Transient two-dimensional IR spectrometer for probing nanosecond temperature-jump kinetics

We have developed a Fourier transform two-dimensional infrared (2D IR) spectrometer to probe chemical reactions and biophysical processes triggered by a nanosecond temperature jump (⁠$T$ jump). The technical challenges for such a spectrometer involve (1) synchronization of a nanosecond $T$-jump laser and femtosecond laser system, (2) overcoming the decreased signal-to-noise ratio from low repetition rate data acquisition, and (3) performing an interferometric measurement through a sample with a density and index of refraction that varies with time delay after the $T$ jump. The first challenge was overcome by synchronizing the two lasers to a clock derived from the Ti:sapphire oscillator, leading to timing accuracy of $2ns$ for delays up to $50ms$⁠. The data collection time is reduced by using undersampling with the improved signal-to-noise ratio obtained from a balanced detection scheme with a dual stripe array detector. Transient dispersed vibrational echo and 2D IR spectroscopy are applied to $N$-methylacetamide and ubiquitin, as examples, and the spectral responses by a temperature elevation and by structural changes of the protein are compared. The synchronization of 2D IR spectroscopy with a nanosecond temperature jump without losing its sensitivity at a low repetition rate opens a new applicability of the nonlinear spectroscopy to probe a variety of molecular structure changes induced by a nanosecond perturbation.