Chunlei Xiao

Experimental and Theoretical Differential Cross Sections for a Four-Atom Reaction: HD + OH → H2O + D

A theoretical analysis of a four-atom reaction has a level of detail and accuracy previously restricted to three-atom systems. Quantum dynamical theories have progressed to the stage in which state-to-state differential cross sections can now be routinely computed with high accuracy for three-atom systems since the first such calculation was carried out more than 30 years ago for the H + H2 system. For reactions beyond three atoms, however, highly accurate quantum dynamical calculations of differential cross sections have not been feasible. We have recently developed a quantum wave packet method to compute full-dimensional differential cross sections for four-atom reactions. Here, we report benchmark calculations carried out for the prototypical HD + OH → H2O + D reaction on an accurate potential energy surface that yield differential cross sections in excellent agreement with those from a high-resolution, crossed–molecular beam experiment.

Transition-State Spectroscopy of Partial Wave Resonances in the F + HD Reaction

Partial View Skilled billiard players can easily predict how spinning of one ball will affect the trajectory of the second ball it strikes in a collision. In principle, quantum mechanics can be used to predict the analogous impact of the angular momentum of reagents on the outcome of a chemical reaction. In practice, however, observation of most chemical reactions—even in the confines of a molecular beam apparatus—encompasses a vast number of collisions over multiple angular momentum distributions. Dong et al. (p. 1501; see the Perspective by Althorpe) have honed their spectroscopic resolution sufficiently to distinguish the impact of subtle angular momentum variations on the reactivity of fluorine with hydrogen atoms. Their data agree with theory and reveal oscillating peaks in reaction probability, termed partial wave resonances. Spectroscopy can distinguish the reaction paths in a collision between an atom and a diatomic system. Partial wave resonances, quasi-bound resonance states with well-defined rotation in the transition state region of a chemical reaction, play a governing role in reaction dynamics but have eluded direct experimental characterization. Here, we report the observation of individual partial wave resolved resonances in the F + HD → HF + D reaction by measuring the collision energy–dependent, angle- and state-resolved differential cross section with extremely high resolution, providing a spectroscopic probe to the transition state of F + HD → HF + D. The agreement of the data with the high-level theoretical calculations confirms the sensitivity of this probe to the subtle quantum mechanical factors guiding this benchmark reaction.

The Extent of Non–Born-Oppenheimer Coupling in the Reaction of Cl(2P) with para-H2

Elementary triatomic reactions offer a compelling test of our understanding of the extent of electron-nuclear coupling in chemical reactions, which is neglected in the widely applied Born-Oppenheimer (BO) approximation. The BO approximation predicts that in reactions between chlorine (Cl) atoms and molecular hydrogen, the excited spin-orbit state (Cl*) should not participate to a notable extent. We report molecular beam experiments, based on hydrogen-atom Rydberg tagging detection, that reveal only a minor role of Cl*. These results are in excellent agreement with fully quantum-reactive scattering calculations based on two sets of ab initio potential energy surfaces. This study resolves a previous disagreement between theory and experiment and confirms our ability to simulate accurately chemical reactions on multiple potential energy surfaces.

Dynamical resonances accessible only by reagent vibrational excitation in the F + HD->HF + D reaction.

Access via Vibration Molecular beam studies over the past decade have elucidated many subtle quantum mechanical factors governing the influence of vibrational excitation on the outcome of elementary chemical reactions. However, these studies have generally had to focus on reagents that can be easily made to vibrate by direct absorption in the infrared (IR). Wang et al. (p. 1499) show that a variation on stimulated Raman pumping can efficiently excite the IR-inactive stretch vibration in the diatomic molecule, hydrogen deuteride (HD). As a result, they can probe the influence of that vibration on the outcome of the HD + F reaction. Through a combined spectroscopic and theoretical investigation, they uncover Feshbach resonances along the reaction coordinate that are only accessible through vibrational preexcitation. A protocol for efficient vibrational excitation enabled discovery of an unusual reaction trajectory in a well-studied system. Experimental limitations in vibrational excitation efficiency have previously hindered investigation of how vibrational energy might mediate the role of dynamical resonances in bimolecular reactions. Here, we report on a high-resolution crossed-molecular-beam experiment on the vibrationally excited HD(v = 1) + F → HF + D reaction, in which two broad peaks for backward-scattered HF(v′ = 2 and 3) products clearly emerge at collision energies of 0.21 kilocalories per mole (kcal/mol) and 0.62 kcal/mol from differential cross sections measured over a range of energies. We attribute these features to excited Feshbach resonances trapped in the peculiar HF(v′ = 4)–D vibrationally adiabatic potential in the postbarrier region. Quantum dynamics calculations on a highly accurate potential energy surface show that these resonance states correlate to the HD(v′ = 1) state in the entrance channel and therefore can only be accessed by the vibrationally excited HD reagent.

Extremely short-lived reaction resonances in Cl + HD (v = 1) → DCl + H due to chemical bond softening

A few very brief pauses in the action Chemical reactions proceed by the cumulative effect of trillions upon trillions of collisions between atoms and molecules. Usually, a given collision bounces the participants right back out again, either in their original form or with the atoms shuffled around into distinct products. In certain cases, the reacting partners experience a brief lull, termed a resonance, before they rearrange. Yang et al. report the discovery of particularly short-lived resonances in certain reactive collisions of chlorine atoms with vibrationally excited hydrogen deuteride (HD). Their results suggest that similar, as yet overlooked, resonances may lurk in other reactions of vibrationally excited molecules. Science, this issue p. 60 Molecular beam spectroscopy reveals unanticipated transient states along particular pathways of a well-studied reaction. The Cl + H2 reaction is an important benchmark system in the study of chemical reaction dynamics that has always appeared to proceed via a direct abstraction mechanism, with no clear signature of reaction resonances. Here we report a high-resolution crossed–molecular beam study on the Cl + HD (v = 1, j = 0) → DCl + H reaction (where v is the vibrational quantum number and j is the rotational quantum number). Very few forward scattered products were observed. However, two distinctive peaks at collision energies of 2.4 and 4.3 kilocalories per mole for the DCl (v′ = 1) product were detected in the backward scattering direction. Detailed quantum dynamics calculations on a highly accurate potential energy surface suggested that these features originate from two very short-lived dynamical resonances trapped in the peculiar H-DCl (v′ = 2) vibrational adiabatic potential wells that result from chemical bond softening. We anticipate that dynamical resonances trapped in such wells exist in many reactions involving vibrationally excited molecules.

A surface femtosecond two-photon photoemission spectrometer for excited electron dynamics and time-dependent photochemical kinetics.

A surface femtosecond two-photon photoemission (2PPE) spectrometer devoted to the study of ultrafast excited electron dynamics and photochemical kinetics on metal and metal oxide surfaces has been constructed. Low energy photoelectrons are measured using a hemispherical electron energy analyzer with an imaging detector that allows us to detect the energy and the angular distributions of the photoelectrons simultaneously. A Mach–Zehnder interferometer was built for the time-resolved 2PPE (TR-2PPE) measurement to study ultrafast surface excited electron dynamics, which was demonstrated on the Cu(111) surface. A scheme for measuring time-dependent 2PPE (TD-2PPE) spectra has also been developed for studies of surface photochemistry. This technique has been applied to a preliminary study on the photochemical kinetics on ethanol/TiO2(110). We have also shown that the ultrafast dynamics of photoinduced surface excited resonances can be investigated in a reliable way by combining the TR-2PPE and TD-2PPE techniques.

The dynamics of the D2 + OH --> HOD + D reaction: a combined theoretical and experimental study.

A combined theoretical and experimental study has been carried out to show the current status of comparison between experiment and theory on the title reaction. Differential cross sections and product relative translational energy distributions at collision energies of 0.25 and 0.34 eV, as well as the collision energy dependence of differential cross section in the backward direction have been measured by using crossed molecular beam experiment with D-atom Rydberg tagging technique. Theoretically, the time-dependent wave packet method has been employed to calculate state-to-state differential cross sections for the title reaction in full dimension. It is found that the experimental observations are in good accord with those of Davis and coworkers at the collision energy of 0.28 eV [Science, 290, 958 (2000)]. The overall agreement between theory and experiment on this benchmark four-atom reaction is good, but not perfect. Further studies, both theoretical and experimental, are called to bring a complete agreement between theory and experiment on the reaction.

Surface Photocatalysis-TPD Spectrometer for Photochemical Kinetics

A surface photocatalysis-TPD apparatus devoted to studying kinetics and mechanism of photocatalytic processes with various signal crystal surfaces has been constructed. Extremely high vacuum (~0.2 nPa) in the ionization region is obtained by using multiple ultrahigh vacuum pumps. Compared with similar instruments built previously by others, the H2, CH4 background in the ionization region can be reduced by about two orders of magnitude, and other residual gases in the ionization region can be reduced by about an order of magnitude. Therefore, the signal-to-noise ratio for the temperature programmed desorption (TPD) and time of flight (TOF) spectra is substantially enhanced, making experimental studies of photocatalytic processes on surfaces much easier. In this work, we describe the new apparatus in detail and present some preliminary studies on the photo-induced oxygen vacancy defects on TiO2(110) at 266 nm by using the TPD and TOF methods. Preliminary results suggest that the apparatus is a powerful tool for studying kinetics and mechanism of photochemical processes.

Highly Efficient Pumping of Vibrationally Excited HD Molecules via Stark-Induced Adiabatic Raman Passage.

A primary prerequisite to study reactivity of vibrationally excited species is to efficiently prepare reacting species in a well-defined vibrational level. Efficient pumping of IR active vibrational modes in a molecule can be achieved by direct IR absorption. For vibrational modes that are only Raman active, however, efficient preparation of vibrationally excited states in those modes is not easily attainable. In this work, we have shown that highly efficient preparation of the HD(v = 1) state using the Stark-induced adiabatic Raman passage (SARP) scheme is feasible. As high as 91% population transfer from v = 0 to 1 of HD has been demonstrated in our experiment. This method provides new opportunities for future experimental studies on the dynamics of vibrational state molecules, especially H2, in both gas-phase and beam-surface reactions.

Efficient Coherent Population Transfer of D2 Molecules by Stark‐Induced Adiabatic Raman Passage

Preparation of a high flux of hydrogen molecules in a specific vibrationally excited state is the major prerequisite and challenge in scattering experiments that use vibrationally excited hydrogen molecules as the target. The widely used scheme of stimulated Raman pumping suffers from coherent population return which severely limits the excitation efficiency. Recently we successfully transferred D2 molecules in the molecular beam from (υ=0, J=0) to ( υ=1, J=0) level, with the scheme of Stark‐induced adiabatic Raman passage. As high as 75% of the excitation efficiency was achieved. This excitation technique promise to be a unique tool for crossed beam and beam‐surface scattering experiments which aim to reveal the role of vibrational excitation of hydrogen molecules in the chemical reaction.