n-alkanes on MgO(100). I. Coverage-dependent desorption kinetics of n-butane.

High-quality temperature-programmed desorption (TPD) measurements of n-butane from MgO(100) have been made for a large number of initial butane coverages (0-3.70 ML, ML-monolayers) and a wide range of heating ramp rates (0.3-10 K/s). We present a TPD analysis technique which allows the coverage-dependent desorption energy to be accurately determined by mathematical inversion of a TPD spectrum, assuming only that the preexponential factor (prefactor) is coverage independent. A variational method is used to determine the prefactor that minimizes the difference between a set of simulated TPD spectra and corresponding experimental data. The best fit for butane desorption from MgO is obtained with a prefactor of 10(15.7+/-1.6) s(-1). The desorption energy is 34.9+/-3.4 kJ/mol at 0.5-ML coverage, and varies with coverage approximately as Ed(theta)=34.5+0.566theta+8.37 exp(-theta/0.101). Simulations based on these results can accurately reproduce TPD experiments for submonolayer initial coverages over a wide range of heating ramp rates (0.3-10 K/s). Advantages and limitations of this method are discussed.