New experimental information from two-photon spectroscopy and comparison with theory
From a specially designed expt., allowing the measurement of 2-photon absorption spectra and 2-photon polarization parameters over a wide spectral range, new exptl. information is obtained on the location of electronically excited states usually not seen in the UV spectrum. By comparison with theory it is shown that in most of the examples investigated up to now the ordering of the low lying excited states is different from what is usually expected. Correlation strongly influences the ordering esp. in those systems which consist of loosely coupled p-systems.