Transient Absorption Spectroscopy (Pump-probe)
Transient absorption spectroscopy uses two laser pulses, a strong pump and a weak probe which are overlapped in the sample. The linear absorption of the sample is recorded as the probe is delayed in time relative to pump using an optical delay line. This technique is used to measure ground state recovery times and the induced absorption of any new species resulting from a photochemical reaction, e.g., the charge separated state for a Donor-Bridge-Acceptor system.

Figure 1.  Basic Pump-probe experimental arrangement and levels electronic levels diagram. Note the thickness of the lines indicates the relative intensity of the laser pulses the probe is weak to avoid unwanted perturbation of the system.

Photon Echo Peak Shift and Transient Grating Spectroscopy
The photon echo peak shift (PEPS) and transient grating (TG) techniques use three laser pulses of equal intensity overlapped in the sample with a triangular geometry (Fig. 2). The signal is measured in a unique direction that is dependent on the pulse sequence for the incoming pulses (Fig. 2). In the scenario where pulse one proceeds in time pulse two and pulse two proceeds pulse three the signal is measured in the phase matching direction ks = k3 + k2 – k1 and the two time variables are referred to as the Coherence and Population times, respectively. The purpose of these experiments is to measure the coherence lifetime of a sample and in the case of PEPS distinguish between slowly varying decoherence processes such as inhomogeneous broadening and the random and usually rapid decoherence processes such as homogeneous broadening.

Figure 2.  Basic experimental arrangement for a three pulse photon echo experiment from the top and side view and the corresponding energy level diagrams for pulse sequence E1-E2-E3. Note photon echo signal is measured in the phase matching direction ks = k3 + k2 – k1 whereas a transient grating signal (t12 = 0) is the sum of ks and ks'.

TCSPC (Bulk)
Time Correlated Single Photon Counting in bulk solutions has and continues to be a workhorse of our long term research into synthetic electron- and charge-transfer systems. The continued development of these systems will lead to the next generation of cheap and durable plastics based optical materials for a myriad applications including display technology, optical switching and energy storage for computing, solar cells and protection from photodegradation, to name a few. TCSPC is powerful tool for determining fluorescence lifetimes on a nanosecond to millisecond timescale. When combined with our cavity-dumped femtosecond oscillator the repition rate can be optimised for the shortest collection time.

Figure 1.  Experimental apparatus for TCSPC measurements in bulk solutions for determining fluorescence lifetimes on a ns timescale. BS = beam splitter, BD = beam dump, Mono = monochrometer, MCP = Multi Channel Plate, CFD = constant fraction discriminator, TAC = time to amplitude converter, ADC = analogue to digital converter.