Biomedical subjects

Dong H Zhang

Publications and source records attributed to Dong H Zhang.

9 recordsLinked to original sources

PubMed2006

A test of the continuous configuration time-dependent self-consistent field (CC-TDSCF) method on the H + CH4 reaction.

The continuous configuration time-dependent self-consistent field (CC-TDSCF) method is employed to calculate the flux-flux autocorrelation functions for the H + CH4 reaction on the potential energy surface recently developed by Manthe and co-workers. We include up to 10 out of the total 12 degrees of freedom in our calculations, only with the doubly degenerate bending modes involving the motion of the hydrogens in nonreacting CH3 group excluded. Comparison of flux-flux autocorrelation functions obtained by using the exact dynamics method and the CC-TDSCF method shows that the CC-TDSCF method is capable of producing very accurate results. Our calculations clearly reveal that the CC-TDSCF method is a powerful approximation quantum dynamics method. It allows us to partition a big problem into several smaller ones. By changing partition systematically, one can investigate the correlations between different degrees of freedom. By grouping modes with strong correlations together as a cluster, one can systematically improve accuracy of the result.

Computer Simulation↗

PubMed2006

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.

Electrons↗

PubMed2006

A full dimensional time-dependent wave packet study for the H4 four-center, collision induced dissociation, and single exchange reactions: reaction probabilities for J=0.

A time-dependent initial state selected wave packet method has been developed to study the H2(v(1)=10-11,j1=0)+H2'(v2=0,j2=0)-->HH'+HH' four-center (4C) reaction, and two other competing reactions: the H2+H2'-->H+H+H2' collision induced dissociation (CID) and the H2+H2'-->H+HH'+H' single exchange (SE) reaction, in full six dimensions. Initial state-specific total reaction probabilities for these three competing reactions are presented for total angular momentum J=0 and the effects of reagent vibration on reactions are examined. It is found that (a) the CID process is the dominant process over the whole energy range considered in this study, but the 4C and SE processes also have non-negligible probabilities; (b) the SE process has a lower threshold energy than the 4C process, but the SE probability increases slower than the 4C probability as collision energy increases; (c) the vibrational excitation of H2(v1) is much more efficient than translational motion for promoting these processes, in particular to the CID process.

Journal Article↗

PubMed2005

Interpolation of diabatic potential-energy surfaces: quantum dynamics on ab initio surfaces.

A method for constructing diabatic potential-energy matrices from ab initio quantum chemistry data is described and tested for use in exact quantum reactive scattering. The method is a refinement of that presented in a previous paper, in that it accounts for the presence of the nonremovable derivative coupling. The accuracy of quantum dynamics on this type of diabatic potential is tested by comparison with an analytic model and for an ab initio description of the two lowest-energy states of H3.

Journal Article↗

PubMed2005

Continuous configuration time-dependent self-consistent field method for polyatomic quantum dynamical problems.

A new continuous configuration time-dependent self-consistent field method has been developed to study polyatomic dynamical problems by using the discrete variable representation for the reaction system, and applied to a reaction system coupled to a bath. The method is very efficient because the equations involved are as simple as those in the traditional single configuration approach, and can account for the correlations between the reaction system and bath modes rather well.

Journal Article↗

PubMed2004

Coherent classical-path description of deep tunneling.

A central challenge to the semiclassical description of quantum mechanics is the quantum phenomenon of "deep" tunneling. Here we show that real time classical trajectories suffice to account correctly even for deep quantum tunneling, using a recently formulated semiclassical initial value representation series of the quantum propagator and a prefactor free semiclassical propagator. Deep quantum tunneling is effected through what we term as coherent classical paths which are composed of one or more classical trajectories that lead from reactant to product but are discontinuous along the way. The end and initial phase space points of consecutive classical trajectories contributing to the coherent path are close to each other in the sense that the distance between them is weighted by a coherent state overlap matrix element. Results are presented for thermal and energy dependent tunneling through a symmetric Eckart barrier.

Journal Article↗

PubMed2002

State-to-State integral cross section for the H+H2O-->H2+OH abstraction reaction.

The initial state selected time-dependent wave-packet method was extended to calculate the state-to-state integral cross section for the title reaction with H2O in the ground rovibrational state on the potential energy surface of Yang, Zhang, Collins, and Lee. One OH bond length was fixed in the study, which is justifiable for the abstraction reaction, but the remaining 5 degrees of freedom were treated exactly. It was found that the H2 molecule is produced vibrationally cold for collision energy up to 1.6 eV. The OH rotation takes away about 4% of total available energy in the products, while the fraction of energy going to H2 rotation increases with collision energy to about 20% at 1.6 eV.

Journal Article↗

PubMed2002

Probing the transition state via photoelectron and photodetachment spectroscopy of H(3)O(-).

The H(3)O(-) anion has stable and metastable structures that resemble configurations in the vicinity of the transition state for the neutral reactions OH + H(2) <--> H(2)O + H. Photoelectron spectroscopy of this anion probes the neutral reaction dynamics in the critical transition-state region. Accurate quantum dynamics calculations of the photoelectron intensity and photodissociation product energies are shown to provide a quantitatively reliable means of interpreting such experimental observations and reveal a detailed picture of the reaction dynamics.

Journal Article↗

PubMed2002

Breakdown of the spectator model for the OH bonds in studying the H+H2O reaction.

The time-dependent wave packet method is used to study the exchange and abstraction processes for the H+H2O reaction with both OH bonds in the H2O reactant treated as reactive bonds in full dimension. The calculation clearly shows that it is necessary to treat both OH bonds in this way in order to accurately investigate the exchange process. However, for the abstraction process, the spectator model works very well. Nonreactive treatment of one OH bond by using a few vibrational basis functions or even freezing the bond can yield very accurate abstraction reaction probability.

Journal Article↗