A 5 3

Surface Science 150 (1985) 451-479 451 North-Holland. Amsterdam

T H E I S O T H E R M A L K I N E T I C S O F H Y D R O G E N A D S O R P T I O N O N T O

I R O N F I L M S O B S E R V E D W I T H T H E C H E M I S O R P T I O N - I N D U C E D

R E S I S T A N C E C H A N G E

M . R . S H A N A B A R G E R

Quantum lnstttute. Umverstt)" of ('ahfornia. Santa Barbara. Santa Barbara. California 93106. USA

Received 27 March 1984; accepted for publication 1 October 1984

A study has been made of the isothermal kinetics for hydrogen chemisorption onto evaporated thin iron films. The measurements were made over a temperature range from about 293 to 460 K and for hydrogen pressures up to about 10-2 N / m 2 (8 x 10- 5 Torr). The chemisorption kinetics were measured using the chemisorption-induced resistance change to monitor the coverage of the chemisorbed hydrogen as it responded to changes in the gas phase pressure. Data from the measurements (adsorption kinetics, desorption kinetics, pressure-jump relaxation kinetics, and equilibrium isotherms) were analyzed in terms of a simple chemical reaction kinetic model which assumes that dissociative chemisorption occurs via an adsorbed molecular state. The absolute rate constants of the model obtained in this analysis were found to be independent of coverage for estimated coverages less than 0.8 monolayers. Based on the model, the desorption process in this temperature range is rate limited by desorption from the adsorbed molecular state to the gas phase.

480 Surface Science 150 (1985) 480 486 North-Holland. Amsterdam

I N F L U E N C E O F T H E E N E R G Y D I S T R I B U T I O N F U N C T I O N O N

T H E R M O D E S O R P T I O N

V e r a D O N D U R a n d D r a g a n F I D L E R

lnstttute of Physical ('hemtstry. Faculty of Sciences and Mathematics. Belgrade Unit'er,~tO'. P.O. Box 550. Belgrade. Yugoslavta

Received 6 June 1984; accepted for publication 15 October 1984

A kinetic model of the desorption process based on the energy distribution function was set up. The influence of known distribution functions, such as those of Dirac. Gauss, Cauchy. Weibull and others, on the kinetics of thermodesorption was investigated. In those cases the energy distribution function is a kinetic parameter. It was shown that the shape and position of the thermodesorption peaks depend on the type of energy distribution function. On the basis of the theoretical kinetic model the dependencies of differential desorption heat upon degree of coverage were calculated. It was shown that the differential desorption heat also depends on the energy distribution function.