Biomedical subjects

Weicheng Shiu

Publications and source records attributed to Weicheng Shiu.

9 recordsLinked to original sources

PubMed2006

State-correlation matrix of the product pair from F + CD(4)--> DF(nu') + CD(3)(0 v(2) 0 0).

The title reaction was investigated under crossed-beam conditions at three different collision energies, E(c) = 8.4, 2.76 and 1.46 kcal mol(-1). The combination of using a (2 + 1) resonance-enhanced multiphoton ionization for tagging state-specific CD(3) products and exploiting a time-sliced velocity imaging for ion detection allows us to reveal the coincident information of the two product pairs in a state-correlated manner. The pair-correlated results are reported for the two product vibrators -- (v(2) = 0, v'), (v(2) = 1, v'), (v(2) = 2, v') and (v(2) = 3, v')-and the dynamics attributes we examined include product state distributions, energy disposals and angular distributions. Together with our earlier communications, a rather complete picture of the correlated dynamics of the title reaction emerges. One of the major findings, the anti-correlated excitations of the two product vibrators at all four energies of this study, can qualitatively be understood by kinematics arguments.

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PubMed2006

Rotationally selected product pair correlation: F+CD4 --> DF(nu')+CD3(nu2 = 0 and 2, N).

The product pair correlation of the title reaction was measured with rotational selection for both the vibrationally ground CD3(nu = 0) and umbrella-excited CD3(nu2 = 2) products. A striking linear relationship was found between the rotational energy of the selected CD3 product and the correlated kinetic energy release (or the average vibrational energy of the DF coproduct). Such a linearly correlated (or anticorrelated) dependence appears to be stronger for CD3(nu2 = 2,N) than for CD3(nu = 0,N). The mechanistic implication of the observation is that the rotational motion N of the CD3 product tends to lie antiparallel to the orbital angular momentum l' of the two departing products. The dependency on the K quantum number--the projection of N on the top axis--is, on the other hand, less significant yet noticeable.

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PubMed2005

Imaging the reaction dynamics of OH + CD4. 2. Translational energy dependencies.

The reaction of OH + CD4 is investigated in a crossed-beam experiment over the collisional energies ranging from reaction threshold of about 5 to 16 kcal/mol. Exploiting a time-sliced ion velocity imaging detection scheme, the coincident information on the two polyatomic product pairs, HOD and CD3, is revealed in a state-correlated manner. The recently discovered vibrational mode-correlation between the two products is found to persist over the full range of collision energies of this study. In addition, the energy dependencies of the correlated cross section, state distribution, and angular distribution are elucidated, providing an unprecedented insight into this important reaction.

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PubMed2005

Imaging the reaction dynamics of OH + CD4. 3. Isotope effects.

Isotope effects in the reaction of hydroxyl radical with methane are investigated in a crossed-beam experiment. By exploiting different combinations of OH/OD + CH4/CD4/CHD3, a total of eight isotopically variant reactions are examined to decipher the dynamical consequence of both primary and secondary isotope effects. The most prominent observation is from isotopic substitution of the transferred atom (primary isotope effect), which yields more stretch-excited water product for the D atom case than H atom. The opposite is found, however, for excitations in bending and combination modes. The secondary isotope effects, from either substituted hydroxyl radical or the three H atoms that are not being abstracted, are relatively minor and manifest themselves in spreading the vibrational distribution of the water coproducts.

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PubMed2005

Mode correlation of product pairs in the reaction OH+CD4-->HOD+CD3.

The hydrogen abstraction reaction from methane by a hydroxyl radical produces two polyatomic molecules. Each product has several vibrational modes that characterize distinct, concerted motions of the constituent atoms of the molecule. This communication describes the first measurement that maps out the coincident information on how the mode of excitation of one product varies with that of the other co-product. Such information on mode correlation of product pairs is particularly appealing in that it provides intuitively a glimpse of the reaction paths by which the chemical transformation occurs.

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PubMed2004

Rotationally selected product pair correlation in F+CD(4)-->DF(nu('))+CD(3)(nu=0,N).

The title reaction was studied in a crossed-beam experiment by imaging of state-selected products. The rotational state selection of the CD(3) products was achieved using (2+1) resonance-enhanced multiphoton ionization. The coincident information on the DF coproducts was revealed in a state-resolved manner from time-sliced velocity map images. Significant dependences of both the correlated differential cross sections and the DF vibrational branching ratios on the "tagged" CD(3) rotation states were found. The dynamical implications of one of the major findings are discussed.

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PubMed2004

Reactive resonance in a polyatomic reaction.

In the course of an extensive investigation aimed at understanding the detailed mechanisms of a prototypical polyatomic reaction, several remarkable observations were uncovered. To interpret these findings, we surmise the existence of a reactive resonance in this polyatomic reaction. The reaction of concern is F+CH4-->HF+CH3, and the abnormal attributes were revealed only near the reaction threshold. The discovery of reactive resonance in a polyatomic reaction is more than just an extension from a typical atom+diatom reaction. As shown here, it holds great promise to disentangle the elusive intramolecular vibrational dynamics of transient collision complex in the critical transition-state region.

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PubMed2004

Imaging the pair-correlated excitation function: The F+CH4-->HF(v')+CH3(nu=0) reaction.

The velocity map ion imaging technique was applied to measure the reaction excitation function for the first time. It was found that the "raw" excitation function was significantly distorted by the density-to-flux transformation of the title reaction. Through a systematic investigation, possible reasons for such a dramatic effect are outlined. In addition, the state-resolved, pair-correlated excitation functions and branching ratios are presented. Effects of imperfect time slicing in the time-sliced velocity imaging technique in general are also discussed.

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PubMed2003

State-specific correlation of coincident product pairs in the F + CD4 reaction.

When a chemical reaction forms two molecular products, even if the state-resolved differential cross section (DCS) for each product is obtained individually, the coincident attributes of the coproducts are still lacking. We exploit a method that provides coincidence information by measuring the state-resolved, pair-correlated DCS. Exemplified by the reaction F + CD4 --> DF + CD3, a time-sliced ion velocity imaging technique was used to measure the velocity distribution of a state-selected CD3 product and to reveal the information of the coincident DF in a state-correlated manner. The correlation of different product state pairs shows a striking difference, which opens up a new way to unravel the complexity of a polyatomic reaction.

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