SURFACE-DIFFUSION AND DESORPTION-KINETICS FOR PERFLUORO-NORMAL-BUTANE ON RU(001)

The surface diffusion and desorption kinetics for perfluoro-n-butane on Ru(001) were examined using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The surface diffusion displayed Arrhenius behavior and was coverage independent. The surface diffusion parameters for perfluoro-n-butane on Ru(001) were E(dif) = 2.9 +/- 0.3 kcal/mol and D0 = 5.9 X 10(-2 +/- 0.2) cm2/s. The desorption parameters for perfluoro-n-butane on Ru(001) were E(des) = 13.8 +/- 0.6 kcal/mol and v(des) = 2.8 X 10(21 +/- 0.1) s-1. In comparison, the surface diffusion parameters for n-butane on Ru(001) were E(dif) = 3.5 +/- 0.2 kcal/mol and D0 = 1.4 X 10(-1 +/- 0.2) cm2/s. The desorption parameters for n-butane on Ru(001) were E(des) = 11.9 +/- 0.5 kcal/mol and v(des) = 3.6 X 10(15 +/- 0.1) s-1. The corrugation ratio, defined as OMEGA = E(dif)/E(des), was determined to be OMEGA = 0.21 for perfluoro-n-butane on Ru(001). This corrugation ratio was substantially different than the corrugation ratio of OMEGA almost-equal-to 0.30 measured for n-butane and various other n-alkanes, cycloalkanes and branched alkanes on Ru(001). The comparison between perfluoro-n-butane and the other alkanes indicates that fluorination lowers the surface corrugation ratio on Ru(001). Likewise, fluorination significantly increases the preexponential for desorption from Ru(001). This study illustrates the magnitude of substituent effects on surface diffusion and desorption kinetics for a physisorbed molecule on a single-crystal metal surface.