Femtosecond-laser photoemission and photodesorption from magnesia supported gold clusters

Nanosize gold clusters were grown by thermal evaporation on ultrathin magnesia films on Mo(100) and investigated by femtosecond (fs)-laser photoemission spectroscopy. The surface work function in this system was found to increase with increasing nominal gold coverage. Methyl bromide (CH3Br) molecules adsorb molecularly on the magnesia surface as well as on the gold clusters. At sub-monolayer coverage the CH3Br molecules were, however, considerably stronger bound to the gold particles than to the magnesia surface as revealed by temperature programmed desorption spectroscopy. To investigate the photoreaction dynamics of the adsorbate molecules fs-laser pump-probe mass spectrometry was applied. In this new approach the methyl fragment appearance was monitored after photoexcitation of the adsorbed CH3Br with fs time resolution. The transient methyl data revealed that the CH3Br dissociation and subsequent methyl desorption proceeded in less than one picosecond. Most interestingly, a part of the transient signal could be unambiguously assigned to the dynamics of methyl desorption from the gold clusters. The appearance of this signal at about 270 fs after photoexcitation illustrates the massive distortion of the CH3Br electronic level structure due to the strong interaction with the gold clusters compared to the bare magnesia surface. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim