Abstract
For the first time physisorbed non dissociated NH 3 molecules and chemisorbed partially dissociated NH 2 fragments, adsorbed at low temperature on Si(001)-2 × 1 surfaces are clearly differentiated by UPS (ultraviolet photoemission spectroscopy) HeII 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 NH 3 molecules prevails and 300 K where only that of NH 2 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 N 1s core level signals in relation with the two species.
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