A414

Surface Science 134 (1983) 161-183 161 North-Holland Publishing Company

T H E A D S O R P T I O N O F B E N Z E N E A N D N A P H T H A L E N E O N T H E

R h ( l l l ) S U R F A C E : A L E E D , A E S A N D T D S S T U D Y

R . F . L I N *, R.J . K O E S T N E R , M . A . V A N H O V E a n d G . A . S O M O R J A I

Materials and Molecular Research Division, Lawrence Berkeley Laboratory and Department of Chemistry, University of California, Berkeley, California 94720, USA

Received 15 March 1983

The adsorption of benzene and naphthalene on the Rh(l l l ) single-crystal surface has been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). Both benzene and naphthalene form two different ordered surface structures separated by temperature-induced phase transitions: benzene transforms from a (3 1~ I 3J structure, which can also be labelled c(2v/3 x4)rect, to a (3x3) structure in the range of 363-395 K, while naphthalene transforms from a (3v~ × 3v~)R30 ° structure to a (3 x 3) structure in the range 398-423 K. Increasing the temperature further, these structures are found to disorder at about 393 K for benzene and about 448 K for naphthalene. Then, a first H 2 desorption peak appears at about 413 K for benzene and 578 K for naphthalen e and is interpreted as due to the occurrence of molecular dissociation. All these phase transitions are irreversible. The ordered structures are interpreted as due to flat-lying or nearly flat-lying intact molecules on the rhodium surface, and they are compared with similar structures found on other metal surfaces. Structural models and phase transition mechanisms are proposed.

184 Surface Science 134 (1983) 184-194 North-Holland Publishing Company

S U R F A C E S E G R E G A T I O N I N L I Q U I D G a - S n A L L O Y S BY A E S

S t e p h e n H A R D Y a n d J o s e p h F I N E

Metallurgy Division and Surface Science Division, National Bureau of Standards, Washington, DC 20234, USA

Received 6 June 1983

The surface segregation of Sn in liquid Ga-Sn alloys has been studied by Auger electron spectroscopy as a function of bulk concentration at 350°C. The Sn was found to be strongly adsorbed at the surface. The surface concentrations of Sn and Ga were calculated from the Auger measurements using inelastic mean free paths and backscattering factors estimated in recent theoretical work. The values found for the surface concentrations are essentially in agreement with those deduced from surface tension measurements using Gibbs adsorption theory. This result supports a monolayer adsorption distribution in the Ga-Sn system.