picture shows a model of the super conducting magnet used in these experiments.

In a chemical reaction nuclei in a molecule have to be replaced with respect to the equilibrium position. This requires chemical bonds to be deformed which in turn requires vibrational energy. The flow of vibrational energy is therefore a pre-requiste of most chemical processes. We have employed various spectroscopic techniques (IR-, Raman- and probe induced fluorescence spectroscopy)  to time resolve this flow on the picosecond time scale.

In the experiment depicted we studied the photolysis of triiodide I3- which results in the formation of vibrationally excited diiodide I2-. This excitation is monitored via the fluorescence induced by the probe pulse. This fluorescence features a pronounced anti-Stokes part which decays with the vibrational excitation.

Light can do "real harm" to a molecule, i.e. photo chemistry can yield products with little structural resemblance to the starting material. Such photo transformation usually involve several reaction steps many of them representing fundamental organic reaction types. We seek to decipher the mechanisms of such reactions by means of femtosecond vibrational spectroscopy.

One reaction recently studied by us is the photo-transformation of ortho-nitrobenzaldhyde (1) to ortho -nitrosobenzoic acid which was one of the first photo reaction ever reported on. In a femtosecond IR experiment the first intermediate of this reaction, a ketene (2), could be identified via its characteristic stretch vibration at 2100 cm-1.

Many photochemical processes involve states with triplet multiplicity and it is thus very important to identify triplet states experimentally. Unfortunately, techniques sensing triplet states, e.g. EPR spectroscopy, do not feature time resolutions better than ~ 10 nanosecond and thus cannot be applied for fast photochemical processes. We apply and seek for techniques to sense triplet states on time scales below 10 ns.

Xanthone is prototypical for the class of aromatic carbonyl compounds which feature very high triplet yield. We have investigated the formation of this triplet state by various approaches. By means of triplet-triplet energy transfer we could demonstrate that intersystem crossing from the photo-excited singlet to a triplet state takes only 1 picosecond. The intersystem crossing kinetics depend on the solvent enviroment, a finding that can literally be seen when looking at the fluorescence of xanthone solutions (see picuture).

(no picture available)
Telephon:
+49-89-2180-9243 (Office)
+49-89-2180-9285 (Lab)
E-Mail: peter.gilch(at) physik.uni-muenchen.de
Curriculum Vitae
Projects
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