Two-photon photoemission spectroscopy: naphthalene on Cu(1 1 1)

Two-photon photoemission (2PPE) spectroscopy is used to investigate the electronic structure of naphthalene dosed on Cu(1 1 1) at 86 K. The three transitions involving unoccupied orbitals were found and identified as having π∗ molecular orbital character––the first lying 0.40 eV above the vacuum level and identified as having π∗b1u character; the second 0.30 eV below the vacuum level and having π∗b3g character; and the third lying 1.10 eV below the vacuum level with π∗b2g character. The results accord with both inverse photoemission spectroscopy (IPES) and electron transmission spectroscopy (ETS) data. There are interesting relatively unique features in the temperature programmed desorption (TPD) spectra indicative of structural reorganization of as-dosed naphthalene. Thermal processing also has a marked influence on 2PPE intensities and is taken as reflecting changes, with respect to the Cu(1 1 1) surface, of the ensemble average orientation of the naphthalene overlayer.