Ultrafast Transient Absorption Signatures of ESIPT

Using probe light from a NOPA and integrated detection allows for a time resolution of 30 fs. figure 4.5 shows the transmission change as a function of the probe wavelength and pump-probe delay that results from such an experiment on HBT [30, 31]. 

After an initial transmission decrease around time zero, a positive signal appears with a small delay, which then dominates. In addition, pronounced signal oscillations are observed with a couple of frequencies in the range up to 700 cm, as can be seen from the Fourier transforms. The transmission decrease at time zero is caused by ESA, which takes place as soon as the molecule is promoted to the Sj 

Already in early investigations transfer times of about 100 fs or less were observed for HBT in tetrachloroethylene [7], 2-(2 -hydroxyphenyl)benzoxazole (HBO) in cyclohexane [33], and for methyl salicylate (MS) [34] and OH BA [35] in gas phase. For a number of ESIPT molecules, very fast transfer times were found even at cryogenic temperatures [36]. Pioneering work was performed on 2-(2 -hydroxy-5 -methylphenyl)benzotriazole (TINUVIN P), and for the first time oscillatory signal contributions associated with the ESIPT were observed [37]. Meanwhile ultrafast ESIPT and subsequent signal oscillations due to coherent wavepacket motions were also found in HBO [38], OHBA [39], and a number of other ESIPT compounds [13, 40-43]. 

The spectral position and shape of the SE reflects the fluorescence of the enol form. The onset of the SE is a clear signature for the proton transfer itself. The signal oscillations result from a wavepacket motion in the excited-state potential of the keto form. Wavepackets in the electronic ground state of the enol configuration which might be excited by impulsive stimulated Raman processes [44, 45] do not contribute since the enol absorption spectrum is at much shorter wavelengths than the applied probe pulses. 

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