Jingang Zhou

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.

Observation of a reactive resonance in the integral cross section of a six-atom reaction: F+CHD3

The title reaction was investigated under crossed-beam conditions at collisional energies ranging from about 0.4 to 7.5 kcal/mol. Product velocity distributions were measured by a time-sliced, velocity-map imaging technique to explicitly account for the density-to-flux transformation factors. Both the state-resolved, pair-correlated excitation functions and vibrational branching ratios are presented for the two isotopic product channels. An intriguing resonance tunneling mechanism occurring near the reaction threshold for the HF+CD3 product channel is surmized, which echoes the reactive resonances found previously for the F+HD-->HF+D reaction and more recently for the F+CH4 reaction.

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.

Mode-correlated product pairs in the F+CHD3→DF+CHD2 reaction

The title reaction was investigated at three different collision energies in a pulsed, crossed-beam apparatus. The (2+1) REMPI spectra of the CHD2 products revealed, in addition to the anticipated 4mn vibronic bands, a hitherto unobserved feature. The new feature was shown and assigned to the 311 band. A time-sliced ion velocity imaging technique was applied to map out the coincident DF attributes of the two product states 42 and 31, whose energy levels lie nearly degenerate. Remarkably similar results were found for the two states in every aspect at all three collision energies. A simple model of Fermi-coupled states was proposed to rationalize this, at first sight, surprising finding. Implications to collisional processes which involve mixed molecular basis states in general are outlined. Possible quantum interference phenomenon is suggested.

Imaging the isotope effects in the ground state reaction of Cl CH4 and CD4

The ground state reactions of Cl + CH4 and CD4 were studied in a crossed-beam experiment, in which the ground state methyl products were probed using a time-sliced velocity imaging technique. By taking the images over the energy range of chemical significance - from reaction threshold to about 10 kcal/mol, the reactive excitation functions as well as the dependences of product angular distributions and of the energy disposals on initial collision energies were obtained. Comparing the dynamical attributes of the two isotopic reactions revealed an interesting isotope effect in product angular distributions. The previous reduced-dimensional quantum scattering calculations of Yu and Nyman appear to reproduce this isotope effect rather well. Yet, a physical understanding of its origin awaits further theoretical investigations.

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.

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.

Deciphering the nature of the reactive resonance in F + CHD3: correlated differential cross-sections of the two isotopic channels

Following up a previous publication (J. Chem. Phys. 121, 813 (2004)) on the observation of a reactive resonance in the integral cross-section of the title reaction, this report elucidates the resonance fingerprints in the pair-correlated differential cross section (CDCS). By using a time-sliced, velocity-mapped (ion) imaging technique, we measured the DCS in a product pair-correlated manner under the crossed-beam conditions. Here, the focus is on the major vibration states of the two isotopic product channels: the CD3(v = 0) + HF(v) and CHD2(v = 0) + DF(v). A distinct ridge structure near reaction threshold that followed by oscillatory forward-backward peaking features is clearly revealed in the E c (collision energy)-evolution of the angular distribution of the paired products. The observed patterns are attributed to the characteristic imprints of a resonant reaction pathway. More intriguing is the observation that these resonance signatures occur at about the same collision energies in both HF + CD3 and DF + CHD2 isotope channels. To account for the experimental findings, the nature of the resonance states is delineated, which invokes two geometrical isomers of the F-H-CD3 and F-D-CHD2 configurations in the transition state region. The facile isomerization process of the resonant complexes causes the resonance wave function being delocalised over the two isomeric configurations, thus yielding similar resonance imprints in both isotopic product channels.