Electronically stimulated dissociation ofNO2on Pt(111)

The electronically stimulated dissociation of ${\mathrm{NO}}_2$ on Pt(111) by 5–800-eV electrons has been studied through state-selective, time-of-flight detection of the NO product above the surface. The NO leaves as a direct dissociation product, whereas the O atom remains on the surface. We find a 10–15-eV dissociation threshold, which is much more energetic than the 3.1-eV gas-phase value. Because of rapid decay via resonant tunneling from substrate levels, the shallower adsorbate valence excitations have lifetimes that are too short to produce observable dissociation. The threshold corresponds to ionization of 3$b_2$ and double ionization of 1$a_2$ levels. Holes in these levels cannot be resonantly filled by substrate electrons and have the longest lifetimes relative to Auger decay. Screening of the hole(s) by the metal proceeds via rehybridization between the substrate levels and the 6$a_1$ orbital of the molecule. The screening charge not only determines the lifetime, but is expected to change the excited-state ONO bond angle and NO bond length. In the gas phase this results in considerable internal excitation of the NO product. On the metal surface, however, vibrational excitation above ν=2 is not observed: The relative populations for ν=0,1,2 are (1.0):(0.96):(0.52). The average rotational energy (800 K for each vibrational level) is only 50% warmer than the zero-point energy of the ground-state bending motion of the adsorbate. Thus the screening charge must be less than unity. The observation of shifts to higher translational energy for higher rotational and vibrational states indicates the presence of different charge-transfer screening densities for the 3$b_2^{\mathrm{-}1}$ and 1$a_2^{\mathrm{-}2}$ excitations.