Femtosecond Systems, Experimental Suggestions

The first requirement for these measurements is that the laser pulse is stable over time. Key faetors for the stability of the laser are the temperature and humidity of the room, whieh must be kept as eonstant as possible every day and espeeially throughout the measurement session, as the thermal stress of the optieal parts ean eompromise stability of the pulses. 

Eaeh transient speetrum corresponds to a certain position of the mirrors on the delay hne, this position determines the time of arrival of the PROBE beam to the sample and consequently the delay of the probe with respect to the pump beam. This is achieved by the motion, using a stepping motor, of a mechanical device (linear positioning stage), controlled by software. To generate a stable, high-quality white light is needed a stable low power (0.05 mJ) 800 nm laser pulse. 

The measurement consists of the following steps (i) determination of the zero line before of the excitation (PROBE before PUMP), (ii) delaying the arrival of the beam on the sample (PROBE after PUMP) and acquisition of the corresponding transient spectrum (iii) displacement of the PROBE to a further delay and so on. The sequence of delays is set by software by the operator before or during the measurement. 

In order to obtain an optimum signal there must be a perfect spatial overlap of the PUMP and PROBE on the sample. The PUMP beam that emerges from the sample should not be detected by optical fibers to prevent damage to them. In contrast, the light of analysis (PROBE) should fit perfectly in the fiber to avoid distortions in the signals. 

For substantially photostable samples in solution it is advisable to keep shaking the sample. Commercially available magnetic stirrers are capable of transmitting the torque to a Teflon coated magnetic bar inside of a sample cell. Due to its small size, the stirring bar fits into 2 mm thick cuvettes. Since the rotation of the stirring bar is achieved with a magnetic field the stirrer can be used with hermetically sealed cells. 

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Recent advances, for example, replacement of the Pockels cell with a PEM system, has provided an improvement in the experimental SNR of an order-of-magnitude [44]. Further, the authors suggest that with their experimental approach, the picosecond laser system now in use could be replaced by one operating with femtosecond pulses. If successful, this would allow extension of CD measurements into a time domain where the initial structural changes which determine the outcome of a sequence of complicated events can be probed. 

The vast majority of papers devoted to tautomerization dynamics deal with ESIPT reactions. Since Weller s suggestion that the large Stokes shift he measured for salicyhc acid fluorescence was caused by rapid proton transfer in the excited state [62], and the development of techniques to study this on a femtosecond timescale, the field has blossomed. Most of the 2000 papers on tautomerization dynamics is on ESIPT, from both an experimental and a theoretical point of view. The number of compounds exhibiting ESIPT is far too large to discuss here. It ranges from molecules as simple as malonaldehyde to systems as complicated as 3-hydroxyflavone or 2-(2 -hydroxyphenyl)benzothiazole. In particular, substituted salicylic acids and ortho-hydroxybenzaldehydes have attracted much attention from both experimentalists and theoreticians. Weller s idea is depicted in Figure 1.10. 

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