Photoemission from Ordered Physisorbed Adsorbate Phases:N2on Graphite and CO on Ag(111)

The measured two-dimensional band structure of physisorbed molecular monolayers has been used to determine the structure of the ordered phase. For the previously reported ordered herringbone structure of ${\mathrm{N}}_2$ on graphite we observe a $k$-dependent splitting in the $3{\sigma }_g$ level which is a consequence of the intermolecular interaction of the two molecules per unit cell. Our measured dispersion is consistent with the structure proposed by recent LEED calculations and with a semi-empirical tight-binding calculation. For CO on Ag(111), which has not been studied by LEED, we find an orientationally ordered phase where there are two molecules per unit cell with the CO lying parallel to the surface.