Novel detection scheme for application in pump–repump–probe spectroscopy
Introduction
In numerous investigations the pump–probe scheme has been used to record the dynamic properties of the investigated system. In the last 25 years femtosecond pump–probe spectroscopy has evolved as one of the most important techniques to investigate ultrafast processes in biology, chemistry and physics [1]. Ti:Sa-based laser systems play a key role in the generation of the pulses required for these investigations [2], [3]. The development of different frequency conversion mechanisms (like second harmonic generation (SHG), white light continuum generation (WL), optic parametric processes) has made accessible a broad frequency range. In pump–probe spectroscopy a laser pulse excites the sample and thereby changes its optical properties. Afterwards another, often less intense, laser pulse records the transmission of the sample as a function of the time-delay between the two pulses. These transmission changes contain the information on the dynamics of the system under investigation. In more advanced experiments pump–dump or pump–repump schemes [4], [5], [6], [7], [8], [9] were used to investigate special properties of reacting systems.
In a recent study we investigated the ring-opening reaction (initiated by the excitation pulse) of a trifluorinated indolylfulgide after a preceding ring-closure reaction (initiated by the pre-excitation pulse) [9]. It was shown that reaction dynamics and quantum efficiency of the ring-opening reaction strongly depend on the delay time Δt1 between pre-excitation and excitation pulse. In this experiment it was difficult to record the dynamics of the small quantum yield with sufficient precision. Therefore we developed a new experimental scheme, where the change in quantum efficiency as a function of pump–repump delay Δt1 can be measured directly.
Section snippets
The laser system
The home-built Ti:Sapphire laser system working at 800 nm with repetition rate of 1 kHz has been described in detail in reference [10]. It provides a fundamental laser pulse that is split into several parts. For the probe and reference pulses a white light continuum (WL) is generated in a 3 mm thick sapphire plate. The excitation pulse at 630 nm (duration 35 fs) with 45 nJ pulse energy is produced in a noncollinear optical parametric amplifier (NOPA) [11], [12] and afterwards compressed in a quartz
Novel experimental detection scheme and results
Fig. 2 shows three experimental setups for the different transient absorption experiments. The classical pump–probe design is depicted on the top (a). In (b) the build-up used in [9] for pump–repump–probing and in (c) the new setup presented in this publication are shown.
Conclusion
In this publication we presented a novel detection scheme for a pump–repump–probe experiment. The splitting of the probe pulse into two identical parts and the special placement of the chopper made it possible to detect the complex transient absorption difference signal in a single measurement. It could be demonstrated that this special experimental setup allows the investigation of the dynamics and the quantum efficiency of the ring-opening reaction of an indolylfulgide after a preceding
Acknowledgements
Supported by Deutsche Forschungsgemeinschaft through the DFG-Cluster of Excellence Munich-Centre for Advanced Photonics and the SFB 749. The authors thank Karola Rück-Braun for providing the indolylfulgide sample and many fruitful discussions.
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Increasing the efficiency of the ring-opening reaction of photochromic indolylfulgides by optical pre-excitation
2010, Chemical Physics LettersCitation Excerpt :The accuracy of the experiment was improved by using a chopper blocking every second excitation pulse. This allowed to measure the difference between the transient absorption signal under ‘pre-excitation pulse only’-condition and the signal under ‘pre-excitation and excitation pulse’-condition with reduced noise [23]. In Fig. 2a transient absorption data for fulgide 1 at a probe wavelength of 547 nm are shown.
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