Molecular beam study of molecular collisions with a hot filament. Pyrolysis of methyl iodide and other organic compounds on hot tungsten

Abstract

A technique has been developed for sampling, under collision-free conditions in a molecular beam, the molecular species from the surface of a filament immersed in a stream of molecules of such an intensity that the molecular bombardment rate was equivalent to pressures of up to 0.02 torr. The molecular beam sample was studied by mass-spectrometric analysis and velocity analysis of the resultant ion beam permitted measurement of the thermal velocity of the species from the filament surface. Study of the effect of surface temperature on the interaction of argon with a tungsten filament indicated that a transition from diffuse (cosinal) reflection to specular reflection to specular reflection occurred as the filament temperature was raised from room temperature to 1500°K. The process was dependent on the state of the tungsten surface and was partly changed by carbonization. Similar phenomena probably occur with organic compounds. Appearance potential studies of the products from pyrolysis of azomethane and nitromethane on a tungsten filament at red heat, indicated the presence of methyl radicals. Iodine atoms were readily detected in the pyrolysis of methyl iodide. Quantitative studies of this process, by measurement of the disappearance of methyl iodide. Quantitative studies of this process, by measurement of the disappearence of methyl iodide, and by measurement of the amount of iodine formed, at various temperatures, indicate an activation energy for pyrolysis of about 45kcal/mole. The observations indicate that methyl iodide molecules undergo mainly specular collisions with the hot surface, during which internal vibrational modes can be excited. After separation from the surface, redistribution of the internal energy leads to decomposition. The iodine atom continues with little change of its direction of motion, but the methyl radical is ejected in an almost random direction with respect of the initial trajectory of the activated complex, the direction depending only on the spatial orientation of the complex at the instant of decomposition.