H/D isotopic exchange between H(2)O and D(2)O molecules was studied at the surface of ice films at 90-140 K by the technique of Cs(+) reactive ion scattering. Ice films were deposited on a Ru(0001) substrate in different compositions of H(2)O and D(2)O and in various structures to study the kinetics of isotopic exchange. H/D exchange was very slow on an ice film at 95-100 K, even when H(2)O and D(2)O were uniformly mixed in the film. At 140 K, H/D exchange occurred in a time scale of several minutes on the uniform mixture film. Kinetic measurement gave the rate coefficient for the exchange reaction, k(140 K)=1.6(+/-0.3) x 10(-19) cm(2) molecule(-1) s(-1) and k(100 K)< or =5.7(+/-0.5) x 10(-21) cm(2) molecule(-1) s(-1) and the Arrhenius activation energy, E(a)> or =9.8 kJ mol(-1). Addition of HCl on the film to provide excess protons greatly accelerated the isotopic exchange reaction such that it went to completion very quickly at the surface. The rapid reaction, however, was confined within the first bilayer (BL) of the surface and did not readily propagate to the underlying sublayer. The isotopic exchange in the vertical direction was almost completely blocked at 95 K, and it slowly occurred only to a depth of 3 BLs from the surface at 140 K. Thus, the proton transfer was highly directional. The lateral proton transfer at the surface was attributed to the increased mobility of protonic defects at the molecularly disordered and activated surface. The slow, vertical proton transfer was probably assisted by self-diffusion of water molecules.

Distribution of electrolyte ions near the surface of water or ice is a subject of interest in both fundamental chemistry and atmospheric chemistry of sea-salt aerosols and ice particles. The ion distribution at the surface of these particles may affect their reaction with ozone and organic species in the troposphere and the generation of reactive halogen species. [1,2] In early studies, the surface of aqueous solutions containing simple electrolytes such as alkali halides was thought to be deficient of ions, as inferred from the surface tension of solutions.[3] Molecular dynamics (MD) simulations have been widely employed since the 1990s to study the molecular details of solvation and segregation of atomic ions in water clusters.[4-8] MD calculations [6][7][8] predict that the large and polarizable anions are more readily available at the surface than the small nonpolarizable cations, in both water slab models [8] and clusters of finite sizes. [6,7,9] Photoelectron spectroscopic studies [10] of anionic water clusters in the gas phase support the surface residence of the larger halide anions by comparing the ionization energies with calculations. Further studies of the anionic clusters [X À (H 2 O) 1-5 , X= F, Cl, Br and I] using vibrational spectroscopy [11][12][13] indicate that the larger halides (Cl À , Br À and I À ) are solvated at the surface of water clusters. The spectra from clusters with the larger halides reveal a hydrogen-bonding network of water molecules, which supports surface solvation of anions, whereas the spectra from the F À -containing clusters lack such hydrogen-bonding features. [12b, 13] A limited number of experimental studies were performed to investigate the ion distribution at the surface of real aqueous solutions, and basically none at ice surfaces. Morgner and co-workers [14] measured He(I) photoelectron spectra from the surface of concentrated aqueous solutions of CsF and observed that the salt concentration is strongly depleted in the surface region. MD simulations from the same group [15] explain the phenomena by the circumstance that the ions near the surface mostly keep their first solvation shell intact. Using Xray photoelectron spectroscopy and scanning electron microscopy, Ghosal et al. [16] observed selective segregation of Br À ions to the surface of a NaCl crystal, which was slightly doped with Br À ions, under conditions of relative humidity, where the condensed water films caused partial dissolution of the crystal surface. Their results provide the first experimental evidence for preferential segregation of large halide anions to the surface of mixed alkali halides. More recently, vibrational sum frequency generation spectroscopy for sodium halide solutions [17] [a] J

We studied diffusion of water molecules in the direction perpendicular to the surface of an ice film. Amorphous ice films of H(2)O were deposited on Ru(0001) at temperature of 100-140 K for thickness of 1-5 bilayer (BL) in vacuum, and a fractional coverage of D(2)O was added onto the surface. Vertical migration of surface D(2)O molecules to the underlying H(2)O multilayer and the reverse migration of H(2)O resulted in change of their surface concentrations. Temporal variation of the H(2)O and D(2)O surface concentrations was monitored by the technique of Cs(+) reactive ion scattering to reveal kinetics of the vertical diffusion in depth resolution of 1 BL. The first-order rate coefficient for the migration of surface water molecules ranged from k(1)=5.7(+/-0.6) x 10(-4) s(-1) at T=100 K to k(1)=6.7(+/-2.0) x 10(-2) s(-1) at 140 K, with an activation energy of 13.7+/-1.7 kJ mol(-1). The equivalent surface diffusion coefficients were D(s)=7 x 10(-19) cm(2) s(-1) at 100 K and D(s)=8 x 10(-17) cm(2) s(-1) at 140 K. The measured activation energy was close to interstitial migration energy (15 kJ mol(-1)) and was much lower than diffusion activation energy in bulk ice (52-70 kJ mol(-1)). The result suggested that water molecules diffused via the interstitial mechanism near the surface where defect concentrations were very high.

We report on a temperature-dependent band gap property of epitaxial MoSe2 ultrathin films. We prepare uniform MoSe2 films epitaxially grown on graphenized SiC substrates with controlled thicknesses by molecular beam epitaxy. Spectroscopic ellipsometry measurements upon heating sample in ultra-high vacuum showed temperature-dependent optical spectra between room temperature to 850 °C. We observed a gradual energy shift of optical band gap depending on the measurement temperature for different film thicknesses. Fitting with the vibronic model of Huang and Rhys indicates that the constant thermal expansion accounts for the steady decrease of band gap. We also directly probe both optical and stoichiometric changes across the decomposition temperature, which should be useful for developing high-temperature electronic devices and fabrication process with the similar metal chalcogenide films.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-017-2266-7) contains supplementary material, which is available to authorized users.