To quantify the temperature changes occurring on laser excitation, we explored the effectiveness of the silanol ν(SiOD) band as a thermometer. This band shows no interaction with water under the conditions of these experiments, so that variations in its frequency and intensity with temperature provide a way to estimate temperature changes during the T jump experiments. Variations in the ν(ZOD) band in dry zeolites and the ν(CO) bands of trifluoroacetic acid have previously been used successfully as thermometers in other studies in the literature. Figure (a) in the ESI shows how the silanol band in ZSM-5 varies with temperature in the FTIR spectra. To remove the contributions to the transient spectra from water desorption, baseline subtraction close to the edges of the ν(SiOD) band is applied. Applying this also to FT-IR difference spectra from the same starting temperature allows a relationship between the silanol mode absorbance change and increase in temperature to be established. The ν(SiOD) band is observed by FT-IR to shift and weaken as a function of increasing temperature. Frequency shifts alone produce a symmetric derivative in the difference spectra, while an absorbance decrease alone would give a negative peak in the difference spectrum. The combination of a frequency shift to lower frequency and an absorbance decrease gives an asymmetric difference band with an enhanced negative lobe on the high frequency side. This is illustrated with simulated spectra in ESI Figure . Figure shows how the intensity of this negative lobe in difference FTIR spectra can be used to construct a calibration curve. A linear baseline is fitted to the difference spectrum as illustrated in Figure (a) and the baseline subtracted spectra plotted as a function of the temperature difference in Figure . The intensity of the negative lobe is a linear function of the temperature difference (Figure )) The linear slope of 1/K provides a conversion factor K used to convert the intensity of the ν(SiOD) difference feature to the corresponding change in sample temperature.