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The dynamics of cesium atom motion above the copper(111) surface following electronic excitation with light was studied with femtosecond (10(-15) seconds) time resolution. Unusual changes in the surface electronic structure within 160 femtoseconds after excitation, observed by time-resolved two-photon photoemission spectroscopy, are attributed to atomic motion in a copper-cesium bond-breaking process. Describing the change in energy of the cesium antibonding state with a simple classical model provides information on the mechanical forces acting on cesium atoms that are "turned on" by photoexcitation. Within 160 femtoseconds, the copper-cesium bond extends by 0.35 angstrom from its equilibrium value.

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Electron dynamics induced by direct photoexcitation of the antibonding state on the Cs͞Cu(111) surface are studied by interferometric time-resolved two-photon photoemission. Femtosecond resolution pump-probe correlation measurements indicate phase and energy decay, respectively, on 15 and 50 fs time scales for the Cs antibonding state at 33 K. Both the polarization and phase dynamics are nonexponential and strongly temperature dependent. The energy dependence of the antibonding state population dynamics is consistent with Cs photodesorption. [S0031-9007(99)08606-8] PACS numbers: 73.50.Gr, 78.40.Kc, Energy and phase relaxation times of electronically excited states of atoms and molecules adsorbed on solid surfaces are important for many physical phenomena including surface scattering, photon and electron stimulated desorption, surface sputtering, and surface photochemistry [1]. According to the Menzel-Gomer-Redhead (MGR) model [2,3], photodesorption occurs when an adsorbate is excited to a dissociative antibonding or affinity state, if the nuclear motion along the reaction coordinate can compete with the electronic deexcitation. Typically, electronic quenching of adsorbates on metal surfaces occurs on a ø10 fs time scale [4][5][6], which has prevented direct, real-time observation of surface photochemical reactions.Alkali atom-metal surface chemisorption is a model system of practical importance in thermionic emission and promotion or poisoning in catalysis [7]. Near a metal surface, alkali atoms experience a repulsive interaction of the valence electron with the induced image charge of the ionic nuclear core. As the atom-surface distance is decreased, the valence electrons shift upward in energy and hybridize with those of the substrate into bonding and antibonding states [8][9][10]. Nordlander and Tully have calculated orbital-dependent energies and electron transfer rates of #2 fs for Cs at 3.01 Å (Cu-Cs bond length [11]) above a jellium surface [8]. In agreement with theory, an unoccupied antibonding state having a linewidth G 350 meV at 300 K, which implies a lifetime t of 2 fs according to the Heisenberg relation Gt h, has been observed in inverse and two-photon photoemission (2PP) spectra of alkali metals on copper [12][13][14]. However, in a recent study of Cs͞Cu(111), Bauer et al. measured a significantly longer lifetime of 11 fs by time-resolved photoemission [14], implying that G has significant contributions from inhomogeneous broadening and/or quasielastic [e.g., electron-phonon ͑e-p͒] scattering. To address this contradiction and to gain a deeper understanding of the bonding, and electronic relaxation of alkali atoms on metal surfaces, it is necessary to determine both the phase and energy relaxation times of the antibonding state on Cs͞Cu(111). In this Letter, the electronic relaxation is measured by the interferometric time-resolved two-photon photoemission (ITR-2PP) technique [15,16], thus providing a direct method to study the MGR photodesorption.The ITR-2PP experiment and data evaluation method ...

The fine particulate matter (PM2.5) problem has attracted much scientific and public attention, due to its effects on visibility, human health, and global climate. There are three factors that have important effect on PM2.5 mass concentration: domestic pollutant emission sources, external sources outside of the country, and the meteorological conditions. Nagasaki is a coastal prefecture located at the westernmost part of Japan, which is an ideal location to study pollutants from long range transport and correlation between PM2.5 and meteorological conditions. In this paper, PM2.5 concentration data and meteorological data were obtained during 1 January 2013~31 December 2013. The spatial distribution depicts that the western part of the study area has the most serious PM2.5 pollution. The correlation analysis results between PM2.5 concentration and meteorological data showed that temperature had a negative, and precipitation had a positive, correlation with PM2.5. There was a threshold in the correlations between humidity and wind speed and PM2.5. The correlation was positive or negative depending on the meteorological variable values, if these were lower or higher than the threshold. From the relationship with wind direction, it can be depicted that the west wind might bring the most pollutants to Nagasaki.