Two-dimensional projected-momentum covariance mapping for coulomb explosion imaging

We introduce projected-momentum covariance mapping, an extension of recoil-frame covariance mapping for 2D ion imaging studies. By considering the two-dimensional projection of the ion momenta as recorded by the detector, one opens the door to a complex suite of analysis tools adapted from three-dimensional momentum imaging studies. This includes the use of different frames of reference to unravel the dynamics of fragmentation and the application of fragment momentum constraints to isolate specific fragmentation channels. The technique is demonstrated on data from a two-dimensional ion imaging study of the Coulomb explosion of the cis and trans isomers of 1,2-dichloroethene, following strong-field ionization by an intense near-infrared femtosecond laser pulse. Classical simulations are used to guide the interpretation of projected-momentum covariance maps. The results offer a detailed insight into the distinct Coulomb explosion dynamics for this pair of isomers and lay the groundwork for future time-resolved studies of photoisomerization dynamics in this molecular system

Waste-to-Energy: Production of Fuel Gases from Plastic Wastes

A new mechanochemical method was developed to convert polymer wastes, polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC), to fuel gases (H2, CH4, and CO) under ball-milling with KMnO4 at room temperature. By using various solid-state characterizations (XPS, SEM, EDS, FTIR, and NMR), and density functional theory calculations, it was found that the activation followed the hydrogen atom transfer (HAT) mechanism. Two metal oxidant molecules were found to abstract two separate hydrogen atoms from the α–CH and β–CH units of substrates, [–βCH2–αCH(R)–]n, where R = H in PE, R = γCH3 in PP, and R = Cl in PVC, resulting in a di-radical, [–βCH•–αC•(R)–]. Subsequently, the two unpaired electrons of the di-radical were recombined into an alkene intermediate, [–βCH = αC(R)–], which underwent further oxidation to produce H2, CH4, and CO gases

The Onset of H + Ketene Products from Vinoxy Radicals Prepared by Photodissociation of Chloroacetaldehyde at 157 nm

We investigate the unimolecular dissociation of the vinoxy radical (CH₂CHO) prepared with high internal energy imparted from the photodissociation of chloroacetaldehyde (CH₂ClCHO) at 157 nm. Using a velocity map imaging apparatus, we measured the speed distribution of the recoiling chlorine atoms, Cl­(²P_3/2) and Cl­(²P_1/2), and derived from this the resulting distribution of kinetic energy, <i>P</i>(<i>E</i>_T), imparted to the Cl + vinoxy fragments upon dissociation. Using conservation of energy, the distribution of kinetic energy was used to determine the total internal energy distribution in the radical. The <i>P</i>(<i>E</i>_T) derived for the C–Cl bond fission presented in this work suggests the vinoxy radicals are mostly formed in the à state. We also took ion images at <i>m</i>/<i>z</i> = 42 and <i>m</i>/<i>z</i> = 15 to characterize the branching between the unimolecular dissociation channels of the vinoxy radical to H + ketene and methyl + CO products. Our results show a marked change in the branching ratio between the two channels from the previous study on the photodissociation of chloroacetaldehyde at 193 nm by Miller et al. (<i>J. Chem. Phys.</i>, <b>2004</b>, 121, 1830) in that the production of ketene is now favored over the production of methyl. To help analyze the data, we developed a model for the branching between the two channels that takes into account how the change in rotational energy en route to the products affects the vibrational energy available to surmount the barriers to the channels. The model predicts the portion of the C–Cl bond fission <i>P</i>(<i>E</i>_T) that produces dissociative vinoxy radicals, then predicts the branching ratio between the H + ketene and CH₃ + CO product channels at each <i>E</i>_T. The model uses Rice–Ramsperger–Kassel–Marcus rate constants at the correct sums and densities of vibrational states while accounting for angular momentum conservation. We find that the predicted portion of the <i>P</i>(<i>E</i>_T) that produces H + ketene products best fits the experimental portion (that we derive by taking advantage of conservation of momentum) if we use a barrier height for the H + ketene channel that is 4.0 ± 0.5 kcal/mol higher than the isomerization barrier en route to CH₃ + CO products. Using the G4 computed isomerization barrier of 40.6 kcal/mol, this gives an experimentally determined barrier to the H + ketene channel of 44.6 kcal/mol. From these calculations, we also predict the branching ratio between the H + ketene and methyl + CO channels to be ∼2.1:1

Feasibility, acceptability, and potential efficacy of an innovative postnatal home-based breastfeeding peer support programme in Hong Kong : a feasibility and pilot randomised controlled trial

As suggested by the World Health Organization, breastfeeding peer support is being introduced worldwide to support women's breastfeeding needs. Evidence has shown that when such support is offered to women, the duration and exclusivity of breastfeeding is increased. We developed an innovative home-based intervention to sustain exclusive breastfeeding in Hong Kong. However, potential barriers must be addressed before a full randomised controlled trial (RCT) is conducted. The aim of this study was to determine the feasibility of a breastfeeding support programme with home-based visits from peer supporters over a six month period among postpartum Chinese women in Hong Kong. We conducted a feasibility and pilot randomised controlled trial. Twenty primiparous women intending to breastfeed their healthy term singleton infant were recruited from a hospital in Kowloon, Hong Kong between February and March 2019. Participants were randomly allocated to the intervention or control group. Participants in the intervention group received five home-based visits with a peer supporter over a six month period, as well as standard care, whereas participants in the control group received standard care only. We assessed feasibility, compliance, and acceptability of the breastfeeding peer support programme. Other outcomes assessed were breastfeeding self-efficacy, duration, and exclusivity. It was feasible to recruit and train existing peer supporters, and peer supporters were able to deliver the intervention, which was acceptable to women, but rates of stopping the intervention and loss to follow-up were high. There was higher retention seen within the first month. Women interviewed at the end of the study reported that the intervention was positive. The cessation risk of any, and exclusive breastfeeding were not statistically different between the intervention and control groups. This study provided valuable information on feasibility of the trial design and intervention. Modifications to the intervention, such as targeting women with lower breastfeeding self-efficacy, or combining home visits with technology and telephone follow-up may be more appropriate in a larger trial. Implementing the programme early during the antenatal phase and tailoring peer support to most appropriately sustain exclusive breastfeeding and other feeding modes should be incorporated in a future home-based peer support arm

Elucidating the Decomposition Mechanism of Energetic Materials with Geminal Dinitro Groups Using 2‑Bromo-2-nitropropane Photodissociation

These experiments photolytically generate two key intermediates in the decomposition mechanisms of energetic materials with nitro substituents, 2-nitropropene, and 2-nitro-2-propyl radicals. These intermediates are produced at high internal energies and access a number of competing unimolecular dissociation channels investigated herein. We use a combination of crossed laser-molecular beam scattering and velocity map imaging to study the photodissociation of 2-bromo-2-nitropropane at 193 nm and the subsequent unimolecular dissociation of the intermediates above. Our results demonstrate that 2-bromo-2-nitropropane has four primary photodissociation pathways: C–Br bond fission yielding the 2-nitro-2-propyl radical, HBr elimination yielding 2-nitropropene, C–N bond fission yielding the 2-bromo-2-propyl radical, and HONO elimination yielding 2-bromopropene. The photofragments are formed with significant internal energy and undergo many secondary dissociation events, including the exothermic dissociation of 2-nitro-2-propyl radicals to NO + acetone. Calculations at the G4//B3LYP/6-311++g­(3df,2p) level show that the presence of a radical at a nitroalkyl center changes the mechanism for and substantially lowers the barrier to NO loss. This mechanism involves an intermediate with a three-center ring rather than the intermediate formed during the traditional nitro–nitrite isomerization. The observed dissociation pathways of the 2-nitro-2-propyl radical and 2-nitropropene help elucidate the decomposition mechanism of larger energetic materials with geminal dinitro groups

Imaging and Scattering Studies of the Unimolecular Dissociation of the BrCH₂CH₂O Radical from BrCH₂CH₂ONO Photolysis at 351 nm

We report a study of the unimolecular dissociation of BrCH₂CH₂O radicals produced from the photodissociation of BrCH₂CH₂ONO at 351/355 nm. Using both a crossed laser-molecular beam scattering apparatus with electron bombardment detection and a velocity map imaging apparatus with tunable VUV photoionization detection, we investigate the initial photodissociation channels of the BrCH₂CH₂ONO precursor and the subsequent dissociation of the vibrationally excited BrCH₂CH₂O radicals. The only photodissociation channel of the precursor we detected upon photodissociation at 351 nm was O–NO bond fission. C–Br photofission and HBr photoelimination do not compete significantly with O–NO photofission at this excitation wavelength. The measured O–NO photofission recoil kinetic energy distribution peaks near 14 kcal/mol and extends from 5 to 24 kcal/mol. There is also a small signal from lower kinetic energy NO product (it would be 6% of the total if it were also from O–NO photofission). We use the O–NO photofission <i>P</i>(<i>E</i>_T) peaking near 14 kcal/mol to help characterize the internal energy distribution in the nascent ground electronic state BrCH₂CH₂O radicals. At 351 nm, some but not all of the BrCH₂CH₂O radicals are formed with enough internal energy to unimolecularly dissociate to CH₂Br + H₂CO. Although the signal at <i>m</i>/<i>e</i> = 93 (CH₂Br⁺) obtained with electron bombardment detection includes signal both from the CH₂Br product and from dissociative ionization of the energetically stable BrCH₂CH₂O radicals, we were able to isolate the signal from CH₂Br product alone using tunable VUV photoionization detection at 8.78 eV. We also sought to investigate the source of vinoxy radicals detected in spectroscopic experiments by Miller and co-workers (J. Phys. Chem. A 2012, 116, 12032) from the photodissociation of BrCH₂CH₂ONO at 351 nm. Using velocity map imaging and photodissociating the precursor at 355 nm, we detected a tiny signal at <i>m</i>/<i>e</i> = 43 and a larger signal at <i>m</i>/<i>e</i> = 15 that we tentatively assign to vinoxy. An underlying signal in the time-of-flight spectra at <i>m</i>/<i>e</i> = 29 and <i>m</i>/<i>e</i> = 42, the two strongest peaks in the literature electron bombardment mass spectrum of vinoxy, is also apparent. Comparison of those signal strengths with the signal at HBr⁺, however, shows that the vinoxy product does not have HBr as a cofragment, so the prior suggestion by Miller and co-workers that the vinoxy might result from a roaming mechanism is contraindicated