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.

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  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