The adsorption and photochemistry of Mo(CO)₆ on Rh(100)

The adsorption and photochemistry of Mo(CO)₆ on Rh(100) has been studied using thermal desorption spectroscopy (TDS), photoinduced desorption (PID), and electron energy loss spectroscopy (EELS). It is found that Mo(CO)₆ adsorbs onto the surface at 80 K dissociatively as carbonyl fragments and CO and adsorbs molecularly after the first monolayer is filled. Photochemistry arising from charge–transfer electronic excitation of adsorbed Mo(CO)₆ is observed to correlate directly with the presence of multilayer adsorption. Results from PID indicate that UV radiation is effective in removing at least two, but not all, of the CO, ligands from each physisorbed Mo(CO)₆ molecule. The rate of deexcitation by the metal surface is not sufficiently fast to quench the observed photochemistry. The effects of coadsorption of Mo(CO)₆ on a presaturated CO ordered overlayer on Rh(100) are also studied. It is found that Mo(CO)₆ adsorbs molecularly without affecting the CO(sat) layer and undergoes photodissociation. The quenching rate of the excited states of Mo(CO)₆ is much slower on the ordered CO surface than on the dissociated surface containing carbonyl fragments and CO.