Institute for physical chemistry

Chair for Molecular spectroscopy and Nanosystems
Professor Karl Kleinermanns Research Group

Dispersed Fluorescence (DF)

The most important technique for measuring the intermolecular vibrations of complexes with aromatic chromophores is still a simple dispersion of the cluster fluorescence obtained from excitation of different S1 vibrational states.
The vibrational frequencies, the anharmonicities, and the Franck-Condon intensity pattern of progressions and combination bands reveal very detailed information about the S0 intermolecular potential. Additionally, the S0 vibrations can be calculated much more reliable and with much less expense than the S1 vibrations.
High quality DF, even of weak transitions, is possible with a monochromator of high dispersion and multichannel detection via an image intensified, gated charge couple device (CCD) camera. With a 1m Czerny-Turner monochromator, a holographic grating with 2400 grooves/mm blazed to obtain UV-spectra around 300 nm in second order and a resolution limiting CCD pixel size of 23 µm a spectral resolution of 1 cm-1 can be achieved. The resulting two-dimensional camera picture (x = dispersion, y = height of the entrance slit) is corrected for the y-curvature from the spherical aberration of the mirrors.
A single dispersed fluorescence spectrum is obtained by summing the fluorescence of a few hundred laser pulses on the CCD chip and substracting the background straylight (gas pulse off) from the same number of laser pulses. 10 - 20 of these spectra are averaged. The pixelnumber - wavelength relation has to be calibrated carefully, e.g. with the scattered light from the jet at different excitation laser wavelengths. Intensity correction can be performed by calibrating the non-uniform sensitivity of the CCD device, e.g. with a tungsten strip filament lamp irradiating an Ulbricht sphere to generate a uniform background. This is especially important if the emission intensities are evaluated to interpret the Franck-Condon pattern of the spectra.


The measured Franck-Condon pattern is determined by geometry changes upon electronic excitation. Thus, it is possible to fit the structural change upon excitation with a Franck-Condon fit routine (Programm: FCFit). The programm uses ab initio calculated structures as well as the force constants for both electronic states as a first approximation.
Additionally the relative line intensities must be digitized. In the fit procedure a distortion along experimentally observed normal coordinates is made in order to get the highest degree of consistency between calculated and experimental data. A minimum is fitted with the aid of a cost function that calculates the weighted sum of squared residuals.

Selected Publications

M. Schmitt, U. Henrichs, H. Müller, K. Kleinermanns,
Structure and vibrations of the phenol dimer, revealed by spectral hole burning and dispersed fluorescence spectroscopy,
J. Chem. Phys. 103 (1995) 9918
 
 
M. Gerhards, W. Perl, S. Schumm, U. Henrichs, C. Jacoby, K. KIeinermanns,
Structure and vibrations of catechol and catechol(H2O) in the S0 and S1 state,
J. Chem. Phys. 104 (1996) 9362
 
 
W. Roth, Ch. Jacoby, A. Westphal, M. Schmitt,
A Study of 2H- and 2D-Benzotriazole in Their Lowest Electronic States by UV-Laser Double Resonance Spectroscopy
J. Phys. Chem. A, 102 (1998) 3048
 
 
D. Spangenberg, P. Imhof, K. Kleinermanns,
The S1 state Geometry of Phenol Determined by Simultaneous Franck-Condon and Rotational Constants Fits,
PCCP, 5 (2003) 2501-2514
 
 
P. Imhof, D. Krügler, R. Brause, K. Kleinermanns,
Geometry change of simple aromatics upon electronic excitation obtained from Franck-Condon fits of dispersed fluorescence spectra,
J. Chem. Phys., 121(6) (2004) 2598-2610
 
 
R. Brause, M. Schmitt, D. Krügler, K. Kleinermanns,
Determination of the excited state structure of 7-Azaindole using a Franck-Condon analysis,
Mol. Phys., in press

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last changed: 22.09.2004 Michael Nispel
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