Weicheng Shiu

Application of time-sliced ion velocity imaging to crossed molecular beam experiments

A three-dimensional (3D) ion velocity imaging method was developed to measure the product velocity distributions in crossed molecular beam experiments. While maintaining conventional two-dimension velocity mapping, the third velocity component was mapped linearly to the ion time of flight. A weak extraction field was used to spread the ion turnaround time to several hundred nanoseconds, which permits good resolution for selection of the longitudinal velocity. A fast gated (⩾5 ns) intensified charge coupled device camera was used to record time-sliced ion images. Calibration of the apparatus was done by measuring O+ images from the multiphoton dissociation/ionization of O2. The resolution in velocity achieved was about 1% (Δv/v) in slicing through the center of a Newton sphere. The overall performance was examined by observing product ion images from the F+CD4→DF+CD3 reaction. To detect CD3+ with kinetic energy release of about 1 eV, 50 ns time slicing provides sufficient velocity resolution, such that res...

Crossed-beam scattering of F+CD4→DF+CD3(νNK): The integral cross sections

The title reaction was investigated in a crossed-beam experiment. A (2+1) resonance-enhanced multiphon ionization technique was used to interrogate the internal-state distributions of the CD3 product at three different collision energies. Only the ν2 (umbrella) mode excitation was observed. Its distribution changes from a monotonically declined distribution at low energy to a slightly inverted one at higher collision energy. Although the rotational excitations of CD3 were small, a strong preference for K=0 was found, indicative of the dominance of the tumbling rotation motion of the CD3 product. The vibration-resolved excitation functions were also measured for ν2=0–3. A reaction barrier of 0.5 kcal/mol was deduced.

Insights into dynamics of the F+CD4 reaction via product pair correlation

To unravel the “extra-atom” complexity of the title reaction, we exploit an experimental approach which, by taking advantage of the correlated information of coincident product pairs, allows us to peel off judiciously the intrinsic complications of a six-atom reaction, extracting the underlying backbone of three-atom dynamics. Examining the collisional energy dependencies of the pair-correlated attributes for a given state(s) of CD3 products from the title reaction, several of major observations can qualitatively be understood, whereas others await further theoretical investigations. An intriguing possibility for the existence of reactive resonances in this six-atom reaction is surmised.

Rotationally selected product pair correlation in F+CD4→DF(ν')+CD3(ν=0,N)

The title reaction was studied in a crossed-beam experiment by imaging of state-selected products. The rotational state selection of the CD3 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” CD3 rotation states were found. The dynamical implications of one of the major findings are discussed.

Rotationally selected product pair correlation: F+CD4→DF(ν′)+CD3(ν2=0and2,N)

The product pair correlation of the title reaction was measured with rotational selection for both the vibrationally ground CD3(ν=0) and umbrella-excited CD3(ν2=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(ν2=2,N) than for CD3(ν=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.

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.

State-correlation matrix of the product pair from F + CD4→ DF(ν′) + CD3(0 v2 0 0)

The title reaction was investigated under crossed-beam conditions at three different collision energies, Ec = 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 CD3 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— (v2 = 0, v′), (v2 = 1, v′), (v2 = 2, v′) and (v2 = 3, v′)—and the dynamics attributes we examined include product state distributions, energy disposals and angular distributions. Together with our earlier communications [J. Lin, J. Zhou, W. Shiu, and K. Liu, Science 2003, 300, 966, and J. Lin, J. Zhou, W. Shiu, and K. Liu, J. Chem. Phys. 2003, 119, 4997, ], 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.