Photodissociation of adsorbed Mo(CO)6 induced by direct photoexcitation and hot electron attachment. II. Physical mechanisms

Abstract

Articles you may be interested inPlasmon-induced photoexcitation of "hot" electrons and "hot" holes in amorphous silicon photosensitive devices containing silver nanoparticles The wavelength dependence of photoinduced hot electron dissociative attachment to methyl bromide adsorbed on gallium arsenide (110) Photodissociation of adsorbed Mo(CO)6 induced by direct photoexcitation and hot electron attachment. I. Surface chemistryPhotodissociation of Mo (CO) 6 adsorbed on potassium-free and potassium-preadsorbed Cu( 111) and SiC 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 m W I cm 2 ), wavelength (250-800 nm), polarization (s andp), and incident angle (20°_70°). Two surface photoreaction mechanisms are considered: (i) direct electronic excitation of the adsorbate and (ii) attachment of photogene rated hot carriers to the adsorbate. The photodissociation spectra obtained on K-free Cu( 111) and SiC 111)7 X 7 exhibit the same resonant structure as the absorption spectrum ofMo(CO)6' Photodissociation ofMo(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.

Nanostructure formation by localized decomposition of Mo(CO)6 on Si(111)-(7×7) surfaces

“…The authors of that work identified the HOMO→LUMO transition as mechanism which leads to subsequent decomposition of the molecule. 23 Since the photon energy is similar to the deposition threshold energy of the electrons in our experiments we conclude that the same dissociation mechanism is effective if Mo͑CO͒ 6 is dissociated locally by electrons from the STM tip. As soon as the electrons arrive at the physisorbed layer with approximately double ͑triple, etc.͒ the energy necessary for excitation of Mo͑CO͒ 6 , one electron can induce two ͑three, etc.͒ dissociation events.…”

“…5. 23 For Mo͑CO͒ 6 physisorbed on Si(111)-(7ϫ7), the highest occupied molecular orbital ͑HOMO͒ (4d) and the lowest unoccupied molecular orbital ͑LUMO͒ (4d*) are 7 and 3.2 eV, respectively, below the vacuum level. Dissociation of Mo͑CO͒ 6 can be induced by excitation of an electron from the HOMO to the LUMO.…”

Laser-induced photochemistry of methane on Pt(111): Excitation mechanism and dissociation dynamics

“…Ho and co-workers 37,56,57 demonstrated that direct excitation plays a major role in photodissociation of Mo͑CO͒ 6 physisorbed on various substrates above 250 nm. The direct excitation in the photochemistry of weakly adsorbed Mo͑CO͒ 6 on various surfaces was confirmed from the close resemblance of the wavelength dependence of the photodissociation yield to that of optical absorption measured in cyclohexane.…”

Dissociation of Thymine by Low-Energy Electrons

“…In their results, SSB was observed as low as at 3-eV incident-electron energy. Since this report, Sanche and other researchers [3][4][5][6][7][8][9] has pioneered the investigation of DNA damage induced by LEE irradiation and explained the observation of DNA damage with dissociative electron attachment (DEA) process. DEA occurs when a free unbound electron attaches to a molecule to form a temporary negative ion, then dissociates into a negative ion and a neutral fragment [3,4]:…”