Ultrafast Systems KRONOS, Руководство пользователя

Страница: 23 / 39

Figure 10: Kinetic profile of Congo Red in 20% water in ethanol.
In Figure 10, the large spike is a result of the Xe flash lamp. Ignoring the large spike and looking at
data after 100 μs, the signal drops from ~19.8 mA to ~19.6 mA. Subsequently, the value increases
back to the baseline over tens of milliseconds. The flash induces a fast photochemical response,
much faster than what the Kronos can resolve, and subsequently generates the ground state of the
cis-form. This absorbs light, causing the detector to register a drop in transmission. Then, the
excited cis-state returns to the trans-state ground-state over many milliseconds.
Adjust averaging as desired, then measure the other samples you have prepared and save their
averaged data sets, adjusting the time scale as needed.
Evaluating the rate constants for the cis-trans conversion of CR as a function of [OH-], you can
obtain a curve similar to the one shown in Figure 11. You should find that the decays follow an
exponential rate law with a constant that is a linear function of [OH-] and the slope of the line (or
the best fit of the regression) is the bimolecular rate constant for the catalytic process.
22