Theoretical and experimental studies of band formation in CO adlayers

We have performed electronic-structure calculations using the extended tight-binding method for two-dimensional isolated layers of CO molecules corresponding to the ordered arrays formed on a Pd(100) surface. We find that the calculated width and shape of the $4\sigma$-derived band is in good agreement with angular-resolved photoemission results from this system. The calculated widths of the $5\sigma$- and $1\pi$-derived bands are much greater than that of the $4\sigma$-derived band, reflecting their more delocalized nature. In the photoemission experiment some dispersion is detected on the $5\sigma +1\mathrm{}\pi$ composite level but it was not possible to separate out the component bands.