The adsorption of 1,3,5,7-Cyclooctatetraene (COT) on Ru(001) is studied by temperature-programmed desorption (TPD), work function measurements, as well as time- and angle-resolved two-photon photoemission (2PPE) spectroscopy. The TPD data show that COT films grow at 115 K in a metastable phase when the coverage is increased from the chemisorbed monolayer to the bulklike molecular multilayer structure. The metastable states desorb at a temperature which is ~ 9 K lower than the multilayer, followed by an irreversible and thermally activated transformation into a stable multilayer phase. This transition occurs at 165±2 K and is accompanied by a pronounced increase in the total electron yield in the 2PPE by more than one order of magnitude as well as the appearance of image potential states. The image states have binding energies of -0.70 eV and -0.24 eV for the n=1 and n=2 states, respectively, and a lifetime of 20±5 fs for both states. Their appearance is interpreted as an indication of island formation in the stable multilayer regime. 2PPE spectroscopy of the image potential states provides a sensitive probe of structural transitions in the adsorbate layers.
1 Current address: Institute of Optics, University of Rochester, NY 14627, USA
The publication is available at link doi:10.1016/j.susc.2005.04.027