Kinematic constraints in reactive collisions

Crossed Molecular Beam Study of the H + HD → H2 + D Reaction at 0.60 and 1.26 eV Using the Near-Threshold Ionization Velocity Map Ion Imaging

By using the 1 + 1' near-threshold ionization velocity map ion imaging technique, state-to-state reactive differential cross sections have been measured for the H + HD → H₂ + D reaction. High-resolution images of the D products, with the rotational states of the H₂ co-products clearly resolved, were acquired at the collision energies of 0.60 and 1.26 eV, respectively. It is found that the angular distribution is predominantly backward-scattering at the collision energy of 0.60 eV. However, at 1.26 eV, where the collision energy is higher, the angular distribution becomes forward-backward-scattering. Notably, at both collision energies, the main peaks of backward-scattered products gradually shift from backward toward sideways direction as the rotational quantum number of H₂ increases. Moreover, in the forward direction, fast angular oscillations, which are induced by specific partial waves have also been observed at 1.26 eV. These features show a strong correlation between the product states and angular distributions and also indicate the unique role of partial waves in quantum reactive scattering.

Imaging the State-to-State Dynamics of the H + D2 → HD + D Reaction at 1.42 eV

High-resolution state-resolved differential cross sections (DCSs) are of great importance in understanding quantum reaction dynamics, and they are the most detailed observables that can be experimentally measured. Here we report a synergic crossed molecular beam and quantum reaction dynamics study on the H + D₂ reaction. With the time-sliced velocity map ion imaging (VMI) technique and the near-threshold ionization scheme, we acquired the product rovibrational state-resolved DCSs of the H + D₂ (v = 0, j = 0) → HD (v', j') + D reaction at a collision energy of 1.42 eV. For HD products with small j' quantum numbers, significant forward scattering with clear angular oscillations was observed. The forward scattering disappears for the rotational states with large j' quantum numbers. Interestingly, as the j' number increases, the peak of the DCS shifts from backward to sideways systematically. The experimental observation agrees very well with theoretical quantum mechanical dynamics results, which reveals that the systematic shift of the peak in the DCS from backward scattering to sideways scattering can be understood very well with the strong correlation between the product rotational quantum number j' and the specific partial waves (J = 3-12), whereas the forward angular oscillations are from the coherent summation of larger partial waves.

Formation of C9H2 and C10H2 from Reactions C3H + C6H2 and C4H + C6H2

The reactions of C₃H and C₄H radicals with C₆H₂ were investigated for the first time. Reactants C₃H, C₄H, and C₆H₂ were synthesized in two beams of C₂H₂ diluted with helium by pulsed high-voltage discharge. We measured translational-energy distributions, angular distributions, and photoionization-efficiency spectra of C₉H₂ and C₁₀H₂ produced from the title reactions in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet photoionization. The C₃H (C₄H) + C₆H₂ reaction releases 42% (33%) of available energy into the translational degrees of freedom of product C₉H₂ (C₁₀H₂) + H and scatters products into a nearly isotropic angular distribution. The photoionization-efficiency spectrum of C₉H₂ (C₁₀H₂) is in good agreement with that of C₉H₂ (C₁₀H₂) produced from the C₇H (C₈H) + C₂H₂ reaction. The ionization threshold, after deconvolution, was determined as 8.0 ± 0.1 eV for C₉H₂ and 8.8 ± 0.1 eV for C₁₀H₂. The combination of measurements of product translational-energy release and photoionization-efficiency spectra indicates productions of ³HC₉H/c-¹HC₃(C)C₅H/c-¹HC₇(C)CH + H and ¹HC₁₀H + H in the two title reactions, which are supported also by quantum-chemical calculations. Ratios branching to the three isomers of C₉H₂ remain unknown. This work demonstrates that long carbon-chain molecules (e.g., C₉H₂ and C₁₀H₂) can be synthesized from reactions of C_mH (e.g., m = 3 and 4) radicals with polyynes (e.g., HC₆H) and gives some valuable implications to planetary, interstellar, and combustion chemistry.

Formation of C3H2, C5H2, C7H2, and C9H2 from reactions of CH, C3H, C5H, and C7H radicals with C2H2

A three-dimensional velocity distribution contour of C_2n+1H₂ produced from the reaction of C_2n−1H (n = 1–4) with C₂H₂ in crossed molecular beams.

Product pair correlation in bimolecular reactions

The vast majority of chemical reactions involve polyatomic species as reactants and/or products. The added degree of complexity offers opportunities to address dynamical questions other than those already encountered in a typical atom + diatom reaction. Product pair correlation is one of them. This article introduces the basic concept, outlines the experimental approach we developed and then highlights some of the applications to bimolecular reaction dynamics. Particular emphasis is placed on the information contents and unique insights gained from this type of measurements, which otherwise would have been lost by the conventional approach.

Surfactants for CO2

For some 15 years the attainment of efficient, nonfluorinated CO2-active surfactants has been a Holy Grail for researchers spanning pure and applied chemical sciences. This article tells the story of small-molecule CO2-active surfactants, from the first tentative observations with fluorinated compounds in 1991 up to recently discovered fluorine-free oxygenated amphiphiles.

State-to-state reaction dynamics: A selective review

A selective review of state-to-state reaction dynamics experiments is presented. The review focuses on three classes of reactions that exemplify the rich history and illustrate the current state of the art in such work. These three reactions are (1) the hydrogen exchange reaction, H+H2-->H2+H and its isotopomers; (2) the H+RH-->H2+R reactions, where RH is an alkane, beginning with H+CH4-->H2+CH3 and extending to much larger alkanes; and (3) the Cl+RH-->HCl+R reactions, principally Cl+CH4-->HCl+CH3. We describe the experiments, discuss their results, present comparisons with theory, and introduce heuristic models.

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(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.

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 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.