The pump and probe beams were orthogonally polarised and combined collinearly on a wire grid polariser (P3 in Figure ), then focused onto the sample using a 12.7 mm focal length ZnSe asphere (Thorlabs). The profiles and divergence characteristics of the focussed beams were examined using a microbolometer camera at the focal plane (see ESI Section 3). An accurate characterisation was critical for simulations. The average FWHM size of the pump and probe at the focal plane were 36 ± 1.4 and 21.5 ± 1 μm respectively. Pulse energies incident on the sample were 71 ±5 μJ (pump) and <0.5 μJ (probe). After the sample, the beams were re-collimated with a second ZnSe asphere. Residual / scattered pump light was rejected with a long-pass filter. A grating spectrometer dispersed the probe light onto a 128-pixel HgCdTe detector (Infrared Associates) with ~4 cm -1 resolution per-pixel. IR spectral intensities were digitised shot-by-shot (Infrared Development Systems FPAS) and correlated against simultaneous probe reference intensity measurements recorded using a small amount of pre-sample probe light sent to a separate spectrometer / detection system to correct for intensity fluctuations using a method similar to that of Feng et al. Spectra were recorded using one of two different measurement schemes. In the first, the sample was held stationary and the pump laser triggered at 20 Hz (chopped to 10 Hz), giving a data collection rate of 10 measurements per second and a 50 ms spectral collection window per measurement due to the need to chop the pump beam. To allow measurement of samples not fully returning to equilibrium conditions in < 50 ms, the sample was mounted on a motorised XY translation stage (two Newport XMS series stages, see ESI Section 2) which performed a triggered 200 μm step movement of the sample in < 50 ms at the end of each measurement, ensuring that each T-jump occurred from an unpumped region of the sample. These step measurements were collected at a rate of 4 measurements per second, giving a 20-40 ms time window stationary interval post pumping while allowing for the movement time of the XY translation stage.