Photodissociation of adsorbed Mo(CO)6 induced by direct photoexcitation and hot electron attachment. I. Surface chemistry

The adsorption and photodissociation of Mo(CO)6 on potassium-free and potassium-preadsorbed Cu(111) and Si(111)7×7 have been studied under ultrahigh vacuum conditions using vibrational spectroscopy and mass spectrometry. It has been observed that Mo(CO)6 adsorbs molecularly on potassium-free Cu(111) and Si(111)7×7 at 85 K and, upon heating, desorbs at 200 K. In the case of Cu(111), heating also leads to thermal dissociation of a fraction of Mo(CO)6 at ∼150 K. The dissociation channel competes favorably with the molecular desorption channel at low Mo(CO)6 exposures. On potassium-preadsorbed Cu(111) and Si(111)7×7 at 85 K, Mo(CO)6 adsorbs in part molecularly and in part dissociatively. The extent of dissociation depends on the K coverage; a high K coverage results in large degree of dissociation. The thermal desorption temperature for the molecularly adsorbed Mo(CO)6 increases to 210 K on the potassium-preadsorbed surfaces. Adsorbed Mo(CO)6 molecules photodissociate nonthermally into CO and carbonyl fragments. The product CO desorbs under irradiation. Photodissociation occurs on the K-free surfaces only under ultraviolet (UV) irradiation. With coadsorbed potassium, however, the photoyield increases substantially in the UV and extends to the visible and infrared (IR) regions. The carbonyl fragments which are present on the K-preadsorbed surfaces before irradiation are found to have a negligible contribution to the CO desorption signal. The photoactive species is identified to be the molecularly adsorbed Mo(CO)6 .