From calibration curves constructed in this way we can then estimate temperature changes occurring in the T jump experiments. Figure shows temperature-time dependence curves obtained in this way. Instantaneous local heating must occur within the 1.5 ns pump pulse duration because vibrationally excited Brønsted sites, water and silanols relax on ~sub 200 ps timescales. T-jump rise times of ~ 4 ns are observed, greater than the measured puse duration of the pump laser. This presumably represents the absorbed energy equilibrating from the location of the O-H sites pumped by the laser pulse to the entire framework within each crystallite. Temperatures stabilise after 3 -10 × 10 -9 s, after which the zeolite remains hot out to the millisecond time range. The magnitude of the T-jump achieved (ΔTMax) varies depending on starting temperature (Figure inset). This is due to the varying levels of water in the sample changing the absorbance in the pump region of the spectrum and therefore the amount of energy absorbed from the pump pulse. The source of this effect is demonstrated by the fact that the samples at 250 and 300 °C are both free of adsorbed water and therefore have virtually identical IR spectra (Figures The cooling behaviour of the samples was analysed by fitting the temperature curves of Figure . Points earlier than 10 -7 s were excluded from the fit in order to capture just the cooling. In all cases the data was found to be best represented by a biexponential function of the form: