|Multiwavelength absorption kinetics.
The real challenge for the detection system is multiwavelength absorption kinetics. Due to the fact that the signal displayed (∆absorption) is not linearly proportional to the signal measured (%transmittance) and, owing to fluctuations in the Is/Ir ratio, it is necessary to read out collected data after every laser shot. This puts high demands on the readout circuit that must handle 1 (pump) + 512 (Is) + 512 (Ir) readouts per millisecond instead of just three in single wavelength experiments. The pump pulse intensity is also measured for gating and intensity scaling purposes. Usually, gating is only applied on the pump pulse, however, if the probe pulses are unusually unstable, gating can also be applied on the probe pulses.
The two diode arrays can be placed above each other at the output of the spectrograph. If a smaller spectrograph is used, for which the two output spectra can not be separated sufficiently, a mirror can be used to achieve a 5-mm separation between the two spectra (see fig).