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Abstract
I seem to have started off on the wrong foot in life, but I am extremely fortunate that I soon found my footing in the company of physical chemists. I consider myself to be very lucky to be doing something that constantly brings me in contact with bright minds, stimulating conversations, and exciting experiments. My work has allowed me to learn astounding facts about the molecules and atoms that make up our surroundings and ourselves. For this article, I focus on one aspect of my research, understanding the fundamental principles of the simple reaction between a hydrogen atom and a hydrogen molecule. Although my group and others have been studying this seemingly simple reaction for well over 30 years, it continues to provoke questions about the properties of matter.
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Affiliation(s)
- Richard N. Zare
- Department of Chemistry, Stanford University, Stanford, California 94305-5080
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Jankunas J, Zare RN, Bouakline F, Althorpe SC, Herráez-Aguilar D, Aoiz FJ. Seemingly Anomalous Angular Distributions in H + D
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Reactive Scattering. Science 2012; 336:1687-90. [DOI: 10.1126/science.1221329] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Affiliation(s)
- Justin Jankunas
- Department of Chemistry, Stanford University, Stanford, CA 94305–5080, USA
| | - Richard N. Zare
- Department of Chemistry, Stanford University, Stanford, CA 94305–5080, USA
| | - Foudhil Bouakline
- Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam-Golm, Germany
| | - Stuart C. Althorpe
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
| | - Diego Herráez-Aguilar
- Departamento de Química Física, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain
| | - F. Javier Aoiz
- Departamento de Química Física, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain
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Kuppermann A. Reactive scattering with row-orthonormal hyperspherical coordinates. 4. Four-dimensional-space Wigner rotation function for pentaatomic systems. Phys Chem Chem Phys 2011; 13:8259-68. [DOI: 10.1039/c0cp02907f] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Bartlett NCM, Jankunas J, Goswami T, Zare RN, Bouakline F, Althorpe SC. Differential cross sections for H + D2→ HD(v′ = 2, j′ = 0,3,6,9) + D at center-of-mass collision energies of 1.25, 1.61, and 1.97 eV. Phys Chem Chem Phys 2011; 13:8175-9. [DOI: 10.1039/c0cp02460k] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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Bouakline F, Althorpe SC, Larregaray P, Bonnet L. Strong geometric-phase effects in the hydrogen-exchange reaction at high collision energies: II. Quasiclassical trajectory analysis. Mol Phys 2010. [DOI: 10.1080/00268971003610218] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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Wang D, Kuppermann A. Analytical Derivation of Row-Orthonormal Hyperspherical Harmonics for Triatomic Systems. J Phys Chem A 2009; 113:15384-410. [DOI: 10.1021/jp906473n] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Desheng Wang
- Arthur Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
| | - Aron Kuppermann
- Arthur Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
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Althorpe SC, Juanes-Marcos JC, Wrede E. The Influence of the Geometric Phase on Reaction Dynamics. ADVANCES IN CHEMICAL PHYSICS 2008. [DOI: 10.1002/9780470259474.ch1] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/10/2023]
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Bouakline F, Althorpe SC, Peláez Ruiz D. Strong geometric-phase effects in the hydrogen-exchange reaction at high collision energies. J Chem Phys 2008; 128:124322. [DOI: 10.1063/1.2897920] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Koszinowski K, Goldberg NT, Zhang J, Zare RN, Bouakline F, Althorpe SC. Differential cross section for the H+D2→HD(v′=1,j′=2,6,10)+D reaction as a function of collision energy. J Chem Phys 2007; 127:124315. [PMID: 17902911 DOI: 10.1063/1.2771157] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We have measured differential cross sections (DCSs) for the HD (v(')=1,j(')=2,6,10) products of the H+D(2) exchange reaction at five different collision energies in the range 1.48< or =E(coll)< or =1.94 eV. The contribution from the less energetic H atoms formed upon spin-orbit excitation of Br in the photolysis of the HBr precursor is taken into account for two collision energies, E(coll)=1.84 and 1.94 eV, allowing us to disentangle the two different channels. The measured DCSs agree well with new time-dependent quantum-mechanical calculations. As the product rotational excitation increases, the DCSs shift from backward to sideward scattering, as expected. We also find that the shapes of the DCSs show only a small overall dependence on the collision energy, with a notable exception occurring for HD (v(')=1,j(')=2), which appears bimodal at high collision energies. We suggest that this feature results from both direct recoil and indirect scattering from the conical intersection.
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Affiliation(s)
- Konrad Koszinowski
- Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA
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Juanes-Marcos JC, Althorpe SC, Wrede E. Effect of the geometric phase on the dynamics of the hydrogen-exchange reaction. J Chem Phys 2007; 126:044317. [PMID: 17286480 DOI: 10.1063/1.2430708] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
A recent puzzle in nonadiabatic quantum dynamics is that geometric phase (GP) effects are present in the state-to-state opacity functions of the hydrogen-exchange reaction, but cancel out in the state-to-state integral cross sections (ICSs). Here the authors explain this result by using topology to separate the scattering amplitudes into contributions from Feynman paths that loop in opposite senses around the conical intersection. The clockwise-looping paths pass over one transition state (1-TS) and scatter into positive deflection angles; the counterclockwise-looping paths pass over two transition states (2-TS) and scatter into negative deflection angles. The interference between the 1-TS and 2-TS paths thus integrates to a very small value, which cancels the GP effects in the ICS. Quasiclassical trajectory (QCT) calculations reproduce the scattering of the 1-TS and 2-TS paths into positive and negative deflection angles and show that the 2-TS paths describe a direct insertion mechanism. The inserting atom follows a highly constrained "S-bend" path, which allows it to avoid both the other atoms and the conical intersection and forces the product diatom to scatter into high rotational states. By contrast, the quantum 2-TS paths scatter into a mainly statistical distribution of rotational states, so that the quantum 2-TS total ICS is roughly twice the QCT ICS at 2.3 eV total energy. This suggests that the S-bend constraint is relaxed by tunneling in the quantum system. These findings on H+H(2) suggest that similar cancellations or reductions in GP effects are likely in many other reactions.
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Affiliation(s)
- Juan Carlos Juanes-Marcos
- Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
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Goldberg NT, Koszinowski K, Pomerantz AE, Zare RN. Doppler-free ion imaging of hydrogen molecules produced in bimolecular reactions. Chem Phys Lett 2007. [DOI: 10.1016/j.cplett.2006.11.073] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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Althorpe SC. General explanation of geometric phase effects in reactive systems: Unwinding the nuclear wave function using simple topology. J Chem Phys 2006; 124:084105. [PMID: 16512706 DOI: 10.1063/1.2161220] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We describe a simple topological approach which was used recently to explain geometric phase (GP) effects in the hydrogen-exchange reaction [Juanes-Marcos et al., Science 309, 1227 (2005)]. The approach is general and applies to any reactive system in which the nuclear wave function encircles a conical intersection (CI) and is confined to one adiabatic surface. The only numerical work required is to add and subtract nuclear wave functions computed with normal and GP boundary conditions. This is equivalent to unwinding the nuclear wave function onto a double cover space, which separates out two components whose relative sign is changed by the GP. By referring to earlier work on the Aharanov-Bohm effect, we show that these two components contain all the Feynman paths that follow, respectively, an even and an odd number of loops around the CI. These two classes of path are essentially decoupled in the Feynman sum, because they belong to different homotopy classes (meaning that they cannot be continuously deformed into one another). Care must be taken in classifying the two types of path when the system can enter the encirclement region from several different start points. This applies to bimolecular reactions with identical reagents and products, for which our approach allows a symmetry argument developed by Mead [J. Chem. Phys. 72, 3839 (1980)] to be generalized from nonencircling to encircling systems. The approach can be extended in order to unwind the wave function completely onto a higher cover space, thus separating contributions from individual winding numbers. The scattering boundary conditions are ultimately what allow the wave function to be unwound from the CI, and hence a bound state wave function cannot be unwound. The GP therefore has a much stronger effect on the latter than on the wave function of a reactive system.
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Affiliation(s)
- Stuart C Althorpe
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
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Juanes-Marcos JC, Althorpe SC. Geometric phase effects in the H+H2 reaction: Quantum wave-packet calculations of integral and differential cross sections. J Chem Phys 2005; 122:204324. [PMID: 15945741 DOI: 10.1063/1.1924411] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We report quantum wave-packet calculations on the H+H(2) reaction, aimed at resolving the controversy over whether geometric phase (GP) effects can be observed in this reaction. Two sets of calculations are reported of the state-to-state reaction probabilities, and integral and differential cross sections (ICSs and DCSs). One set includes the GP using the vector potential approach of Mead and Truhlar; the other set neglects the phase. We obtain unequivocal agreement with recent results of Kendrick [J. Phys. Chem. A 107, 6739 (2003)], predicting GP effects in the state-to-state reaction probabilities, which cancel exactly on summing the partial waves to yield the ICS. Our results therefore contradict those of Kuppermann and Wu [Chem. Phys. Lett. 349 537 (2001)], which predicted pronounced GP effects in the cross sections. We also agree with Kendrick in predicting that there are no significant GP effects in the full DCS at energies below 1.8 eV, and in the partial (0<or=J<or=10) DCS at energies above this. However, we find that in the full DCS above 1.8 eV (which was not reported by Kendrick), there are GP effects, which may be experimentally measurable.
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Aoiz * FJ, BaÑares L, Herrero VJ. The H+H2reactive system. Progress in the study of the dynamics of the simplest reaction. INT REV PHYS CHEM 2005. [DOI: 10.1080/01442350500195659] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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Kuppermann A. Hyperspherical Harmonics for Tetraatomic Systems. 2. The Weak Interaction Region. J Phys Chem A 2004. [DOI: 10.1021/jp048874l] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Aron Kuppermann
- Arthur Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
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Xia Y, Zhao M, Li F, Huang B, Tan Z, Liu X, Ji Y, Mei L. Recombination and Exchange Reactions of Hydrogen and Dihydrogen Molecular Condensation in Single-Walled Carbon Nanotubes. J Phys Chem B 2004. [DOI: 10.1021/jp049370q] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Affiliation(s)
- Yueyuan Xia
- Department of Physics, Shandong University, Jinan, Shandong, 250100 P. R. China, Department of Optoelectronics, Shandong University, Jinan,Shandong, 250100 P. R. China, and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 P. R. China
| | - Mingwen Zhao
- Department of Physics, Shandong University, Jinan, Shandong, 250100 P. R. China, Department of Optoelectronics, Shandong University, Jinan,Shandong, 250100 P. R. China, and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 P. R. China
| | - Feng Li
- Department of Physics, Shandong University, Jinan, Shandong, 250100 P. R. China, Department of Optoelectronics, Shandong University, Jinan,Shandong, 250100 P. R. China, and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 P. R. China
| | - Boda Huang
- Department of Physics, Shandong University, Jinan, Shandong, 250100 P. R. China, Department of Optoelectronics, Shandong University, Jinan,Shandong, 250100 P. R. China, and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 P. R. China
| | - Zhenyu Tan
- Department of Physics, Shandong University, Jinan, Shandong, 250100 P. R. China, Department of Optoelectronics, Shandong University, Jinan,Shandong, 250100 P. R. China, and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 P. R. China
| | - Xiangdong Liu
- Department of Physics, Shandong University, Jinan, Shandong, 250100 P. R. China, Department of Optoelectronics, Shandong University, Jinan,Shandong, 250100 P. R. China, and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 P. R. China
| | - Yanju Ji
- Department of Physics, Shandong University, Jinan, Shandong, 250100 P. R. China, Department of Optoelectronics, Shandong University, Jinan,Shandong, 250100 P. R. China, and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 P. R. China
| | - Liangmo Mei
- Department of Physics, Shandong University, Jinan, Shandong, 250100 P. R. China, Department of Optoelectronics, Shandong University, Jinan,Shandong, 250100 P. R. China, and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 P. R. China
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Pomerantz AE, Ausfelder F, Zare RN, Althorpe SC, Aoiz FJ, Banares L, Castillo JF. Disagreement between theory and experiment in the simplest chemical reaction: Collision energy dependent rotational distributions for H+D2→HD(ν′=3,j′)+D. J Chem Phys 2004; 120:3244-54. [PMID: 15268478 DOI: 10.1063/1.1641008] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We present experimental rotational distributions for the reaction H + D2 --> HD(nu' = 3,j') + D at eight different collision energies between 1.49 and 1.85 eV. We combine a previous measurement of the state-resolved excitation function for this reaction [Ayers et al., J. Chem. Phys. 119, 4662 (2003)] with the current data to produce a map of the relative reactive cross section as a function of both collision energy and rotational quantum number (an E-j' plot). To compare with the experimental data, we also present E-j' plots resulting from both time-dependent and time-independent quantum mechanical calculations carried out on the BKMP2 surface. The two calculations agree well with each other, but they produce rotational distributions significantly colder than the experiment, with the difference being more pronounced at higher collision energies. Disagreement between theory and experiment might be regarded as surprising considering the simplicity of this system; potential causes of this discrepancy are discussed.
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Affiliation(s)
- Andrew E Pomerantz
- Department of Chemistry, Stanford University, Stanford, California 94305-5080 USA
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Carlos Juanes-Marcos J, Althorpe SC. On the role of the conical intersection in H+H2 reactive scattering. Chem Phys Lett 2003. [DOI: 10.1016/j.cplett.2003.10.041] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
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Ayers JD, Pomerantz AE, Fernández-Alonso F, Ausfelder F, Bean BD, Zare RN. Measurement of the cross section for H+D2→HD(v′=3,j′=0)+D as a function of angle and energy. J Chem Phys 2003. [DOI: 10.1063/1.1595092] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
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Affiliation(s)
- Desheng Wang
- Arthur Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
| | - Aron Kuppermann
- Arthur Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
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Chao SD, Skodje RT. Time delay as a tool to identify the signatures of reactive resonance: F+HD and F+H2 reactions. J Chem Phys 2003. [DOI: 10.1063/1.1582172] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Abstract
This review discusses recent quantum scattering calculations on bimolecular chemical reactions in the gas phase. This theory provides detailed and accurate predictions on the dynamics and kinetics of reactions containing three atoms. In addition, the method can now be applied to reactions involving polyatomic molecules. Results obtained with both time-independent and time-dependent quantum dynamical methods are described. The review emphasises the recent development in time-dependent wave packet theories and the applications of reduced dimensionality approaches for treating polyatomic reactions. Calculations on over 40 different reactions are described.
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Harich SA, Dai D, Wang CC, Yang X, Chao SD, Skodje RT. Forward scattering due to slow-down of the intermediate in the H + HD --> D + H(2) reaction. Nature 2002; 419:281-4. [PMID: 12239562 DOI: 10.1038/nature01068] [Citation(s) in RCA: 162] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Quantum dynamical processes near the energy barrier that separates reactants from products influence the detailed mechanism by which elementary chemical reactions occur. In fact, these processes can change the product scattering behaviour from that expected from simple collision considerations, as seen in the two classical reactions F + H(2) --> HF + H and H + H(2) --> H(2) + H and their isotopic variants. In the case of the F + HD reaction, the role of a quantized trapped Feshbach resonance state had been directly determined, confirming previous conclusions that Feshbach resonances cause state-specific forward scattering of product molecules. Forward scattering has also been observed in the H + D(2) --> HD + D reaction and attributed to a time-delayed mechanism. But despite extensive experimental and theoretical investigations, the details of the mechanism remain unclear. Here we present crossed-beam scattering experiments and quantum calculations on the H + HD --> H(2) + D reaction. We find that the motion of the system along the reaction coordinate slows down as it approaches the top of the reaction barrier, thereby allowing vibrations perpendicular to the reaction coordinate and forward scattering. The reaction thus proceeds, as previously suggested, through a well-defined 'quantized bottleneck state' different from the trapped Feshbach resonance states observed before.
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Affiliation(s)
- Steven A Harich
- Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
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Aoiz FJ, Bañares L, Castillo JF, Sokolovski D. Energy dependence of forward scattering in the differential cross section of the H+D2→HD(v′=3,j′=0)+D reaction. J Chem Phys 2002. [DOI: 10.1063/1.1490920] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Abstract
Recent studies of state-resolved angular distributions show the participation of reactive scattering resonances in the simplest chemical reaction. This review is intended for those who wish to learn about the state-of-the-art in the study of the H + H2 reaction family that has made this breakthrough possible. This review is also intended for those who wish to gain insight into the nature of reactive scattering resonances. Following a tour across several fields of physics and chemistry where the concept of resonance has been crucial for the understanding of new phenomena, we offer an operational definition and taxonomy of reactive scattering resonances. We introduce simple intuitive models to illustrate each resonance type. We focus next on the last decade of H + H2 reaction dynamics. Emphasis is placed on the various experimental approaches that have been applied to the search for resonance behavior in the H + H2 reaction family. We conclude by sketching the road ahead in the study of H + H2 reactive scattering resonances.
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Affiliation(s)
- Félix Fernández-Alonso
- Istituto di Struttura della Materia-Consiglio Nazionale delle Ricerche, Area della Ricerca di Roma-Tor Vergata, 00133 Rome, Italy.
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Bean BD, Ayers JD, Fernández-Alonso F, Zare RN. State-resolved differential and integral cross sections for the reaction H+D2→HD(v′=3,j′=0–7)+D at 1.64 eV collision energy. J Chem Phys 2002. [DOI: 10.1063/1.1462576] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Althorpe SC, Fernández-Alonso F, Bean BD, Ayers JD, Pomerantz AE, Zare RN, Wrede E. Observation and interpretation of a time-delayed mechanism in the hydrogen exchange reaction. Nature 2002; 416:67-70. [PMID: 11882892 DOI: 10.1038/416067a] [Citation(s) in RCA: 137] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Extensive theoretical and experimental studies have shown the hydrogen exchange reaction H+H2 --> H2+H to occur predominantly through a 'direct recoil' mechanism: the H--H bonds break and form concertedly while the system passes straight over a collinear transition state, with recoil from the collision causing the H2 product molecules to scatter backward. Theoretical predictions agree well with experimental observations of this scattering process. Indirect exchange mechanisms involving H3 intermediates have been suggested to occur as well, but these are difficult to test because bimolecular reactions cannot be studied by the femtosecond spectroscopies used to monitor unimolecular reactions. Moreover, full quantum simulations of the time evolution of bimolecular reactions have not been performed. For the isotopic variant of the hydrogen exchange reaction, H+D2 --> HD+D, forward scattering features observed in the product angular distribution have been attributed to possible scattering resonances associated with a quasibound collision complex. Here we extend these measurements to a wide range of collision energies and interpret the results using a full time-dependent quantum simulation of the reaction, thus showing that two different reaction mechanisms modulate the measured product angular distribution features. One of the mechanisms is direct and leads to backward scattering, the other is indirect and leads to forward scattering after a delay of about 25 femtoseconds.
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