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A primary dissociation channel of Br 2 elimination is detected following a single-photon absorption of (COBr) 2 at 248 nm by using cavity ring-down absorption spectroscopy. The technique contains two laser beams propagating in a perpendicular configuration. The tunable laser beam along the axis of the ring-down cell probes the Br 2 fragment in the B 3Π + ou-X 1Σ g + transition. The measurements of laser energy- and pressure-dependence and addition of a Br scavenger are further carried out to rule out the probability of Br 2 contribution from a secondary reaction. By means of spectral simulation, the ratio of nascent vibrational population for v = 0, 1, and 2 levels is evaluated to be 1:(0.65 ± 0.09):(0.34 ± 0.07), corresponding to a Boltzmann vibrational temperature of 893 ± 31 K. The quantum yield of the ground state Br 2 elimination reaction is determined to be 0.11 ± 0.06. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state (COBr) 2 via internal conversion. A four-center dissociation mechanism is followed synchronously or sequentially yielding three fragments of Br 2 + 2CO. The resulting Br 2 is anticipated to be vibrationally hot. The measurement of a positive temperature effect supports the proposed mechanism. © 2011 American Institute of Physics.

The hydrogen spillover mechanism on B-doped graphene was explicitly investigated by first-principles calculations. By the incorporation of boron into graphene, our theoretical investigation shows that B doping can substantially enhance the adsorption strength for both H atoms and the metal cluster on the substrate. The firmly bound catalytic metal on B-doped graphene can effectively dissociate H2 molecules into H atoms, and the H atom is more likely to migrate from the bridge site of the H-saturated metal to the supporting graphene sheet. Further investigation on the BC3 sheet gives sufficiently low activation barriers for both H migration and diffusion processes; thus, more H atoms are expected to adsorb on BC3 substrate via H spillover under ambient conditions compared with the undoped graphene case. Our result is in good agreement with recent experimental findings that microporous carbon has an enhanced hydrogen uptake via boron substitution, implying that B doping with spillover is an effective approach in the modification of graphitic surface for hydrogen storage applications.

Using a combination of the bond order–length–strength correlation theory, the spin-polarized tight binding method, the first-principles calculations, and the atomistic photoelectron distillation experiments, we investigated the mechanisms of edge-selective generation and hydrogenated modulation of Dirac-Fermi polarons (DFPs) surrounding the atomic vacancies at a graphite surface and at the edges of graphene nanoribbons (GNR). We found that: (i) the \{DFPs\} with a high-spin density at a zigzag-GNR edge and at an atomic vacancy result from the isolation and polarization of the dangling σ-bond electrons of √3d (d is the C–C bond length) distance along the edge by the locally and densely entrapped bonding electrons; (ii) along an armchair-GNR edge and a reconstructed-zigzag-GNR edge, however, the formation of quasi-triple-bond between the nearest edge atoms of d distance prevents the \{DFPs\} from generation; and (iii) hydrogenation reduces the spin density substantially and turns the asymmetric dumb-bell-like density into the spherical-like pz density. A further C 1s photoelectron spectroscopic purification has confirmed that the generation of the \{DFPs\} is associated with two extra peaks of energy states located at the bottom and the top edge of the C 1s band.

The rotational energy transfer (RET) by Ar collisions within the SH X 2Π (v′′ = 0, J′′ = 0.5-10.5) state is characterized. The integral cross sections as a function of collision energy for each rotational transition are calculated using a quantum scattering method in which the constructed potential energy functions are based on a ground state potential energy surface (PES) reported previously. On the other hand, a laser-induced excitation fluorescence technique is employed to monitor the relaxation of the rotational population as a function of photolysis-probe delay time following the photodissociation of H2S at 248 nm. The rotational population evolution is comparable to its theoretical counterpart based on calculated Λ-resolved RET rate constants. The propensity in Λ-resolved RET transitions is found to approximately resemble the case of OH(X 2Π, v′′ = 0) + Ar. The Λ-averaged RET collisions are also analyzed and result in several propensity rules in the transitions. Most propensity rules are similar to those observed in the collisions of SH(A 2Σ+) by Ar. However, the behavior of the conserving ratio, defined as rate constants for spin-orbit conserving transition divided by those for spin-orbit changing transition, shows distinct difference from those described by Hund’s case (b). © the Owner Societies.

By employing time-resolved Fourier transform infrared emission spectroscopy, the fragments HCl (v=1-3), HBr (v=1), and CO (v=1-3) are detected in one-photon dissociation of 2-bromopropionyl chloride (CH3CHBrCOCl) at 248 nm. Ar gas is added to induce internal conversion and to enhance the fragment yields. The time-resolved high-resolution spectra of HCl and CO were analyzed to determine the rovibrational energy deposition of 10.0A ±0.2 and 7.4A ±0.6 kcal mol-1, respectively, while the rotational energy in HBr is evaluated to be 0.9A ±0.1 kcal mol-1. The branching ratio of HCl(v>0)/HBr(v>0) is estimated to be 1:0.53. The bond selectivity of halide formation in the photolysis follows the same trend as the halogen atom elimination. The probability of HCl contribution from a hot Cl reaction with the precursor is negligible according to the measurements of HCl amount by adding an active reagent, Br2, in the system. The HCl elimination channel under Ar addition is verified to be slower by two orders of magnitude than the Cl elimination channel. With the aid of ab initio calculations, the observed fragments are dissociated from the hot ground state CH3CHBrCOCl. A two-body dissociation channel is favored leading to either HCl+CH3CBrCO or HBr+CH2CHCOCl, in which the CH 3CBrCO moiety may further undergo secondary dissociation to release CO. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.