Mitigation of thermal artifacts in 100 kHz ultrafast 2D IR spectroscopy.

Ytterbium lasers make possible shot-to-shot data collection of two-dimensional infrared (2D IR) spectra at 100 kHz and higher repetition rates. At those rates, the power absorbed by the sample is appreciable and creates a steady state temperature rise and an accumulated thermal grating artifact in the spectra that can obscure weak or low concentration IR chromophores. We report the magnitude of the temperature rise, the pulse ordering by which it is created, and ways to mitigate it. Using a calibrant molecule, we measured a steady-state temperature up to 32.5 and 45 °C for laser light at 4 µm in H2O and 6 µm in D2O, respectively, for a typical optical density used in 2D IR experiments. The temperature reached a steady state in ∼60 s. The temperature rise scales with the integrated optical density of the sample across the laser spectrum. By cooling the sample cell, we returned the steady state temperature to room temperature within the laser focus. For samples that undergo rotation, the accumulated thermal grating artifact is removed using a perpendicular ⟨XXYY⟩ polarization because the permuted time-orderings of the thermal grating artifact has the orientational response ⟨XYXY⟩, which decays to zero during the delay between consecutive laser pulses. The procedure described in this study can be used to characterize and minimize the thermal effects in experiments where repetition rate and/or pulse energy cause an appreciable temperature rise.