A I 4

molecular orbital theory. Four-fold sites are found favored on the (100) and (110) surfaces and the di-o bridging site is favored on Fe( l l l ) . We calculate carbon bond stretches of 0.36, 0.30, and 0.26 A, CH bending angles away from the surfaces of 65 °, 60 °, and 65 °, decreasing adsorption energies, and increasing carbon bond scission energies for Fe(100), (110), and (111), respectively. The bonding in chemisorbed acetylene is compared with the ethylene molecule. Conditions for twisting of the acetylene molecule are discussed, but no twisting is observed at the most stable chemisorp- tion sites on these surfaces. The relative activities of these surfaces toward acetylene are analyzed and a relationship is found between surface atom density and activity, such that the (110) and (111) surfaces are less active than the (100) surface because of their respective higher and lower surface iron atom packing densities. Finally, probable surface coverages and structures are discussed on the basis of calculated interactions between two adsorbed acetylene molecules and adsorbed acetylene and a CH fragment on the large clusters.

Surface Science 136 (1984) 419 436 419 North-Holland, Amsterdam

SECONDARY I O N E M I S S I O N F R O M U H V - D E P O S I T E D A M I N O A C I D

O V E R L A Y E R S O N M E T A L S

W . L A N G E , M . J I R I K O W S K Y a n d A . B E N N I N G H O V E N

Physikalisches lnstitut, Universiti~t Miinster, Domagkstrasse 75, 4400 Miinster, *ted. Rep. of Gerrnan.~

Received 20 July 1983; accepted for publication 28 September 1983

The secondary ion emission from organic solids has been studied under UHV conditions in a combined SIMS-AES instrument equipped with an evaporation source for volatile organic compounds. The instrument allows controlled formation of amino acid overlayers in the submono- layer, monolayer and multilayer range on metal surfaces, which can previously be characterized by their secondary ion and Auger electron emission. Secondary ion emission from leucine, glutamic acid, methionine, phenylalanine and glycine overlayers on Ag, Au, Ni and Cu have been investigated. In the submonolayer and monolayer range the emission strongly depends on the chemical nature of the metal substrate. No influence in the multilayer range was observed, The highest ( M + H) + yields were found for noble metal substrates. Protonated and deprotonated molecules ( M + H ) + and ( M - H ) - show a completely different behaviour concerning their intensity change during enhanced ion bombardment or heating: the damage cross section for (M + H) + emission from metal surfaces exceeds that for ( M - H) by a factor of about 5. During heating of the sample, the (M + H) + emission disappears at about 100 K below the disappearance temperature of ( M - H) .