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Rotational energy transfer (RET) by Ar collisions within the v′ = 0 level of the SH A2Σ+ state is probed using a laser-induced dispersed fluorescence technique, following photodissociation of H2S at 248 nm. The Ar pressure is adjusted appropriately to allow for significant observation of the single-collision induced RET process. The spin-resolved and spin-averaged rate constants are then evaluated with the aid of a kinetic model under single-collision conditions. The theoretical counterparts are calculated using a quantum scattering method, in which a newly fitted potential energy function is based on ab initio potential energy surface reported previously. The experimental and theoretical kinetic data are essentially consistent in the trend of N and ΔN dependence. Several propensity rules are found in the RET collisions. For instance, for ΔN = 1, 2, and 3, the rate constants decrease with increasing N or ΔN. Given a fixed ΔN, the rate constants of the same initial N in the downward transition appear to be larger than those in the upward transitions. In ΔN = 0, the F2 → F1 transitions prevail over the F 1 → F2 transitions (F1 = N + 1/2, F 2 = N - 1/2), whereas in ΔN ≠ 0, the fine-structure- conserving collisions are more favored than the fine-structure-changing collisions. The principle of microscopic reversibility is also examined for both experimental and theoretical kinetic data, showing that translational energies of the RET collisions are close to thermal equilibrium at room temperature. The propensity rules may be rationalized according to this principle. © 2010 the Owner Societies.

Evanescent-wave cavity ring-down spectroscopy is applied to investigate the adsorption behavior of rhodamine B at three different interfaces. The adsorption equilibrium constant (Kads) and adsorption free energy of rhodamine B at the silica/methanol interface are determined to be (1.5 ± 0.2) × 104 M-1 and -23.8 ± 0.4 kJ/mol by use of a Langmuir isotherm model. A Langmuir-based kinetic model is also developed to determine the corresponding adsorption and desorption rate constants of (1.02 ± 0.03) × 102 M-1 s-1 and (7.1 ± 0.2) × 10-3 s-1, from which Kads is obtained to be (1.45 ± 0.09) × 104 M-1, in agreement with the value determined under equilibrium conditions. Similarly, when rhodamine B is at the chlorotrimethylsilane-immobilized silica/methanol interface, the adsorption and desorption rate constants are determined to be (1.7 ± 0.2) × 102 M-1 s-1 and (5.0 ± 1.0) × 10-3 s-1· The subsequent Kads is (3.6 ± 0.4) × 104 M-1, which is larger than that at the silica/methanol interface. The former adsorption is dominated by hydrophobic interaction, while the latter is subject to electrostatic attraction. When rhodamine B is at the silica/water interface, there exist three chemical forms, including zwitterion (R+B -), cation (RBH+), and lactone (RBL). A combination of double-layer and Langmuir competitive models is used to fit the adsorption isotherm as a function of solution pH, yielding Kads of (2.5 ± 0.2) × 104 M-1 and (1.1 ± 0.2) × 105 M-1 for R+B- and RBH +, respectively. RBL is considered to have the same Kads value as R+B-. © 2010 American Chemical Society.

The orientation dependence for the Br atom formation in the reaction of the oriented OH radicals with HBr molecules at 0.26 eV collision energy has been observed for the first time using the hexapole electric field, and we found that the reaction cross-section for O-end attack is more favorable than that for H-end attack by a factor of 3.4 ± 2.3. © the Owner Societies.

In one-photon dissociation of gaseous acetyl chloride at 248 nm, time-resolved Fourier-transform infrared emission spectroscopy is used to detect the fragments of HCl, CO, and CH2 in the presence of Ar or O 2. The high-resolution spectra of HCl and CO are analyzed to yield the corresponding internal energy deposition of 8.9 ± 1.1 and 6.2 ± 0.9 kcal/mol. The presence of the CH2 fragment is verified by detecting the CO2 product resulting from the reaction of CH 2 and the added O2. The probability of the HCl formation via a hot Cl reaction with the precursor is examined to be negligible by performing two experiments, the CH3COCl pressure dependence and the measurement of Br2 with Cl reaction. The HCl elimination channel under the Ar addition is verified to be slowed by 2 orders of magnitude, as compared to the Cl elimination channel. The observed fragments are proposed to dissociate on the hot ground electronic state via collision-induced internal conversion. A two-body dissociation channel is favored leading to HCl and CH2CO, followed by secondary dissociation. © 2010 American Chemical Society.

In one-photon dissociation of propionyl chloride at 248 nm, time-resolved Fourier-transform infrared emission spectroscopy is used to detect the fragments of HCl and CO in the presence of Ar. The inert gas Ar plays a role to enhance the internal conversion. The time-dependence of high-resolution HCl spectra yields a bimodal rotational distribution in the early stage. The total rotational and vibrational energy partitioned in HCl are evaluated to be 1.7 ± 0.3 and 8.8 ± 1.9 kcal/mol, respectively. The CO appearance indicates that HCl may be eliminated through a five-center mechanism accompanied with three-body dissociation of C2H2, HCl, and CO. A four-center mechanism forming HCl and CH3CHCO also contributes to the HCl fragment with a feature of rotational bimodality. However, the probability for the HCl contribution from the hot Cl reaction is negligible. The reaction with CH4 is carried out to evaluate the HCl and Cl elimination rate constants. © 2010 Elsevier B.V. All rights reserved.

Kinetics of photoinduced electron transfer (ET) from oxazine 1 dye to TiO2 nanoparticles (NPs) surface is studied at a single molecule level by using confocal fluorescence microscopy. Upon irradiation with a pulsed laser at 630 nm, the fluorescence lifetimes sampled among 100 different dye molecules are determined to yield an average lifetime of 2.9 ± 0.3 ns, which is close to the value of 3.0 ± 0.6 ns measured on the bare coverslip. The lifetime proximity suggests that most interfacial electron transfer (IFET) processes for the current system are inefficient, probably caused by physisorption between dye and the TiO2 film. However, there might exist some molecules which are quenched before fluorescing and fail to be detected. With the aid of autocorrelation analysis under a three-level energy system, the IFET kinetics of single dye molecules in the conduction band of TiO2 NPs is evaluated to be (1.0 ± 0.1)×104 s-1 averaged over 100 single molecules and the back ET rate constant is 4.7 ± 0.9 s-1. When a thicker TiO2 film is substituted, the resultant kinetic data do not make a significant difference. The trend of IFET efficacy agrees with the method of fluorescence lifetime measurements. The obtained forward ET rate constants are about ten times smaller than the photovoltage response measured in an assembled dye-sensitized solar cell. The discrepancy is discussed. The inhomogeneous and fluctuation characters for the IFET process are attributed to microenvironment variation for each single molecule. The obtained ET rates are much slower than the fluorescence relaxation. Such a small ET quantum yield is yet feasibly detectable at a single molecule level. © 2010 American Chemical Society.

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The clean-off reaction of AgO added rows by CO on Ag(110) and Au/Ag(110) bimetallic surfaces was studied by scanning tunneling microscopy (STM) and compared with density functional theory (DFT). This combined study of a model system illustrated the complexity of catalytic enhancement in bimetallic systems. By analyzing in situ time-lapsed STM image series, we found that CO oxidation on a Au-enriched Ag(110) surface leads to an exponential depletion of oxygen with time and a reaction rate that is synergistically enhanced by the presence of Au. First principles calculations indicate that the local atomic configuration around the active reaction sites at the chain ends and the preference of An atom substitution into the subsurface second Ag layer are of critical importance. By calculating CO adsorption energies and reaction barriers for plausible reaction pathways, a detailed description of the CO oxidation reaction emerges, For the optimal reaction pathway, a large (similar to 0.09 eV) barrier reduction and a small barrier of similar to 0.01 eV were found for the Eley-Rideal (ER) mechanism. In contrast, a small (similar to 0.03 eV) barrier reduction and a moderate barrier of similar to 0.23 eV were obtained for the Langmuir-Hinshelwood (LH) mechanism. The ER transitional state was also found to be lower in energy. We conclude that, irrespective of whether the ER mechanism is actually rate dominating, it is definitively enhanced.

Whether photoemission probes surface or bulk properties has long been a topic of interest and debate. This work employs angle-resolved photoemission to map the electronic structure of Ag films of varying thicknesses prepared on Si(111). As expected, the discrete quantum-well states or subbands observed at small thicknesses merge into a continuum as the film thickness approaches the bulk limit. However, a number of discrete states remain isolated within gaps or pockets in the bulk continuum. While these Ag surface states have been predicted previously by calculations, most are experimentally identified herein only for the first time. Copyright (C) EPLA, 2009