Spectral and photophysical properties of a few aromatic thioketones in their S2-and T1-state, in particular those determined by their interactions with solvents, are discussed. The reasons for a drastically different behaviour of thioketones in interactions with benzene as well as saturated hydrocarbons and perfluoroalkanes are analysed in more detaił. Results of the time-resolved transient absorption measurements in the pico-and nanosecond time scale are given. An analysis of these results proves that a product of the decay of the S2-state of xanthione in benzene is a new transient (τile > 60 ps) individuum which, regarding the system properties and the conditions of the experiment, has been identified as an exciplex in S 2 -state formed as a result of an efficient interaction with benzene molecule. Also in xanthione/alkane systems the involvement of a transient individuum was proved. Most probably, this individuum was a thioketyl radical which could be formed by hydrogen abstraction from a hydrocarbon molecule by xanthione in the S2 -state. Such individua act as intermediates in passing excitation to the triplet states of thioketones and are effective channels of the S2-state decay. The changes observed in the transient absorption spectra of xanthione in C 6 Η6 in the time range of 10 -7 -10 -6 s can be attributed to the formation of an excimer in the T1-state as a result of the T1-state selfquenching.
A new experimental double beam system for dynamic studies of absorption transitions from initially populated excited states is described. In this system a high power picosecond YAG:Nd3 laser (Quantel YG 570 C) equipped with generators of second, third and fourth harmonic serves as an excitation source. The fundamental (X= 1064 nm) beam is used for generation of the picosecond continuum which serves as an analyzing light. The temporal relation between incident beams is determined using both variable and fixed delay lines. A M-532.22 translation stage (POLYTEC) is used for micropositioning of movable cube-corner prism in the optical delay line. A photomultiplier or an OMA detector are applied for measurements of the transient absorption dynamics. A PC 286 microcomputer controls the sequence of operation of all elements of the system and also processes the experimental data. The results of initial tests of this system are presented. .INTRODUCTIONOptical absorption spectroscopy is a basic method for determination of the energy of energetic levels and spectroscopic assignment of transitions as well as for measurement of the most important photophysical parameters characterizing atomic and molecular energetic states and transitions between these states. However, absorption spectral transitions may originate only from populated states. This means that in normal conditions these transitions may occur exclusively from the ground state and, even more precisely, from its thermally populated vibronic levels.In order to extend the absorption spectroscopy to the range of transitions originating from excited states, the studied system must be exposed to a simultaneous irradiation by two light beams: the exciting beam which transfers a substantial part of the population from the ground state to an excited state and the second (analyzing) beam, which induces absorption transitions to one of the higher excited states"2. Such an experimental method enables also the possibility of investigation of the dynamics of transient absorption as well as the measurement of the lifetime of energetic levels by both, the real time (in the nanosecond range) and temporal/spatial coincidence ("pulse-probe" experiments in the picosecond scale), techniques3'4. THE EXPERIMENTAL SYSTEMA high power picosecond YAG:Nd3 laser system (Quantel YG 570 C) generates both beams used in our experiment: O-8194-1499-9/95/$6.OO SPIE Vol. 2202 / 373 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/15/2015 Terms of Use: http://spiedl.org/terms
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