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Photodissociation of the group VIB (Cr,Mo,W) hexacarbonyls has been studied at 248 nm using molecular beam photofragment spectroscopy. One, two and three photon processes have been observed. Analysis of the product velocity distributions shows that the photodissociation mechanism consists of sequential CO eliminations with the nth photon channel best described as the single photon photodissociation of the stable products of the n-1st photon channel. The product translational energy distribution for the first CO elimination step is quantitatively similar for all three hexacarbonyls and characteristic of a repulsive translational energy release. The product translational energy distributions of all subsequent CO elimination steps are accurately described by a simple, microcanonical model. Qualitative molecular orbital considerations suggest that the large product translational energy observed in the first CO elimination step results from a repulsive σ interaction between the closed shell CO ligand and an excited molecular orbital which has a significant admixture of metal (n+1)pz, (n+1)s and ndz2 orbitals. This repulsive interaction is absent in the remaining CO elimination steps because there are vacancies in the coordination shell along the z axis. [ABSTRACT FROM AUTHOR]

A molecular beam of zinc–diethyl (ZnEt2) is photodissociated at 248 and 193 nm and the velocity distributions of the photofragments are measured by time-of-flight techniques. One and two photon processes are observed. The dominant one photon process at both wavelengths is the dissociation of (ZnEt2)2 to form two ZnEt2 monomers. The absence of secondary dissociation of the ZnEt2 photofragments at both excitation wavelengths and the small fraction of the available energy partitioned to product translation implicates dissociation to an excited electronic potential energy surface correlating to one electronically excited ZnEt[ATOTHER]@B|[/ATOTHER]2 monomer. The mass spectrum of the ZnEt2 photofragments is the same as measured for ‘‘cold’’ ZnEt2 monomers in the molecular beam, suggesting that the electronically excited ZnEt[ATOTHER]@B|[/ATOTHER]2 monomers have fluoresced prior to ionization in the mass spectrometer. A small photodissociation signal of uncomplexed ZnEt2 is observed only at low expansion pressures. The sensitive dependence of this monomeric photodissociation signal to the Ar pressure of the adiabatic expansion suggests that ground state vibrational excitation is required for monomeric photodissociation at 248 nm.In contrast to the dimer single photon photodissocation channel, when ZnEt2 monomers are photodissociated, a significant fraction of the available energy appears as product translation. A qualitative molecular orbital analysis can explain the observed fast photoproduct velocity if dissociation occurs via a repulsive triplet state which correlates to electronic ground state products. The two photon process observed is assigned to single photon photodissociation of the electronically excited ZnEt[ATOTHER]@B|[/ATOTHER]2 monomers produced in the dimer photodissociation step. The photofragment velocity distributions for the two photon channel can be quantitatively modeled by sequential ethyl eliminations on... [ABSTRACT FROM AUTHOR]

A crossed laser-molecular beam study of the one and two photon dissociation mechanism of bis (cyclopentadienyl) iron (ferrocene, FeCp2) has been performed at 193 and 248 nm. By combining electron bombardment mass spectroscopy with time-of-flight (TOF) measurements, the photodissociation mechanism at 193 nm is shown to have two distinct mechanisms. (1) FeCp2+hν→FeCp*+Cp; (2) FeCp+2hν→FeCp+Cp, FeCp→Fe+Cp. For the first mechanism, which accounts for less than 5% of the photodissociation events, the FeCp* velocity distribution is quantitatively consistent with a statistical dissociation producing FeCp in an excited, ligand field electronic state. The velocity distributions of the Cp and Fe fragments produced by the second mechanism (FeCp is an unstable intermediate) are also in excellent agreement with microcanonical calculations for both Cp elimination steps using the known metal–ligand bond energies of ferrocene. For the second mechanism, dissociation occurs on the lowest potential energy surface for each Cp elimination. Although one photon is energetically sufficient to remove one Cp ligand from ferrocene, RRKM calculations of the lifetime indicate that Cp elimination is extremely slow for dissociation along the ground electronic state potential energy surface. Hence, after internal conversion to the ground electronic state, the large photon absorption cross section (∼4 Å2) for the experimental irradiation conditions allows additional photons to be absorbed until the dissociation rate exceeds the up pumping rate. The large photon energy causes the dissociation rate to increase by many orders of magnitude for each additional photon absorbed. Consequently, there is strong selectivity for the total number of photons absorbed. Both mechanisms, occurring on two different electronic potential energy surfaces, suggest that dissociation induced by excitation of the ligand-to-metal charge transfer states accessed at 193... [ABSTRACT FROM AUTHOR]

A combined plasma photolysis (CPP) reactor that utilizes a dielectric barrier discharge (DBD) plasma and 207nm UV radiation from discharge-driven KrBr∗ excimers was designed and constructed. Gas streams containing CS2 were treated with stand-alone DBD and CPP at atmospheric pressure. In comparison to DBD, CPP greatly enhanced the removal efficiency at the same applied voltage, waste gas concentration and gas residence time. Thus the applied voltage could be reduced to a certain extent in the plasma processing of industrial wastes. Influences of the KrBr∗ radiation, inlet CS2 concentration and gas residence time on CS2 removal by CPP were also studied. In addition, the likely reaction mechanisms for the removal of CS2 by CPP are suggested based on the byproducts analysis. The enhanced removal efficiency and reaction mechanisms implied that the CPP process would probably be suitable for the removal of a large number of gaseous pollutants. [Copyright &y& Elsevier]

A molecular beam study of the Cl2+GaAs(s) reaction has been performed for surface temperatures in the range of 300–550 K. The gas phase neutral reaction products are identified by mass spectroscopy using electron bombardment ionization. Detailed analysis of the surface temperature dependence of the mass spectrum of the observed reaction products indicates that only three neutral reaction products are formed in this temperature range: GaCl3, AsCl3, and As4. At low (high) surface temperatures, only AsCl3 (As4) is observed. The ratio of the etching rates of Ga and As is independent of the surface temperature and within the range expected for stoichiometric etching. The change in the mode of As removal with surface temperature for the incident Cl2 flux implies that surface diffusion is important at surface temperatures above 400 K. [ABSTRACT FROM AUTHOR]

The reaction microscopic mechanism of mixed aqueous solutions of nitrobenzene and hydrogen peroxide under different conditions was studied by the laser flash photolysis technique (266 nm). The main characteristic peaks in these transient absorption spectra were attributed and the build-up/decay trends of several transient species were investigated. The transient absorption peaks of 285 nm and 305 nm were attributed to when nitrobenzene aqueous solutions were irradiated by 266 nm UV light and the decay rate constant was k 285 nm = 1.22 × 104 s−1 and k 305 nm = 1.05 × 104 s−1 in the presence of N2. k 285 nm and k 305 nm increased to 1.32 × 104 s−1 and 2.05 × 104 s−1, respectively, in the presence of O2. OH radical can be produced through the 266 nm laser flash photolysis of hydrogen peroxide in aqueous solutions. The rate constant of the reaction between OH radical and nitrobenzene was measured to be (3.6–6.0) × 109 (L · mol−1 · s−1). The intermediate of the C6H5NO2-OH adduct can undergo secondary self-decay with a reaction rate of 2k/ϵl = 1.82 × 106 s−1. The C6H5NO2-OH adduct is able to react with O2 to form C6H5NO2-OHO2 with a rate constant of (6.6 ± 0.56) × 107 (L · mol−1 · s−1). [ABSTRACT FROM PUBLISHER]

Abstract: This paper discusses the photodegradation of aqueous organometallic compounds through the use of a microwave electrodeless UV lamp radiating mainly 206nm UV light. Acetylferrocene (AFc), triphenyltin chloride (TPT), and C.I. reactive blue 13 (RB-13) were chosen as research targets. The effects of the initial concentration of organometallic compounds, irradiation time, and initial pH value on degradation efficiency were investigated. The degradation efficiency of compounds followed the order: AFc>TPT>RB-13. TOC removal efficiency of AFc, TPT and RB-13 reaches 53.2%, 25.6% and 19.7%, respectively, with 60min of irradiation. The mechanism of photolysis, based on the analysis of photoproducts using gas chromatography–mass spectrometry (GC–MS), is presented at the end of the paper. The results will help to promote further studies on the ability of microwave discharge electrodeless lamps to degrade aqueous pollutants. [Copyright &y& Elsevier]

Abstract: The photolysis of simulating low concentration of hydrogen sulfide malodorous gas was studied under UV irradiation emitted by self-made microwave discharge electrodeless lamps (i.e. microwave UV electrodeless mercury lamp (185/253.7nm) and iodine lamp (178.3/180.1/183/184.4/187.6/206.2nm)). Experiments results showed that the removal efficiency () of hydrogen sulfide was decreased with increasing initial H2S concentration and increased slightly with gas residence time; H2S removal efficiency was decreased dramatically with enlarged pipe diameter. Under the experimental conditions with pipe diameter of 36mm, gas flow rate of 0.42standardls−1, was 52% with initial H2S concentration of 19.5mgm−3 by microwave mercury lamp, the absolute removal amount (ARA) was 4.30μgs−1, and energy yield (EY) was 77.3mgkWh−1; was 56% with initial H2S concentration of 18.9mgm−3 by microwave iodine lamp, the ARA was 4.48μgs−1, and the EY was 80.5mg kWh−1. The main photolysis product was confirmed to be with IC. [Copyright &y& Elsevier]

The 355nm photon-initiated microscopic reaction mechanisms of the mixed aqueous solution of nitrobenzene and nitrous acid in the presence or absence of O2 were studied by the laser flash photolysis technique. The main transient absorption peaks in the recorded spectra were assigned and the growth/decay trends of several transient species were investigated. It was found that the OH radical formed from the photolysis of nitrous acid triggered most of the subsequent radical reactions. The rate constant of the reaction between OH radical and nitrobenzene was measured to be (3. 4±0. 1)×109 lmol−1 s−1. The product from this reaction, namely C6H5NO2–OH adduct, was found to react with O2 to yield C6H5NO2–OHO2 adduct with a rate constant of (1. 6±0. 2)×109 lmol−1 s−1. Final steady-state products were identified by GC/MS analysis and were in accordance with the transient spectroscopic results. The possible reaction pathways were proposed. [Copyright &y& Elsevier]

Abstract: The reaction mechanism of 3-chlorophenol with OH, H in aqueous solution was studied by transient technology. The 3-chlorophenol aqueous solutions have been saturated with air or N[sub 2] previously. Under alkaline condition, the reaction of OH radical with 3-chlorophenol produces 3-chlorinated phenoxyl radical, with the absorption peaks at 400 nm and 417 nm. Under neutral condition, the reaction of OH radical with 3-chlorophenol produces OH-adduct with the maximal absorption at about 340 nm. And in acid solution, the reaction of H with 3-chlorophenol produces H-adduct with the maximal absorption at about 320 nm. 3-chlorophenol is compared with 4- and 2-chlorophenols from the free radical pathways. The results show that the positions of chlorine on the aromatic ring strongly influence the dehalogenation and degradation process. [ABSTRACT FROM AUTHOR]