Effect of laser heating on the local temperature and composition in Raman spectroscopy:: A study of Ba(NO3)2 and BaO3 decomposition

The effect of laser heating on the local concentration of a white Ba(NO3)(2) or BaO2 sample in a stationary in situ Raman cell, as determined by Raman spectroscopy, was addressed in this study. The decomposition of Ba(NO3)(2) and crystalline BaO2, both supported on MgO, was followed as a function of the temperature of the cell with the use of Raman spectroscopy and product analysis by gas chromatography. At a laser power level of 25 mW and with the sample at 580 degrees C, the decomposition curves were offset by about 3 degrees C, rather than by a temperature difference of more than 15 degrees C that might be expected from laser heating. Diffusion of NO3- and O-2(2-) ions from the surrounding region into the 100 mu m-size spot heated by the laser apparently compensates for the loss of materials at the higher temperature. As a consequence, for in situ catalytic studies, the concentrations determined by Raman spectroscopy may be more representative of the sample than one might expect from the local temperature.