PHOTOINDUCED DESORPTION OF POTASSIUM ATOMS FROM A 2-DIMENSIONAL OVERLAYER ON GRAPHITE

We present an experimental and theoretical investigation of K atom des orption from the basal plane of graphite at 83 K induced by low energy photons (3-6 eV). The 2D potassium overlayer is characterized by low energy electron diffraction (LEED), high-resolution electron energy lo ss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and w ork function measurements. At monolayer coverage (5.2 x 10(14) atoms c m(-2)), the dependence of the cross section on photon energy has a thr eshold at (h) over bar omega approximate to 3.0 eV and rises up to a m aximum of 1.8 +/- 0.4 x 10(-20) cm(2) at 4.8 eV. The coverage dependen ce of the photoyield reflects the existence of two phases of adsorbed K, dilute ionized photo-active and close-packed photo-neutral, respect ively. The observed photodesorption is a single-photon, nonthermal eve nt, consistent with a substrate-mediated mechanism. The desorption res ults from attachment of optically excited hot electrons to the empty 4 s state of ionized potassium. The theory predicts in this case a Gauss ian line shape of the photoyield vs photon energy. Fitting the model p arameters to the experimental data, we determine (i) the energy and sl ope of the excited state potential energy curve, and (ii) the position and width of the potassium-induced 4s resonance. The present findings combined with other available data for potassium on graphite are used to construct 1D potential energy curves along the surface normal for K+ and K-0. The calculated cross sections for s- and p-polarized Light are in qualitative agreement with the measurements. (C) 1997 American Institute of Physics.