PHOTODISSOCIATION OF ADSORBED MO(CO)6 INDUCED BY DIRECT PHOTOEXCITATION AND HOT-ELECTRON ATTACHMENT .2. PHYSICAL-MECHANISMS

Photodissociation of Mo(CO)6 adsorbed on potassium-free and potassium-preadsorbed Cu(111) and Si(111) 7 x 7 at 85 K has been studied under ultrahigh vacuum conditions. The photodissociation yield has been measured as a function of photon power (0.5-30 mW/cm2), wavelength (250-800 nm), polarization (s and p), and incident angle (20-degrees-70-degrees). Two surface photoreaction mechanisms are considered: (i) direct electronic excitation of the adsorbate and (ii) attachment of photogenerated hot carriers to the adsorbate. The photodissociation spectra obtained on K-free Cu(111) and Si(111) 7 x 7 exhibit the same resonant structure as the absorption spectrum of Mo(CO)6. Photodissociation of Mo(CO)6 on K-free surfaces is thus determined to be dominated by direct electronic excitation of the adsorbate, which proceeds via a single-photon process. A new photodissociation channel is opened on K-preadsorbed surfaces. The photoyield increases substantially in the UV and extends to the visible and near IR. By studying the wavelength and polarization dependences of the photoyield, it is firmly established that the new photodissociation channel is due to interaction of photogenerated hot carriers with the adsorbate. The photogenerated hot electrons tunnel through the potential barrier between the adsorbed Mo(CO)6 and substrate and attach to the Mo(CO)6 molecules. This mechanism is energetically possible in the presence of K due to a substantial up-shift in the Fermi level associated with the decrease in the work function. The negative ions formed by electron attachment are unstable and undergo dissociation.