Abstract
Femtosecond visible pulses at 800 nm are observed to induce permanent spectral changes in the CO internal stretch region of CO/Pt(1 1 1) at 300 K, using sum frequency generation vibrational spectroscopy. By contrast, IR femtosecond, and visible picosecond pulses do not induce any change. Defects, disordering, water coadsorption or laser induced CO dissociation can be ruled out to account for the observed changes, which seem to be due to the formation of a metastable structure. With this structure, the CO coverage seems to increase reversibly in equilibrium with gas phase CO, which normally occurs on various surfaces, but not on Pt(1 1 1). An unusually high frequency for linearly bonded CO at 300 K is obtained above ≈10 −7 mbar. More experimental work using other techniques is required to confirm the occurrence of a metastable structure and to characterize it. Nevertheless the available spectroscopic and structural data concerning CO on Pt(1 1 1) do not support an interpretation where the laser induced metastable structure would remain close to the regular c(4×2) structure.
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