Comparative study of C-H stretch and bend vibrations in methane activation on Ni(100) and Ni(111)

State-resolved measurements on clean Ni(100) and Ni(111) surfaces quantify the reactivity of CH4 excited to v=3 of the nu(4) bend vibration. A comparison with prior data reveals that 3 nu(4) is significantly less effective than the nu(3) C-H stretch at promoting dissociative chemisorption, even though 3 nu(4) contains 30% more energy. These results contradict statistical theories of gas-surface reactivity, provide clear evidence for vibrational mode specificity in a gas-surface reaction, and point to a central role for C-H stretching motion along the reaction path to dissociative chemisorption.