Photochemistry on ultrathin metal films

Enhanced photodesorption cross sections are observed for molecules, such as NO2 and SO2, adsorbed onto ultrathin Ag films supported on Si(1 0 0) substrates. For 266 nm light excitation (hν = 4.7 eV) the cross section begins to increase when the film thickness becomes smaller than 20 nm and is by a factor of 3-4 larger at 5 nm film thickness when compared to a Ag(1 1 1) crystal as substrate. The effect is somewhat more pronounced for SO2 than for NO2 albeit the general feature is similar. Time-of-flight spectra of the photodesorbed molecules suggest that the dynamics of the photodesorption process is independent of the film thickness as the velocities of the fragments are independent of the latter. The data suggests that the quantum efficiency increases as the Ag film thickness is decreased by an order of magnitude. A similar effect is observed for 355 nm (hν = 3.5 eV) photo excitation. The increased photodesorption cross section is evidence for photon absorption in the Si substrate producing electrons with no or little momenta parallel to the surface at energies where this is not allowed in Ag(1 1 1).