UPS DIFFERENTIATION BETWEEN MOLECULAR NH3 AND PARTIALLY DISSOCIATED NH2 FRAGMENTS ADSORBED AT LOW-TEMPERATURE ON SI(001) SURFACES

For the first time physisorbed non dissociated NH3 Molecules and chemisorbed partially dissociated NH, fragments, adsorbed at low temperature on Si(001)-2 x 1 surfaces are clearly differentiated by UPS (ultraviolet photoemission spectroscopy) He II spectra. This result is obtained by a careful scanning of the relevant UPS features as a function of T(s) between 80 K where the contribution of the condensed NH3 molecules prevails and 300 K where only that of NH2 is prominent. Actually, at 80 K, for sufficiently high exposures, condensed molecules mainly overlay a weak coverage of dissociated species capping themselves some of the Si electronic surface states. The passage at 120 K enables us to observe simultaneously the two species, providing an indubitable proof for their differentiation. This assignment is ascertained by correlated binding energy variations of the N1s core level signals in relation with the two species.