Resonant photodissociation of Mo(CO)₆ adsorbed on graphite and Ag(111)

The adsorption and photodissociation of Mo(CO)₆ on the basal plane of graphite and Ag (111) are studied by photoinduced desorption, high-resolution electron-energy-loss spectroscopy, electronic electron-energy-loss spectroscopy (EELS), and thermal desorption spectroscopy. Mo(CO)₆ is found to absorb in pure molecular form, without dissociation, on each surface at 85-90 K. Electronic EEL spectra confirm that the electronic structure of the molecule remains relatively unperturbed on the surface. Similar to the gas phase, electronic transitions of the molecule, including the ligand-field transition and the metal-to-ligand charge transfer, were observed for absorbed Mo(CO)₆. Upon low-power UV irradiation (λ < 360 nm), the adsorbed molecules readily photodissociate and release CO. The mechanism of photodissociation on each surface is separately identified to be direct photoelectronic excitation of the adsorbed Mo(CO) ₆ by careful photon power and wavelength-dependence studies. Linear power dependence was found in each case, indicating the initial excitation is due to the absorption of a single photon. Resonances in the photodissociation spectra of the molecules were observed at ∼290 nm and 325 nm, due to the metal-to-ligand charge transfer and the ligand-field transition, respectively. For graphite, the relative photoyield was observed to closely match the Mo(CO)₆ absorption spectrum. In contrast, a relative enhancement in the photoyield was observed for Ag(111) at ∼325 nm due to the increase of the surface electric field associated with the onset of the d band to the Fermi-level transition in this wavelength region. In addition, small quantities of the fragments from Mo(CO)₆ were observed as a result of bombardment of the surface by low-energy (20 eV) electrons. The photodissociation yields are very sensitive to the adsorbate coverage. For a coverage of about 0.25 monolayer or less, there is no observable photodissociation for Mo(CO)₆ on Ag. On both Ag and graphite, the photodissociation yield increases as the coverage approaches and exceeds one monolayer; photodissociation competes efficiently with relaxation into the substrate.