The H, CH 2 CHO, CO, and OH products of the reaction of O( 3 P) atom with alkenes were studied by laserinduced fluorescence (LIF) under single-collision conditions. The average kinetic energies of the H atoms were 10-12 kcal/mol. The CO and OH rotational state populations were characterized by near room temperature Boltzmann distributions. The relative LIF intensities of the various products provide vivid proof of the following mechanism for the reaction of O( 3 P) atoms with molecules of the form R′RCdCH 2 , where R′ and R are H or an alkyl group. The O atom attaches itself to the less substituted carbon atom forming a triplet ketocarbene. There is a barrier to the release of an H atom, and the rate of release must compete with the rate of intersystem crossing. If an H atom is not released, following the intersystem crossing an H atom migrates to the adjacent C atom forming an energized aldehyde, R′RCH-CHdO. The aldehyde dissociates unimolecularly forming the pair of radicals R′ • and the substituted vinoxy • RCH-CHdO or the pair R′RCH and HCO. Some of the latter have enough internal energy to dissociate to H and CO. In a side reaction O( 3 P) abstracts H atoms but only from allylic C-H bonds. The most remarkable observation is that chemical reactions that do not involve the side chains such as release of H atoms or breakup of HCO depend sensitively on the length of these chains.

In part I of this work the relative velocities and anisotropies of the atomic H and D fragments from methane photolysis at 10.2 eV were measured. In this paper the relative abundance of the methyl and methylene fragments are reported. A complete set of quantum yields for the different photodissociation channels of each isotopomer is obtained by combining the two sets of data. Previously it was found that H atoms are almost four times more likely than D atoms to be ejected; now it is found that hydrogen molecule photofragments are much richer in H atoms than in D. Overall, the heavier D atoms are more likely than the H atoms to remain attached to the carbon atom. An implication for astrophysics is discussed. The VUV absorption spectra of CH4 and CH3D are almost identical both at room temperature and 75 K. There is, as expected, no variation in the absorption spectrum with temperature. Evidence is given that all or almost all of the methylene is produced in the a 1A1 and not in the ground B13 state.

Ab initio quantum chemical calculations for the molecular dissociation channel of acetaldehyde are reported. The enthalpy change for the dissociation of acetaldehyde into methane and carbon monoxide was calculated to be exoergic by 1.7 kcal/mol. The transition state for this unimolecular dissociation, confirmed by normal mode analysis, was found to have an activation energy of 85.3 kcal/mol. Experimental measurements are reported for the vibrational and rotational state distribution of the CO product. No vϭ1 CO is found and the rotational temperature is 1300 Ϯ90 K. The reaction coordinate at the transition state implies that the CO product is vibrationally cold and rotationally hot. This conclusion, which requires quantum dynamics calculations to confirm definitively, does agree with and aids in explaining the experimental results.

The wavelength 205.14 nm is absorbed by many hydrogen-containing molecules, which then dissociate to form hydrogen atoms. These in turn can absorb two more 205.14 nm photons and reach the 3s or the 3d state. They can absorb a third photon and form hydrogen ions or decay to the ground state by stepwise fluorescence, first emitting the Balmer α line at 656.2 nm (3s or 3d→2p)and then the Lyman α line (2p→1s)at 121.6 nm. Thus the hydrogen atom kinetic energy can be probed in three different ways. This method broadens greatly the possibilities of investigating photodissociations leading to hydrogen atom products. It has the advantage of simplicity and the disadvantages of a single-color experiment. The method is tested with a molecule that has been extensively investigated, H2S, and then applied to three other molecules, formic acid (HCO2H), vinyl radical (C2H3), and allyl radical (C3H5). H2S has a perpendicular transition with a large release of kinetic energy. Studies of the latter molecules lead to the conclusion that formyl, carboxyl, vinyl, and allyl radicals absorb 205.1 nm light and release hydrogen atoms with a large fraction of the available energy. The dissociation pathways of formic acid are clarified.

The reaction of O(3P) atoms with CH3 radicals is shown to produce CO (in addition to the major product CH2O) which is detected by laser induced fluorescence. The rotational and vibrational temperatures of the CO product are about 2000 K. The results are explained by the assumption that the reaction takes place mainly by an indirect mechanism in which a methoxyl radical is formed and then dissociates unimolecularly.

The HCO product of the reaction of O(3P) with ethene has been detected by cavity ring-down spectroscopy using its A−X transition. For propene a somewhat smaller yield of HCO was obtained but the overall rate constant is much larger. The yield of HCO in this reaction is quite small (∼0.05). Moreover, a large number of other alkenes were tried with negative results. The failure of the 1,2 H atom shift followed by breaking the 1,2 bond implies that the unimolecular decomposition has found a more favorable channel. The proposed mechanism is as follows. For an alkene of the form RCH2CHCH2 the first step is attachment of the O(3P) to the terminal carbon atom, C1. Then, intersystem crossing occurs and finally a H atom shifts from C3 to C2 and not from C1 to C2. In this way a molecule of formaldehyde and an alkene shorter by one carbon atom are formed.

Abstract The synapse is one of the fundamental elements in human brain performing functions such as learning, memorizing, and visual processing. The implementation of synaptic devices to realize neuromorphic computing and sensing tasks is a key step to artificial intelligence, which, however, has been bottlenecked by the complex circuitry and device integration. We report a high-performance charge-trapping memory synaptic device based on two-dimensional (2D) MoS2 and high-k Ta2O5–TiO2 (TTO) composite to build efficient and reliable neuromorphic system, which can be modulated by both electrical and optical stimuli. Significant and essential synaptic behaviors including short-term plasticity, long-term potentiation, and long-term depression have been emulated. Such excellent synaptic behaviors originated from the good nonvolatile memory performance due to the high density of defect states in the engineered TTO composite. The 2D synaptic device also exhibits effective switching by incident light tuning, which further enables pattern recognition with accuracy rate reaching 100%. Such experimental demonstration paves a robust way toward a multitask neuromorphic system and opens up potential applications in future artificial intelligence and sensing technology.

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