Higher excited states, energy dependent relaxation

The fluorescence and absorption spectra of DTT-A.V-dioxidc 20a with polar covalent bonds was studied in THF, toluene, and decalin. The spectral line and peak energy are almost independent of the solvent polarity. The fluorescence spectra of the decalin and toluene solutions (almost the same polarity) are red-shifted by about 5 nm, with respect to the THF solution of higher polarity. No evident solvatochromism was observed. The absorbance and fluorescence excitation spectra (at the fluorescence peak wavelength) for DTT-3, 3 -dioxide 20a (normalized to peak value) was compared. The fluorescence excitation signal is, in fact, dependent both on the density of the excited state (as the absorbance) and on the efficiency of the relaxation from the excited state of the emitting one <2005PCB6004>. 

The excited state absorption starts on the lower 5d level and ends on a level in the conduction band. Therefore this two photon transition results in photoionization of the Ce ion. The conduction band minimum lies only 10,000 cm above the lowest (relaxed) 5d level. The higher energy part of the ground state absorption spectrum (X < 300 nm in Fig. 18) is due to direct (one-photon) photoionization of the Ce " ion. The excited state absorption of Ce is strongly host dependent. InCaF2 Ce andLiYF4 Ce it is situated in the near ultraviolet (46, 47). 

The spectra distribution of fluorescence is dictated by the Franck-Condon factors defined as the square of the second term in Eq, 1, and the position of the centre of pavity of the fluorescence depends upon any geometry changes between pound and excited states. The latter point is illustrated in Fig. 2 from which it can be seen that the most probable transition in absorption is to higher energies than that for fluorescence if the potential surface of the excited state undergoes some non-zero displacement nith respect to the ground state, and assuming that vibrational relaxation is... 

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