Electronically Nonadiabatic Vibrational Excitation of N₂ Scattered from Pt(111)

Abstract

Molecular beam surface scattering is used to compare vibrational excitation of N 2 molecules in collisions with clean Pt(111) and Au(111) surfaces under UHV conditions. Direct single-bounce collisions are dominant under all conditions of this work, as evidenced by narrow specular angular scattering distributions and translational incidence energy dependent rotational temperatures. N 2 (v = 0 → 1) vibrational excitation is observed for Pt(111), but not Au(111). The excitation probabilities, ranging from ∼10 −4 to ∼10 −3 , follow an Arrhenius surface temperature dependence and increase with translational incidence energy with zero threshold. The observations are the typical fingerprint of nonadiabatic vibrational excitation due to an electron mediated excitation mechanism, identified in previous work on NO and CO scattering from metal surfaces. The unfavorable electron affinity of the N 2 molecule (EA = −2.3 eV) makes this observation surprising and we discuss possible excitation mechanisms. ■ INTRODUCTIONThe ability to predict whether or not a reaction will happen when a molecule collides with a surface is essential if catalysts are to be designed from first principles. Real catalytic systems are often extremely complex and results from simple model systems, which allow a detailed picture of the surface−molecule interaction and energy transfer to be built, are essential to test theoretical predictions and develop deeper understanding.Molecular beam scattering from well characterized singlecrystal surfaces has provided valuable information about the dynamic processes occurring during the collisions, demonstrating that while some are electronically adiabatic (mechanical translational to vibrational (T−V) excitation, for example, vibrational excitation of ammonia on Au(111) 1 ), other scattering events can only be adequately explained by electronically nonadiabatic mechanisms, where vibrational excitation occurs with the relaxation of a hot electron−hole pair (EHP) of the metal.Electronically nonadiabatic vibrational energy transfer exhibits dynamical fingerprints. Early work on vibrational excitation of NO directly scattered from Ag(111) 2,3 found a strong surface temperature dependence and the lack of a translational energy threshold for vibrational excitation, providing important evidence that the necessary energy comes from the surface, and not from the kinetic energy of the molecule. Energy transfer by a mechanical phonon-tovibration excitation mechanism is unlikely to be efficient given the mismatch in the energy of the high frequency molecular vibration and the low frequency phonons, leaving coupling to EHPs as the most likely energy source for vibrational (EHP-V) excitation. More recently, striking examples have shown that many quanta of vibrational energy can be transferred to or from the colliding molecule due to electronically nonadiabatic mechanisms. These effects can be seen in multiquantum vibrational relaxation 4 or excitation 5,6 and in the emission of electrons from low work function surfaces. 7 Meas...

Computational approaches to dissociative chemisorption on metals: towards chemical accuracy

“…Recent experiments on N 2 have looked at its vibrationally inelastic scattering from Pt (111). 209 Older experiments have studied its DC on Ru(0001) 210 and on tungsten surfaces. [211][212][213] Recent experiments on O 2 have investigated the effect of orientational alignment 214,215 and of the molecular spin state 215 on its DC on metals, and its DC on Ru/Cu (111) and Cu (111).…”

Ion and velocity map imaging for surface dynamics and kinetics

“…Dynamical processes occurring during the interaction of atoms and molecules with surfaces are intimately related to the reaction rates which are important to understanding heterogeneous catalysis. Depending upon the nature of the surface and the type, energy, and orientation of the atom or molecule, a wide range of processes may occur at the gas-surface interface, ranging from direct scattering (H/Au(111), 1 NO/Ag(111), 2 N 2 /Au(111) 3 ) to trapping-desorption (CO/Pt(111) 4 ), dissociative adsorption (CH 4 /Pt(111), 5 H 2 O/Ni(111), HCl/Au(111) 6 ), and associative desorption (H 2 /Cu(111) 7 ).…”

“…1 Deviations from Maxwellian velocity distributions and from cos(θ) (with respect to the surface normal) angular distributions for argon atoms a) daniel.harding@mpibpc.mpg.de desorbing from Pt(111) provided velocity and incidence angle dependent sticking probabilities, again via detailed balance. 8 The surface temperature dependence of the vibrational excitation probabilities of NO, 9 N 2 , 3 and CO 10 on Au(111), combined with the absence of a threshold in the incidence energy dependence provided a fingerprint for the electronically nonadiabatic vibrational excitation. From these examples, it is clear that the experimental observation of the dynamical fingerprints of the different processes controlling surface scattering require accurate measurement of the laboratory-frame speed and angular distributions of scattered molecules as a function of the surface temperature (T s ) and the molecules' incidence energy and initial quantum state.…”

Vibrationally inelastic scattering of HCl from Ag(111)

“…[7][8][9][10] Molecular dynamics simulation using electronic friction to characterize BOA failure successfully reproduced translational inelasticity of H and D collisions at metal surfaces as well as the magnitude of the isotope effect in chemicurrent experiments. 10 There is strong evidence for BOA failure in vibrationally inelastic scattering of diatomic molecules colliding at noble metal surfaces-see work on NO/Ag(111), [11][12][13][14] NO/Cu(110), 15 NO/Au(111), 16,17 CO/Au(111), [18][19][20][21] N 2 /Pt(111), 22 and CO/Ag(111). 23 The magnitude of the electronically nonadiabatic interaction is believed to be enhanced when a transient negative ion can be formed.…”