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Description
Polarization-dependent sum-frequency generation (SFG) spectroscopy was applied to study the adsorption of carbon monoxide (CO) on the annealed and sputtered Ir(111) single crystal surfaces at various CO coverages [1]. Coverage was adjusted by varying the substrate temperature (300−575 K) and/or gas pressure (10−7 to 1.0 mbar). The cleanliness of Ir(111) was confirmed by Auger electron spectroscopy, and its long-range order and CO overlayer structures were characterized by low energy electron diffraction (LEED). With increasing coverage (gas pressure), molecularly adsorbed CO exhibited three overlayer structures: (√3×√3)𝑅30° →(2√3×2√3)𝑅30° →(3√3×3√3)𝑅30°. Under all conditions, only linearly bonded (on-top) CO was observed by SFG. Using different polarizations, PPP and SSP spectra were acquired with a high signal-to-noise ratio, whereby tilt angles of CO on Ir(111) could be determined for the first time by SFG. It was found that not only the vibrational frequency of on-top CO, but also the tilt angle was strongly coverage-dependent for CO on the annealed surface. The higher the coverage was, the larger the vibrational frequency and the tilt angle were. At about 0.7 ML coverage, a CO tilt angle of at least 20° was observed, in good agreement with density functional theory (DFT) calculations. In addition, the molecular hyperpolarizability ratio (R) of CO (at 0.13 ML in UHV) was determined to be 0.08. Based on the combined SFG/DFT results, it may change to 0.29 at 0.77 ML coverage. However, on the sputtered surface, CO preferred to stand upright with little coverage dependence. After heating to ~600 K in mbar CO pressure, LEED indicated disordered CO adsorption, but SFG showed red-shifted CO both on the annealed and sputtered surface, with reduced intensity. The irreversibility of SFG spectra may point to CO dissociation around 600 K.
[1] X. Li, G. Rupprechter, Catalysis Science & Technology, 11 (2021) 12.
| Speaker Country | Austria |
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