Two-photon photoemission spectroscopy of pyridine adsorbed on Cu(1 1 1)

Two-photon photoemission (2PPE) spectroscopy is employed to investigate the electronic structure of image potential states and unoccupied molecular resonances of pyridine adsorbed on Cu(1 1 1). The two lowest unoccupied states of pyridine, π* 3b1 and π* 2a2, are observed at and above a coverage of two monolayers (ML), at 3.4 and 4.0 eV above the Fermi level, respectively. Adsorption of pyridine lowers the work function from 4.87 eV for clean Cu(1 1 1) to 2.55 eV for 1 ML pyridine/Cu(1 1 1), due to the strong electron donation to the substrate. The work function increases again to 2.80 eV at 2 ML and stays roughly constant at higher coverages. At coverages between 0.6 and 1.0 ML thermal desorption spectra show pronounced changes of the peak shape and desorption temperature, while at the same time 2PPE spectra exhibit two (n=1) image potential states, originating from two different (coexisting) island patches with different local work functions. This is attributed to the occurrence of an orientational phase transition in the first adsorption layer with increasing coverage, as has been observed for pyridine adsorbed on other metal surfaces.