Crossed beam scattering with H-Rydberg Tagging technique

Dynamical Resonances in the F+H2/HD Reaction

    The transition state, an intermediate region between reactants and products, controls the process of a chemical reaction. Detecting and characterizing isolated reaction resonances in the transition state region:
      (1) give us deep insight into how elementary chemical steps actually occur;
      (2) have been a major challenge in both experiment and theory.
    One-dimensional views of two model reactions, a reaction with a simple barrier and a typical reaction with a dynamical resonance:

      (a) the potential energy curve along the reaction coordinate for a model reaction with a simple barrier;
      (b)the calculated reaction probability and time delay for the model reaction in panel a;
      (c) the potential energy curve along the reaction coordinate for a model reaction with a dynamical resonance;
      (d) the calculated reaction probability and time delay for the model reaction in panel c. (Friedman R.S. et al, Chem. Phys. Lett. 1991, 183, 539–543)

   Recently, we have studied the dynamics of F+H2/HD reaction based on:
      (1) experiment: Crossed molecular beam;
           H-atom Rydberg tagging technique;
      (2) theory: Full quantum scattering calculation;
           Accurate potential energy surfaces (PES).
    Through the detailed investigations, an accurate physical picture of reaction resonances in this benchmark reaction has been established, providing an excellent case of dynamical resonances in elemetary chemical reactions.

Feshbach Resonances in F+H2(j=0)

    Experimentally, two striking phenomena were observed:

      (1)the pronounced forward scattering for the HF(ν′=2) product;
      (2)the forward scattering signal for HF(ν′=2) peaking at 0.52 kcal/mol.
    Quantum scattering calculations were carried out on two new surfaces: XXZ-PES & FXZ-PES. Both calculations can well reproduce experimental observations.


Effect of H2 Rotational Excitation on Resonances in F+H2(j=1)

    We have also investigated the effect of a single quantum rotational excitation of H2 on the dynamics of the F + H2 reaction in the low collision energy region.

  The experimental DCS for the HF product from the F+H2(j=1) reaction:

      (a)at collision energy of 0.56 kcal/mol, no forward scattering signal was observed;
      (b)while DCS at 0.19 kcal/mol shows a large forward scattering peak.


    Collision energy dependent forward scattering signal shows a peak around 0.16~0.18 kcal/mol.
    This result is similar to that of F + H2 (j=0), except the peak was shifted to lower energy by about 0.35 kcal/mol, which is roughly equal to the rotational energy of the H2 j=1 level.
    This research shows that, reactant rotational excitation has a profound effect on the dynamics of the resonance-mediated reaction.
    The forward scattering in F+H2(j=0,1) is a result of constructive interference of two Feshbach resonances, which are transiently trapped in the HF(ν′=3)-H′ vibrational adiabatic potential well.



F+HD Reaction: Probing the Resonance Potential with Spectroscopic Accuracy

    In the F+H2 reaction, the dynamics described by both PESs show excellent agreement with the experimental observations.
    However, these two PESs actually have some noticeable differences in the resonance region.
    So: Which PES is more accurate?
    Experimentally, we investigated the F + HD (j=0) --> HF + D reaction in the collision energy range from 0.3 to 1.2 kcal/mol. Three intriguing experimental observations were found. Quantum dynamical calculations on both PESs were performed.

    The backward scattering signal for HF(v=2, j=0 to 3): Calculation on FXZ-PES shows better agreement with experimental data.

    The dynamics of F+HD at low collision energy is exclusively determined by the ground resonance state (003). So it is very sensitive to the position of the ground state.
    Because of this isotope effect, we show that the FXZ-PES is a better PES with spectroscopic accuracy in the transition state region.

Is the HF(ν′=3) Forward Scattering due to Dynamical Resonances?

    Since Lee's landmark crossed-beam experiment in 1984, numerous efforts have been paid to address the intriguing forward scattering in the F+H2-->HF(v'=3)+H reaction. Is the HF(v'=3) forward scattering a result of dynamical resonances? Or does it come from other mechanisms?
Typical cases for HF(v=3) channel:

A & B: The resonance state is below the HF(v′=3) reaction threshold and
 therefore could not be responsible for the HF(v′=3) forward scattering.
      C: The ground resonance state trapped in the HF(v′=3)–H potential well

 could form the HF(v′=3) product via tunneling through the centrifugal
 barrier. (shape resonance)
       D: Forward scattering of HF(ν′=3) happens due to slow-down over the exit

 barrier.(slow-down mechanism)
    Our study reveals that, HF(v’=3) forward scattering is caused predominantly by slow-down mechanism, with a small contribution from shape resonance.


(1) M. Qiu et al. Observation of Feshbach resonances in the F + H2 --> HF + H reaction. Science, 2006, 311, 1440–1443.
(2) Z. Ren et al. Probing the resonance potential in the F atom reaction with hydrogen deuteride with spectroscopic accuracy. Proc. Natl. Acad. Sci. U.S.A. 2008, 105,12662-12666.
(3) X. Wang et al. HF(v’ = 3) forward scattering in the F + H2 reaction: Shape resonance and slow-down mechanism. Proc. Natl. Acad. Sci. U.S.A . 2008, 105, 6227–6231.
(4) Z. Ren et al. Probing Feshbach resonances in F + H2(j = 1) --> HF + H: Dynamical effect of single quantum H2-rotation. J. Chem. Phys. 2006,125, 151102.
(5) X. Yang et al. Dynamical resonances in the fluorine atom reaction with the hydrogenmolecule. Acc. Chem. Res. 2008, 41, 981-989.

The Extend of Non-Born-Oppenheimer Coupling in F+D2/Cl+H2

    Born-Oppenheimer(B-O) approximation states that nuclear motion will occur on potential energy surfaces (PESs) that correspond to the energies of rapidly rearranging electrons as a function of the position of more slowly moving nuclei. Thus, a particular electronic energy state of the reactants will evolve along a single PES.
    The interaction between the orbital and spin angular momenta of Cl/F atom results in two possible spin-obit states, 2P3/2 and 2P1/2 .
    In the case of F+D2(or Cl+H2) reaction, the excited spin-orbit state will react only if the B-O approximation breaks down, allowing the excited spin-orbit reactants transfer down from a non-reactive PES to the reactive PES, by means of what is call a non-adiabatic transition.
    Because of the non-adiabatic transition, to what extend the B-O approximation is applicable to chemical reaction, is both a puzzle and focus for theorists and experimentalists of chemical reaction dynamics.


Non-Born-Oppenheimer Coupling in F+D2

    Energetics of F+D2DF+D reaction, after inclusion of spin-orbit splitting, with a schematic representation of the three FH2 electronically adiabatic PESs. The potential energy surfaces for the two Π states correlate, in the product arrangement, with DF(a3Π)+D, which lies at least 38 kcal/mol above the F(2P3/2)+D2 asymptote. Consequently, at low to moderate energies, only the lowest PES(2Σ+1/2) is BO-allowed channel for this reaction.

    With H-atom Rydberg tagging time-of-flight technique, we found that reaction occurs not only in BO-allowed channel, i.e. F(2P3/2)+D2 channel, but also in BO-forbidden channel, i.e. F*(2P1/2)+D2 channel.

    (A)Collision energy dependence of the overall integral reactive cross sections, summed over product vibrational and rotational levels, for the F/F*+D2( j =0) reactions for 0.25 ≤ Ec ≤ 1.2 kcal/mol.
    (B)The ratio of the cross sections shown in (A). The solid curves are the results of our theoretical calculations, whereas the points indicate the experimental results.
    The excellent agreement between experiment and theory confirms the fundamental understanding of the factors controlling electronic nonadiabaticity in abstraction reactions.


Non-Born-Oppenheimer Coupling in Cl+H2

    An previous experimental molecular-beam study of the Cl + H2 reaction by means of Doppler-selected time-of-flight detection of the H-atom product suggested that reactivity of the excited spin-orbit(Cl*) state becomes increasingly dominant as the collision energy increases. This experimental result is in contrary with the prediction of theory. Although many experiments have been designed to resolved this disagreement and these subsequent experiments suggested the excited spin-orbit state Cl* played only a minor role, in agreement with theory , their experimental resolution is not adequate to give a compelling agreement with theory.

    In order to resolve this disagreement, we investigated the Cl/Cl*+H2 reaction. Form velocity spectra of the H-atom product from the Cl (2P3/2)/Cl* (2P1/2)+H2 reaction in the backward direction at different collision energies, we found that, compared with Cl*, the relative contribution from Cl becomes large at higher collision energy.

    (A)Collision-energy dependence of the differential reactive scattering cross sections, in the backward direction, summed over product vibrational and rotational levels, for the Cl/Cl* + p-H2 reactions for 4 ≤ Ec ≤ 6kcal/mol. The squares and circles are the experimental data, and the lines display the theoretical results from calculations on the CW set of PESs.
 (B) The ratio of the cross sections shown in (A). The red curve and the green circles indicate, respectively, the results of our theoretical calculations on the CW and XJ PESs, whereas the diamonds indicate the experimental results.
    From the figure above, we can see that the reactive backward DCS for the BO-allowed Cl-atom reaction increases much more rapidly with increasing collision energy than that for the BO-forbidden reaction of Cl*.
    The excellent overall agreement between our molecular beam experiments and the results of quantum-reactive scattering calculations on multiple PESs indicates that the theoretical formulation includes correctly the essential physics governing the nonadiabatic processes of importance in the Cl + H2 reaction.


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