CO Adsorption on Pd(100) Revisited by Sum Frequency Generation: Evidence for Two Adsorption Sites in the Compression Stage

摘要

Sum frequency generation (SFG) and low-energy electron diffraction (LEED) have been used to revisit CO adsorption on Pd(100) from very low coverages up to saturation at 300 K. Below 0.5 ML, variations of SFG frequency and intensity with coverage are consistent with IRAS results from the literature. Novel observations are done above 0.5 ML, where the CO adlayer compression takes place. The existing compression model postulates the coexistence of compressed and uncompressed CO. We observe two bands in the spectral region of bridge sites and assign them to compressed and uncompressed CO. Both types of CO behave very differently: the molecular hyperpolarizability at compressed sites is smaller by a factor of 2 than at uncompressed sites. The frequency of uncompressed CO red-shifts during compression as the partial coverage decreases, while that of compressed CO continues to blue-shift as coverage increases. In the time domain, the coexistence of compressed and uncompressed sites results in oscillations in the decay of SFG intensity. A strong decrease from 690 to 222 fs of the phase relaxation time of uncompressed CO is observed during compression, indicating a stronger coupling to the substrate. These results are complemented by calculations of dipole-dipole interactions and DFT VASP calculations. While continuing blue-shift of compressed sites reflects a combination of increasing dipolar coupling and chemisorption change with coverage like below 0.5 ML, the very large red-shift amplitude of uncompressed CO indicates a large chemical contribution opposite to compressed CO. DFT VASP calculations allow us to follow the surface structure evolution from 0.5 to 0.67 ML and CO frequency changes with coverage. Pd atoms below compressed CO rows are pushed up, and compressed CO is tilted by 8- 9° with respect to the surface normal. A frequency split between compressed and uncompressed CO is found in agreement with experimental data. These results suggest that while compressed CO is less strongly bonded as compression proceeds the remaining uncompressed CO is more strongly bonded.