Noble metal nanoclusters containing dozens to hundreds of metal atoms are of great interest because of their unique optical properties. Classical electrodynamics fails to describe the optical properties of clusters smaller than a few nanometers, so quantum-mechanical models are needed to describe these clusters. However, it is challenging to identify which features of the computed excited states indicate plasmon-like character, particularly in a way that is generalizable to the ligand-protected clusters that are commonly studied in experiments. We present an analytical method to identify plasmonlike excited states using three quantifiable indicators that must be considered in combination: (1) large superatomic character, (2) large collectivity among single-particle excitations, and (3) large additivity of contributions of these single-particle excitations to the transition dipole moment. Visualizing these three indicators on a single plot enables rapid classification of hundreds of excited states into plasmon-like, collective, single-particle, or interband categories or as intermediate between these categories. This method is used to identify excited states with plasmon-like character in both bare and ligand-protected Ag clusters at the TDDFT level. Using these three nearly orthogonal indicators in combination provides more information than any one criterion can provide.

The recent discovery of stable Ag nanoclusters presents new opportunities to understand the detailed electronic and optical properties of the metal core and the ligands using ultrafast spectroscopy. This paper focuses on Ag 32 and Ag 15 (with thiolate ligands), which are stable in solution. The steady state absorption spectra of Ag nanoclusters show interesting quantum size effects, expected for this size regime. Using a simple structural model for Ag 32 , TDDFT calculations show absorption at 480 nm and 680 nm that are in reasonable correspondence with experiments. Ag 32 (SG) 19 and Ag 15 (SG) 11 have quantum yields up to 2 orders of magnitude higher than Au nanoclusters of similar sizes, with an emission maximum at 650 nm, identified as the metal−ligand state. The emission from both Ag nanoclusters has a common lifetime of about 130 ps and a common energy transfer rate of K EET ≥ 9.7 × 10 9 s −1 . A "dark state" competing with the emission process was also observed and was found to be directly related to the difference in quantum yield (QY) for the two Ag clusters. Two-photon excited emission was observed for Ag 15 (SG) 11 , with a cross-section of 34 GM under 800 nm excitation. Femtosecond transient absorption measurements for Ag 32 recorded a possible metal core state at 530 nm, a metal−ligand state at 651 nm, and ground state bleaches at 485 and 600 nm. The ground state bleach signals in the transient spectrum for Ag 32 are 100 nm blue-shifted in comparison to Au 25 . The transient spectrum for Ag 15 shows a weak ground state bleach at ∼480 nm and a broad excited state centered at 610 nm. TDDFT calculations indicate that the electronic and optical properties of Ag nanoclusters can be divided into core states and metal− ligand states, and photoexcitation generally involves a ligand to metal core transition. Subsequent relaxation leaves the electron in a core state, but the hole can be either ligand or core-localized. This leads to emission/relaxation that is consistent with the observed photophysics.

We present a combined theoretical and experimental study of CO hydrogenation on a Ni(110) surface, including studies of the role of gas-phase atomic hydrogen, surface hydrogen, and subsurface hydrogen reacting with adsorbed CO. Reaction mechanisms leading both to methane and methanol are considered. In the reaction involving surface or subsurface hydrogen, we investigate four possible pathways, using density functional theory to characterize the relative energetics of each intermediate, including the importance of further hydrogenation versus C-O bond breaking, where the latter may lead to methane production. The most energetically favorable outcome is the production of methanol along a pathway involving the sequential hydrogenation of CO to a HCO* intermediate, followed by a final hydrogenation to give methanol. In addition, we find that subsurface hydrogen noticeably alters reaction barriers, both passively and through the energy released by diffusion to the surface. Indeed, the effective reaction barriers are even lower than for CO methanolation on Cu(211) and Cu(111) than for Ni(110). In studies of gas-phase H atoms impinging on a CO-adsorbed Ni(110) surface, Born-Oppenheimer molecular dynamics simulations show that direct impact of H is unlikely to result in hydrogenation of CO. This means that Eley-Rideal or hot-atom mechanisms are not important; thus, thermal reactions involving subsurface hydrogen are the primary reaction mechanisms leading to methanol. Finally, we demonstrate experimentally for the first time the production of methanol and formaldehyde from CO hydrogenation on Ni(110) and confirm the role of subsurface hydrogen in the mechanism of this reaction.

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The reactions of NO(y) species in the atmosphere with sea spray aerosol replace halogen anions with nitrate. These experiments show the effect of increasing the nitrate content of model sea spray aerosol particles on the morphology changes and the phase transitions driven by changes in relative humidity (RH). The components of the model particles include H2O, Na+, Mg2+, Cl-, NO3-, and SO4(2-). Tandem differential mobility analyzer (TDMA) measurements yield the water content and efflorescence relative humidity (ERH) of these particles, and probe molecule spectroscopic measurements reveal subsequent phase transitions and partially characterize the salt composition on the surface of dry particles. The results show three effects of increasing the nitrate composition: decreasing the EFH (46 to 29%), production of a metastable aqueous layer on the surface of effloresced particles, and decreasing the sulfate content near the surface of dry particles. For the mixtures studied here, the initial crystallization event forms a core of NaCl. For particles that contain a substantial metastable aqueous layer following efflorescence, probe molecule spectroscopy shows a second crystallization at a lower RH. This subsequent phase transition is likely the formation of Na2SO4. Homogeneous nucleation theory (HNT) using a semiempirical formulation predicts the ERH of all mixtures within 2.0% RH, with a mean absolute deviation of 1.0%. The calculations suggest that structures associated with highly concentrated or supersaturated magnesium ions strongly affect the interfacial tension between the NaCl crystal nucleus and the droplet from which it forms.