Conversion of large-amplitude vibration to electron excitation at a metal surface

Gaining insight into the nature and dynamics of the transition state is the essence of mechanistic investigations of chemical reactions1, yet the fleeting configuration when existing chemical bonds dissociate while new ones form is extremely difficult to examine directly2. Adiabatic potential-energy surfaces—usually derived using quantum chemical methods3 that assume mutually independent nuclear and electronic motion4—quantify the fundamental forces between atoms involved in reaction and thus provide accurate descriptions of a reacting system as it moves through its transition state5,6. This approach, widely tested for gas-phase reactions7, is now also commonly applied to chemical reactions at metal surfaces8. There is, however, some evidence calling into question the correctness of this theoretical approach for surface reactions: electronic excitation upon highly exothermic chemisorption has been observed9, and indirect evidence suggests that large-amplitude vibrations of reactant molecules can excite electrons at metal surfaces10,11. Here we report the detection of ‘hot’ electrons leaving a metal surface as vibrationally highly excited NO molecules collide with it. Electron emission only occurs once the vibrational energy exceeds the surface work function, and is at least 10,000 times more efficient than the emissions seen in similar systems where large-amplitude vibrations were not involved12,13,14,15,16,17,18. These observations unambiguously demonstrate the direct conversion of vibrational to electronic excitation, thus questioning one of the basic assumptions currently used in theoretical approaches to describing bond-dissociation at metal surfaces.