The determination of desorption rate constants by monitoring of the time dependence of the decay of infrared bands— dynamic infrared spectroscopy

A method which we have termed “dynamic infrared spectroscopy” that allows the determination of the value of the desorption rate constant of a specified adsorbate by monitoring the decay with time of the intensity of a given infrared band at a fixed temperature is described. By assuming a value of 1013 s−1 it is possible to determine the value of the desorption activation energy from the value of the rate constant. The method is demonstrated for the evaluation of the desorption rate constant and the activation energy of desorption of a bidentate formate species adsorbed on polycrystalline copper. The measurement of the rate constant at different temperatures allows evaluation of the desorption activation energy without assuming a value of 1013 s−1 for the desorption pre-exponential term. The method is valid only for a first-order desorption process. The method can be extended to second-order desorption processes by calibrating the intensity of the given infrared band against the coverage.