CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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The spectrum has been shown in three parts between 0.1-0.4 ps, 1-4 ps, and 10-100 ps. The
spectrum clearly shows that in the time scale up to 0.4 ps, there is an ultrafast increase in the
absorption in the entire spectral region indicating the event of ultrafast electron injection. In
the time delay upto 1ps we have observed rapid decrease in the absorption signals indicating
a very fast back electron transfer reaction. Again from 1-100ps time domain the signal
appears to increase, indicating another injection process. It is interesting to observe that there
appears multiple injection process in the present case and the time constants of which
becomes clear from the decay kinetics at individual wavelengths. It is reported in the
literature [3.16] that there is a drastic increase in the molar extinction co-efficient of the dye
cation absorption in dye- metal ion system in presence of surfactant assembly.
Dynamics of electron transfer dynamics was carried out by monitoring at 550 nm is
represented in Figure 3.5. The kinetics was monitored under low fluence and therefore we do
not induce any nonlinearity in the transient absorption studies. The kinetics clearly reveals
that there is a pulse width limited electron injection and very fast decay followed by a further
injection and a very slow back electron transfer. To ascertain that the initial ultrafast kinetics
0-1.5ps is not due to window/spike we have recorded kinetics with only TiO 2 dissolved in
toluene without the dye, there we did not observe any signal. This observation confirmed that
the ultrafast components are genuine and not due to experimental artifacts. The kinetic decay
trace at 550nm reveals that the injection event is multi-exponential with 3 injection time
constants and with two back electron transfer (BET) time constants of 0.4 ps and > 1 ns
respectively. The dynamics at 900 nm which monitors the absorption arising due to the
conduction band electrons, interestingly reveal the similar dynamics in terms of the time
constant albeit with different contribution to various injection times and also the back