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 CCD camera. With a 1 m 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 wavenumber 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 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. 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

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

R. Brause, M. Schmitt, D. Krügler, K. Kleinermanns,
Determination of the excited state structure of 7-azaindole using a Franck-Condon analysis,
Mol. Phys., 102 (2004) 1615-1623

R. Brause, D. Krügler, M. Schmitt, K. Kleinermanns,
Determination of the excited state structure of 7-azaindole-water cluster using a Franck-Condon analysis,
J. Chem. Phys., 123 (2005) 224311