We present a combined experimental and theoretical investigation of formaldehyde ($H_{2}CO$) dissociation to H2and CO at energies just above the threshold for competing H elimination. High-resolution ...state-resolved imaging measurements of the CO velocity distributions reveal two dissociation pathways. The first proceeds through a well-established transition state to produce rotationally excited CO and vibrationally cold H2. The second dissociation pathway yields rotationally cold CO in conjunction with highly vibrationally excited H2. Quasiclassical trajectory calculations performed on a global potential energy surface for$H_{2}CO$suggest that this second channel represents an intramolecular hydrogen abstraction mechanism: One hydrogen atom explores large regions of the potential energy surface before bonding with the second H atom, bypassing the saddle point entirely.
Ground state radical dissociation pathways of HDCO were investigated. Frequencies of rovibrational lines from a variety of vibrational bands of the first singlet excited state over a broad excitation ...energy range were determined and subsequently used to excite singly deuterated formaldehyde, HDCO. Translational energy release of H and D atoms measured by velocity map imaging was then used to determine accurate threshold energies of 30,448 ± 5 cm
−1
for H + DCO and 30,949 ± 3 cm
−1
for D + HCO.
Remote sensing by nuclear quadrupole resonance Garroway, A.N.; Buess, M.L.; Miller, J.B. ...
IEEE transactions on geoscience and remote sensing,
06/2001, Letnik:
39, Številka:
6
Journal Article
Recenzirano
Detection of explosives has the flavor of those mathematical problems that are not invertible. It is easier to hide explosives than to find them. Many approaches have been proposed and executed for ...the remote detection of explosives, contraband materials, weapons of mass destruction, currency, etc. Most detection technologies suffer from a common problem: the features they look for, such as discontinuities in electrical conductivity, are not unique properties of the target but are contained, to some degree, in the more benign surroundings. Such a degeneracy leads to "clutter" in the response. For example, resolving the false alarms generated by this clutter can determine the rate of advance of a conventional electromagnetic metal detector employed as a landmine detector. One approach that provides a "unique" signature is nuclear quadrupole resonance (NQR) (the technique is also called QR, to avoid confusion with strictly nuclear techniques). This paper outlines the important physical principles behind the use of NQR for remote detection, indicates areas of applicability, and presents results of field trials of a prototype landmine detection system.
We present an experimental investigation of the UV photochemistry of diacetylene under collisionless conditions. The H loss channel is studied using DC slice ion imaging with two-color ...reduced-Doppler detection at 243 nm and 212 nm. The photochemistry is further studied deep in the vacuum UV, that is, at Lyman-alpha (121.6 nm). Translational energy distributions for the H + C₄H product arising from dissociation of C₄H₂ after excitation at 243, 212, and 121.6 nm show an isotropic angular distribution and characteristic translational energy profile suggesting statistical dissociation from the ground state or possibly from a low-lying triplet state. From these distributions, a two-photon dissociation process is inferred at 243 nm and 212 nm, whereas at 121.6 nm, a one-photon dissociation process prevails. The results are interpreted with the aid of ab initio calculations on the reaction pathways and statistical calculations of the dissociation rates and product branching. In a second series of experiments, nanosecond time-resolved phototionization measurements yield a direct determination of the lifetime of metastable triplet diacetylene under collisionless conditions, as well as its dependence on excitation energy. The observed submicrosecond lifetimes suggest that reactions of metastable diacetylene are likely to be less important in Titan's atmosphere than previously believed.
The reaction of ground-state carbon atoms with acetylene was studied under single-collision conditions in crossed beam experiments to investigate the chemical dynamics of forming cyclic and linear ...C3H isomers (c-C3H and l-C3H, respectively) in interstellar environments via an atom-neutral reaction. Combined state-of-the-art ab initio calculations and experimental identification of the carbon-hydrogen exchange channel to both isomers classify this reaction as an important alternative to ion-molecule encounters to synthesize C3H radicals in the interstellar medium. These findings strongly correlate with astronomical observations and explain a higher c-C3H/l-C3H ratio in the dark cloud TMC-1 than in the carbon star IRC+10216.
The photodissociation of cyanoacetylene, one of the key minor constituents in Titan’s atmosphere, was studied in a molecular beam under collisionless conditions using direct current slice ion imaging ...at 121.6, 193.3, and 243.2 nm. The experimental results were augmented by high-level theoretical calculations of stationary points on the ground-state and second excited singlet potential surfaces, and by statistical calculations of the dissociation rates and product branching on the ground-state surface. Results at 121.6 and 243.2 nm are nearly identical, suggesting that the 243.2 nm photodissociation is the result of a two-photon process. The translational energy distributions show only a modest fraction of the available energy in translation and are consistent with barrierless dissociation from the ground state. The results at 193.3 nm are quite distinct, showing up to half of the available energy in translation, implying dissociation with an exit barrier. The 193 nm result is ascribed to dissociation on the S1 potential energy surface. The theoretical calculations show significant rates for H loss on the ground state at 193 nm and significant branching to CN + CCH at 157 nm and higher.
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