Coverage-Induced Orientation Change: CO on Ir(111) Monitored by Polarization-Dependent Sum Frequency Generation Spectroscopy and Density Functional Theory

Xia Li; Verena Pramhaas; Christoph Rameshan; Peter Blaha; Günther Rupprechter

doi:10.1021/acs.jpcc.0c04986

Coverage-Induced Orientation Change: CO on Ir(111) Monitored by Polarization-Dependent Sum Frequency Generation Spectroscopy and Density Functional Theory

J Phys Chem C Nanomater Interfaces. 2020 Aug 20;124(33):18102-18111. doi: 10.1021/acs.jpcc.0c04986. Epub 2020 Jul 30.

Authors

Xia Li¹, Verena Pramhaas¹, Christoph Rameshan¹, Peter Blaha¹, Günther Rupprechter¹

Affiliation

¹ Institute of Materials Chemistry, Technische Universität Wien, 1060 Vienna, Austria.

Abstract

Polarization-dependent sum frequency generation (SFG) spectroscopy was applied to study the adsorption of carbon monoxide (CO) on the well-ordered (annealed) Ir(111) single-crystal surface at various CO coverages. Coverage was adjusted by varying the substrate temperature (300-575 K) and/or gas pressure (10^-7 to 1.0 mbar). Under all conditions investigated, only a single absorption band at 2038-2094 cm^-1 was observed, characteristic of linearly bonded (on-top) CO. 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. 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, which is 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.