Steven A. Harich

Observation of Feshbach resonances in the F + H2 --> HF + H reaction.

Reaction resonances, or transiently stabilized transition-state structures, have proven highly challenging to capture experimentally. Here, we used the highly sensitive H atom Rydberg tagging time-of-flight method to conduct a crossed molecular beam scattering study of the F + H2 → HF + H reaction with full quantum-state resolution. Pronounced forward-scattered HF products in the v′ = 2 vibrational state were clearly observed at a collision energy of 0.52 kcal/mol; this was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF(v′ = 3)-H′ vibrationally adiabatic potential, with substantial enhancement by constructive interference between the two resonances.

Forward scattering due to slow-down of the intermediate in the H + HD --> D + H(2) reaction.

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 + H2 → HF + H and H + H2 → H2 + 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 + D2 → 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 → H2 + 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.

Photodissociation of D2O at 121.6 nm: A state-to-state dynamical picture

Photodissociation dynamics of H2O at 121.6 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique and by quasiclassical trajectory (QCT) calculations. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OH radical product as well as the state resolved angular anisotropy parameters were determined. The dissociation energy D00(H–OH) is determined to be 41151±5 cm−1. Two clear alternations in the OH(X,v=0) rotational distribution have been observed, with each alternation corresponding to an oscillation in the anisotropy distribution. These oscillations had been attributed to the dynamical interference between the two conical intersection pathways. Further theoretical modeling in this work strongly supports this argument. Very highly vibrationally excited OH(X) products (up to v=9) have also been observed. These are ascribed to interconversion of H–O–H bending (H–H vibration) and O–H vibration in...

The O(1D)+H2 reaction at 56 meV collision energy: A comparison between quantum mechanical, quasiclassical trajectory, and crossed beam results

Quantum mechanical and quasiclassical trajectory reactive scattering calculations have been performed for the O(1D)+H2 (v=0,j=0) reaction on the Dobbyn–Knowles ab initio 1 1A′ and 1 1A″ potential energy surfaces (PES) at the mean collision energy Ecol=56 meV (1.3 kcal/mol) of a crossed beam experimental study based on H-atom Rydberg “tagging” time-of-flight detection. Novel data from this latter experiment are presented and compared with the theoretical results at the level of state-resolved integral and differential cross sections and product recoil energy distributions. A good overall agreement with small discrepancies is found between the experimental data and the results of the two theoretical approaches. The main conclusion of the present work is that the contribution of the ground state 1 1A′ PES to the global reactivity accounts for the experimental observations and that, at the title collision energy, the participation of the 1 1A″ PES in the reaction is negligible for all practical purposes.

New low background crossed molecular beam apparatus: Low background detection of H2

A low background and almost hydrocarbon free (∼1×10−14 Torr) molecular beam apparatus with an improved universal detector, based on electron bombardment ionization, has been constructed for crossed molecular beam research. Extremely high vacuum (∼1×10−12 Torr) for the detector’s ionization region is achieved using multiple ultrahigh vacuum pumps. In addition to a home-made liquid nitrogen cryopump and a turbomolecular pump, a two stage cryogenic He cold head (∼10 K) is used to pump the detector’s ionization region. Using this arrangement, the H2 background in the detector can be reduced by about two orders of magnitude in comparison with previously built similar instruments. Therefore, the signal-to-noise for detecting H2 product detection sensitivity is substantially enhanced, making experimental studies of H2 elimination channels in photodissociation processes much easier. Backgrounds at m/e=28 (CO+), 16 (CH4+,O+), 15 (CH3+), 14 (CH2+), and 13 (CH+) in the ionization detection region are also significan...

A fully state-and angle-resolved study of the H+HD→D+ H2 reaction: Comparison of a molecular beam experiment to ab initio quantum reaction dynamics

We present the results of a joint experimental and theoretical investigation of the reaction dynamics of the H+HD→D+H2 chemical reaction. The experiment was performed using a crossed molecular beam apparatus that employed the Rydberg-atom time-of-flight detection scheme for the product D atom. The photolysis of a HI precursor molecule produced a beam source of hot H atoms, which, when crossed with a cold HD beam, yielded two well-defined center-of-mass collision energies, EC=0.498 and 1.200 eV. The resolution of the experiment was sufficient to allow the measurement of the rovibrationally state-resolved differential cross section from the ground state of the HD reagent. The reaction was modeled theoretically using a converged coupled channel scattering calculation employing the BKMP2 potential energy surface: The S matrix was computed on a grid of 56 energies in the range EC=0.245–1.551 eV. It is found that the experimental and theoretical state-to-state differential cross sections are in quantitative agr...

Dynamics of the O(1D)+CH4 reaction: Atomic hydrogen channel vs molecular hydrogen channel

The O(1D)+CH4 reaction has been investigated using a new universal crossed molecular beam apparatus. Both the atomic hydrogen channel (CH3O/CH2OH+H) and the molecular hydrogen channel (H2CO/HCOH+H2) have been experimentally observed in this reaction. The experimental results suggest that the main atomic hydrogen channel in the O(1D)+CH4 reaction should be CH2OH (hydroxymethyl)+H, while the CH3O (methoxy)+H channel is at most a minor process. From the product angular distribution measurements, it is clear that the radical products (CH2OH and/or CH3O) in the hydrogen atom channel are only slightly backward scattered relative to the O(1D) beam direction, indicating that this product channel mainly goes through a long-lived intermediate pathway. The slightly backward scattered products are possibly due to other reaction mechanisms. For the molecular hydrogen channel, the product angular distribution obtained from simulation also seems isotropic, implying that this channel also likely goes through a long-lived...

Quantum state specific dynamics for the O(1D)+HD→OD+H reaction

Rydberg “tagging” time-of-flight (TOF) techniques has been applied to the investigation of the O(1D)+HD→OD+H reaction with very high translational energy resolution and sensitivity. High resolution TOF spectra for the H atom product at different laboratory angles have been measured for the first time, making the determination of the OD product quantum state distributions at different scattering angles possible. Preliminary experimental results show that different vibrationally excited OD products at low rotational levels have strikingly different angular distributions, indicating that intriguing dynamics are involved in this reaction. The experimental results in this work also provide a solid test ground for quantitative theoretical investigations of this benchmark system for insertion mechanism.

Photodissociation dynamics of H2O at 121.6 nm: Effect of parent rotational excitation on reaction pathways

Photodissociation dynamics of H2O at 121.6 nm through the B 1A1′ state have been studied using the high-resolution H atom Rydberg tagging technique. Experimental evidences show two different dissociation pathways to form the ground OH (X,v=0) products: dissociation through the B–X conical intersection, and dissociation through B–A Coriolis coupling. By preparing the H2O molecules at higher rotational temperatures, dissociation through the B–A Coriolis coupling pathway can be enhanced.

State-to-state dynamics of H+HD→H2+D at 0.5 eV: A combined theoretical and experimental study

The state-to-state dynamics of the H+HD→H2+D reaction at a collisional energy of 0.5 eV was studied experimentally using a crossed molecular beam apparatus using the H atom Rydberg “tagging” time-of-flight detection scheme, and modeled theoretically using a converged quantum scattering calculation on the ground potential energy surface. The experimental results agreed very well with the theoretical calculations without considering the geometrical phase at this collisional energy. The results indicate that the H+HD reaction at this energy is dominated by a simple rebound mechanism along a collinear reaction path. Both experimental and theoretical results show that the rotational state distribution of the H2 product is strongly influenced by nuclear spin statistics.