(Featured Article in the Journal of Chemical Physics)
Peter Hamm introduces a new application of 100 kHz Yb-laser systems, transient 2D IR spectroscopy, using the photocycle of bacteriorhodopsin as a demonstration object. This approach utilizes the high repetition rate of these lasers in two ways: First, it directly determines the time resolution of the experiment (10 µs) and, at the same time, enables the measurement of many 2D IR spectra within a reasonable averaging time. Furthermore, the very good stability of the Yb-laser/OPA system allows for hours of measurement time without the need for any readjustments. We have not seen the same level of stability for Ti:S-based laser systems. Data can be taken overnight, and the only limiting aspect at this point is the need to refill the MCT detector with liquid N₂.

Currently, 100 kHz is the upper limit for this type of experiment, despite the fact that Yb-laser systems with higher repetition frequencies are available. The pulse shaper, the response time of the MCT detector, and the AD-converter limit the repetition frequency to that value.

The timescales covered by this type of experiment, 10 µs to a few tens of milliseconds, are perfectly suitable for the investigation of a wide range of photo-active proteins. It also matches that of diffusion controlled bimolecular processes, such as protein–ligand interactions or chemical reactions, in the context of artificial photosynthesis. Even when the goal is “only” transient 1D IR spectroscopy, the system outperforms step-scan FTIR spectroscopy or more modern setups based on quantum cascade lasers in terms of measurement time; 1 min was sufficient to collect transient 1D IR data. Admittedly, the present experimental setup is more expensive than, for example, a quantum cascade laser spectrometer, but not by orders of magnitudes (by a factor of 2–3). At the same time, due to the high intensities of femtosecond pulses, an Yb-laser system is much more versatile, as one may reach a very wide frequency range from THz to x rays by some means of nonlinear optics. The improvement over Ti:S lasers is enormous, suggesting that these types of lasers might become the standard for time-resolved mid-IR spectroscopy on all conceivable timescales, from femtoseconds to hours.