THEORY OF DIPOLE-FORBIDDEN VIBRATIONAL-EXCITATION OF MOLECULES ADSORBED ON METAL-SURFACES IN THE BORN APPROXIMATION

The probability of vibrational excitation by slow electrons of a periodic array of molecules adsorbed on a metal surface is calculated in the Born approximation. By including both dipole and quadrupole terms, a mechanism is derived for the diffuse scattering of electrons by dipole-forbidden transitions. A comparison of this scattering probability with recent measurements reported for scattering from the β1 phase of H on W(100) serves to illustrate the magnitude of this effect in terms of the dipole and quadrupole derivatives. Dipole-forbidden scattering by this mechanism is expected to be two to three orders of magnitude weaker than dipole scattering in the specular direction, but may well make a significant contribution when considering scattering out of the specular direction. © 1979 American Institute of Physics.