Formaldehyde on the Ag(111) surface: Dissociation and polymerization induced by ultraviolet photons and low-energy electrons
The ultraviolet photochemistry of monolayer formaldehyde (H$\sb2$CO) adsorbed on the Ag(111) surface was investigated in ultrahigh vacuum using electron energy loss spectroscopy and temperature programmed desorption. Dissociation of H$\sb2$CO to form methylene (CH$\sb2$) and atomic oxygen occurs at 35 K. Above 65 K, CH$\sb2$ and O induce polymerization of H$\sb2$CO. The polymerization cross-section was measured at photon energies from 3.50 to 4.32 eV. The cross-section at 3.50 eV was also measured as a function of light polarization and angle of incidence. These experiments showed that the electronic excitation of the adsorbed formaldehyde occurs by a substrate-mediated process, and not by direct absorption of the ultraviolet photons by the molecule. Ultraviolet photons absorbed by the silver promote substrate electrons to energies 0.9 to 0.1 eV below the vacuum level. These sub-vacuum level hot electrons can access an electron affinity state of adsorbed H$\sb2$CO. The image potential of the metal lowers the energy of this state several electron volts from its gas-phase value. The hot substrate electrons attach to adsorbed H$\sb2$CO, forming $\rm H\sb2CO\sp-$ ions, which dissociate to CH$\sb2$ and O. In a related experiment, low-energy gas-phase electrons were observed to induce polymerization of H$\sb2$CO on Ag(111) at 85 K, presumably also by an electron attachment mechanism. The polymerization induced by low-energy electrons has a cross-section 670 times higher than the cross-section for UV photons. The lower cross-section for the photon-induced polymerization can be accounted for by consideration of the band structure, optical properties, and electron mean free path of silver, as well as the tunneling probability of electrons through the surface barrier. The observed cross-section for UV-photon-induced polymerization of H$\sb2$CO on Ag(111) is thus shown to be reasonable for a reaction initiated by electron attachment. ^
Physical chemistry|Molecular physics|Condensed matter physics
Fleck, Louise Elizabeth, "Formaldehyde on the Ag(111) surface: Dissociation and polymerization induced by ultraviolet photons and low-energy electrons" (1994). Dissertations available from ProQuest. AAI9503759.