Vibrational excitation and surface reactivity: An examination of the ν3 and 2ν3 modes of CH4

Abstract

The effect of vibrational excitation upon the chemisorption of methane on thin films of rhodium has been investigated. Vibrational modes of the gas are selectively excited by infrared radiation. The rate of reaction is measured by following the pressure decrease in a closed system containing CH4 in contact with the metal surface. Neither irradiation with a He–Ne laser at 3.39 μ to excite the ν3 mode, nor using an arc source with wavelengths in the range overlapping 2ν3, produces any detectable changes in the rate. From these experiments we estimate that the reaction probability of CH4 is <1×10−4 in the first excited state of ν3, and <7×10−2 in 2ν3. Other excitations must be involved in overcoming the barrier of ?7 kcal/mole which has previously been found for the chemisorption of CH4 on rhodium.