A molecular beam study of the interaction of CO molecules with a Pt(111) surface using pulse shape analysis

Abstract

CO adsorption/desorption on a clean Pt(111) surface has been studied using molecular beam relaxation spectroscopy (MBRS). In contrast to conventional MBRS experiments, lock-in tecniques (or Fourier analysis) have been used here only for a qualitative survey. Pulse shape analysis, which allows the deduction of more detailed information from the experimental data, is discussed in detail, compared to conventional Fourier analysis and used for the results presented here. Detailed analysis of the shape of the MBRS pulse waveform has been used to determine the rate constant for CO desorption, the fraction of scattered signal attributable to chemisorption and the time-of-flight distribution of the non-chemisorbed fraction. The rate constant for CO desorption was measured with MBRS in the temperature range 530–650 K. Complementary measurements under quasi-equilibrium conditions using thermal energy atom (helium) scattering (TEAS) were also performed to extend the desorption rate constant determination down to 430 K, allowing accurate determinations of k over six orders of magnitude. On the clean surface (coverage < 1%), the rate constant for CO desorption obtained was k = 1.5 × 10 5 T 3 s exp(−28.8/ RT s ) s −1 (or in Arrhenius form, k = 4.3 × 10 14 exp(−32.0/ RT s ) s −1), with R in kcal mol . The MBRS measurements have also afforded identification of three distinct interactions of CO molecules with the clean Pt(111) surface: chemisorption, direct scattering and a third interaction showing all characteristics which are expected for physisorption. Approximately 3% of the molecules incident at zero coverage desorb from this state independently of temperature in the range 530–650 K. The angular distribution of the directly scattered molecules shows a peak with its maximum shifted away from the specular direction toward the surface normal. The time-of-flight distribution of the directly scattered molecules is similar to that of the incident beam, though somewhat broadened. Both the shift away from the specular direction and the broadening are ascribed to inelastic effects. Measurements of the relative intensities of the chemisorbed and physisorbed signals as a function of surface temperature provide no support to the assignment of the clean surface physisorption state as a precursor to chemisorption.