No abstract

The ultrafast laser excitation of matters leads to nonequilibrium states with complex solid-liquid phase-transition dynamics. We used electron diffraction at mega-electron volt energies to visualize the ultrafast melting of gold on the atomic scale length. For energy densities approaching the irreversible melting regime, we first observed heterogeneous melting on time scales of 100 to 1000 picoseconds, transitioning to homogeneous melting that occurs catastrophically within 10 to 20 picoseconds at higher energy densities. We showed evidence for the heterogeneous coexistence of solid and liquid. We determined the ion and electron temperature evolution and found superheated conditions. Our results constrain the electron-ion coupling rate, determine the Debye temperature, and reveal the melting sensitivity to nucleation seeds.

We describe experimental observations and theoretical analysis of the coarsening of distributions of twodimensional nanoclusters, either adatom islands or vacancy pits, on metal surfaces. A detailed analyses is provided for Ag(111) and Ag(100) surfaces, although we also discuss corresponding behavior for Cu(111) and Cu(100) surfaces. The dominant kinetic pathway for coarsening can be either Ostwald ripening (OR), i.e., growth of larger clusters at the expense of smaller ones, or Smoluchowski ripening (SR), i.e., diffusion and coalescence of clusters. First, for pristine additive-free surfaces, we elucidate the factors which control the dominant pathway. OR kinetics generally follows the predictions of mesoscale continuum theories. SR kinetics is controlled by the size-dependence of cluster diffusion. However, this size-dependence, together with that of nanostructure shape relaxation upon coalescence, often deviates from mesoscale predictions as a direct consequence of the nanoscale dimension of the clusters. Second, we describe examples for the above systems where trace amounts of a chemical additive lead to dramatic enhancement of coarsening. We focus on the scenario where "facile reaction" of metal and additive atoms leads to the formation of mobile additivemetal complexes which can efficiently transport metal across the surface, i.e., additive-enhanced OR. A suitable reaction-diffusion equation formulation is developed to describe this behavior. ReceiVed: July 19, 2008; ReVised Manuscript ReceiVed: December 25, 2008 We describe experimental observations and theoretical analysis of the coarsening of distributions of twodimensional nanoclusters, either adatom islands or vacancy pits, on metal surfaces. A detailed analyses is provided for Ag(111) and Ag(100) surfaces, although we also discuss corresponding behavior for Cu (111) and Cu(100) surfaces. The dominant kinetic pathway for coarsening can be either Ostwald ripening (OR), i.e., growth of larger clusters at the expense of smaller ones, or Smoluchowski ripening (SR), i.e., diffusion and coalescence of clusters. First, for pristine additive-free surfaces, we elucidate the factors which control the dominant pathway. OR kinetics generally follows the predictions of mesoscale continuum theories. SR kinetics is controlled by the size-dependence of cluster diffusion. However, this size-dependence, together with that of nanostructure shape relaxation upon coalescence, often deviates from mesoscale predictions as a direct consequence of the nanoscale dimension of the clusters. Second, we describe examples for the above systems where trace amounts of a chemical additive lead to dramatic enhancement of coarsening. We focus on the scenario where "facile reaction" of metal and additive atoms leads to the formation of mobile additivemetal complexes which can efficiently transport metal across the surface, i.e., additive-enhanced OR. A suitable reaction-diffusion equation formulation is developed to describe this behavior.

Photochemistry of the molecularly and dissociatively adsorbed forms of methanol on the vacuum-annealed rutile TiO 2 (110) surface was explored using temperature-programmed desorption (TPD) both with and without coadsorbed water. Methoxy, and not methanol, was confirmed as the photochemically active form of adsorbed methanol on this surface. UV irradiation of methoxy-covered TiO 2 (110) led to depletion of the methoxy coverage and to formation of formaldehyde and a surface OH group. Coadsorbed water did not promote either molecular methanol photochemistry or thermal decomposition of methanol to methoxy. However, terminal OH groups (OH t ), prepared by coadsorption of water and oxygen atoms, thermally converted molecularly adsorbed methanol to methoxy at 120 K thus enabling photoactivity. While chemisorbed water molecules had no influence on methoxy photochemistry, water molecules hydrogen-bonded in the second layer to bridging oxygen (O br ) sites inhibited methoxy photodecomposition to formaldehyde. From this, we conclude that O br sites accept protons from methoxy to form formaldehyde. These results provide new fundamental understanding into the hole-scavenging role of methanol in photochemical processes on TiO 2 -based materials and into how water influences this photochemistry.

The photoactivity of methanol on the rutile TiO2(110) surface is shown to depend on the ability of methanol to diffuse on the surface and find sites active for its thermal dissociation to methoxy and hydroxy species. Temperature programmed desorption (TPD) results show that the extent of methanol photodecomposition to formaldehyde is negligible on the clean TiO2(110) surface at 100 K due to a scarcity of sites that can convert (photoinactive) methanol to (photoactive) methoxy. The extent of photoactivity at 100 K significantly increases when methanol is coadsorbed with oxygen, however only those molecules able to adsorb near (next to) a coadsorbed oxygen species are active. Preannealing coadsorbed methanol and oxygen to above 200 K prior to UV irradiation results in a significant increase in photoactivity. Scanning tunneling microscopy (STM) images clearly show that the advent of increased photoactivity in TPD correlates with the onset of methanol diffusion along the surface’s Ti4+ rows at ∼200 K. These results demonstrate that optimizing thermal processes (such as diffusion or proton transfer reactions) can be critical to maximizing photocatalytic reactivity on TiO2 surfaces.

We have examined the chemical and photochemical properties of molecular oxygen on the (110) surface of rutile TiO 2 at 100 K using electron energy loss spectroscopy (EELS), photon stimulated desorption (PSD), and scanning tunneling microscopy (STM). Oxygen chemisorbs on the TiO 2 (110) surface at 100 K through charge transfer from surface Ti 3+ sites. The charge-transfer process is evident in EELS by a decrease in the intensity of the Ti 3+ d-to-d transition at ∼0.9 eV and formation of a new loss at ∼2.8 eV. On the basis of comparisons with the available homogeneous and heterogeneous literature for complexed/adsorbed O 2 , the species responsible for the 2.8 eV peak can be assigned to a surface peroxo (O 2 2− ) state of O 2 . This species was identified as the active form of adsorbed O 2 on TiO 2 (110) for PSD. The adsorption site of this peroxo species was assigned to that of a regular five-coordinated Ti 4+ (Ti 5c ) site based on comparisons between the UV exposure-dependent behavior of O 2 in STM, PSD, and EELS data. Assignment of the active form of adsorbed O 2 to a peroxo species at normal Ti 5c sites necessitates reevaluation of the simple mechanism in which a single valence band hole neutralizes a singly charged O 2 species (superoxo or O 2 − ), leading to desorption of O 2 from a physisorbed potential energy surface.

Coarsening (i.e., ripening) of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals (Cu, Ag, Au), both clean and with an adsorbed chalcogen (O, S) present. For the clean surfaces, three aspects are summarized: (1) the balance between the two major mechanisms-Ostwald ripening (the most commonly anticipated mechanism) and Smoluchowski ripening-and how that balance depends on island size; (2) the nature of the mass transport agents, which are metal adatoms in almost all known cases; and (3) the dependence of the ripening kinetics on surface crystallography. Ripening rates are in the order (110)>(111)>(100), a feature that can be rationalized in terms of the energetics of key processes. This discussion of behavior on the clean surfaces establishes a background for understanding why coarsening can be accelerated by adsorbates. Evidence that O and S accelerate mass transport on Ag, Cu, and Au surfaces is then reviewed. The most detailed information is available for two specific systems, S/Ag (111) and S/Cu(111). Here, metal-chalcogen clusters are clearly responsible for accelerated coarsening. This conclusion rests partly on deductive reasoning, partly on calculations of key energetic quantities for the clusters (compared with quantities for the clean surfaces), and partly on direct experimental observations. In these two systems, it appears that the adsorbate, S, must first decorate-and, in fact, saturate-the edges of metal islands and steps, and then build up at least slightly in coverage on the terraces before acceleration begins. Acceleration can occur at coverages as low as a few thousandths to a few hundredths of a monolayer. Despite the significant recent advances in our understanding of these systems, many open questions remain. Among them is the identification of the agents of mass transport on crystallographically different surfaces e.g., 111, 110, and 100. KeywordsAmes Laboratory, Materials Science and Engineering, Mathematics, Physics and Astronomy Disciplines Biological and Chemical Physics | Materials Science and Engineering | Mathematics | Physical Chemistry CommentsThe following article appeared in Journal of Vacuum Science and Technology A 28, no. 6 (2010) Coarsening ͑i.e., ripening͒ of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals ͑Cu, Ag, Au͒, both clean and with an adsorbed chalcogen ͑O, S͒ present. For the clean surfaces, three aspects are summarized: ͑1͒ the balance between the two major mechanisms-Ostwald ripening ͑the most commonly anticipated mechanism͒ and Smoluchowski ripening-and how that balance depends on island size; ͑2͒ the nature of the mass transport agents, which are metal adatoms in almost all known cases; and ͑3͒ the dependence of the ripe...

A well-ordered, self-organized dot-row structure appears after adsorption of S on Ag(111) at 200 K. This dotrow motif, which exhibits fixed spacing between dots within rows, is present over a wide range of coverage. The dots are probably Ag 3 S 3 clusters with adsorbed S in the spaces between dots. Dynamic rearrangements are observed. Small domains of aligned dot-rows form during adsorption and grow quickly after adsorption ends. The domains also exhibit large equilibrium fluctuations after adsorption. The dot-row structure disappears reversibly upon heating above 200 K and transforms reversibly to an "elongated island" structure upon cooling below 200 K. DFT supports the assignment of the dots as Ag 3 S 3 trimers and also lends insight into the possible origins of other structures observed in this complex system. KeywordsAg (111) ReceiVed: NoVember 9, 2007; In Final Form: January 8, 2008 A well-ordered, self-organized dot-row structure appears after adsorption of S on Ag(111) at 200 K. This dot-row motif, which exhibits fixed spacing between dots within rows, is present over a wide range of coverage. The dots are probably Ag 3 S 3 clusters with adsorbed S in the spaces between dots. Dynamic rearrangements are observed. Small domains of aligned dot-rows form during adsorption and grow quickly after adsorption ends. The domains also exhibit large equilibrium fluctuations after adsorption. The dot-row structure disappears reversibly upon heating above 200 K and transforms reversibly to an "elongated island" structure upon cooling below 200 K. DFT supports the assignment of the dots as Ag 3 S 3 trimers and also lends insight into the possible origins of other structures observed in this complex system.