arXiv subjects

Cangtao Yin

Publications and source records attributed to Cangtao Yin.

11 recordsLinked to original sources

arXiv2026

Charge-partition pathways in strong-field photoionization of carbonyl sulfide monomers and dimers

Strong-field photoionization of molecules and molecular clusters gives rise to a rich variety of fragmentation pathways governed by charge localization and redistribution on ultrafast timescales. Here, we report a velocity-map imaging study of the strong-field photoionization and fragmentation of carbonyl sulfide (OCS) monomers and dimers driven by 150 femtosecond (fs) laser pulses at 775~nm. The images of the total kinetic-energy and angular distributions of the OCS$^{2+}$, S$^+$, and CO$^+$ fragments were interpreted with the help of electronic-structure calculations of the potential energy surfaces for OCS$^+$ and OCS$^{2+}$. We identify distinct dissociation pathways of singly and doubly ionized OCS, including two-body breakup channels of OCS$^+$ into $\mathrm{S}^+ + \mathrm{CO}$ and $\mathrm{CO}^+ + \mathrm{S}$, dissociation of OCS$^{2+}$ into $\mathrm{S}^+ + \mathrm{CO}$$^+$ as well as higher-order three-body fragmentation. In addition, the images of the OCS$^{2+}$ channel exhibit near-zero-momentum components, low-energy isotropic features, and highly anisotropic contributions at high kinetic energies that cannot be explained by monomer ionization alone. Analysis of the KER distributions and angular anisotropies indicates that these features originate from the breakup of multiply charged OCS dimers ((OCS)$_2^{2+}$, (OCS)$_2^{3+}$, and (OCS)$_2^{4+}$) through charge-separation channels. Our results illustrate how dynamic signatures of strong-field fragmentation evolve from intramolecular dissociation in isolated molecules to intermolecular charge separation in weakly bound clusters providing a unified picture of charge-driven dissociation dynamics beyond the single-molecule limit.

physics.chem-ph↗

arXiv2026

Reaction Pathway Detection using Machine-Learned Energy Potentials -- Decomposition of Energized CF$_3$CHOO

Characterization of the decomposition products of energized Criegee intermediates is essential for assessing their impact on the chemical evolution of the atmosphere. Here, a generic and microscopically resolved approach is used to determine the molecular fragmentation pathways and products for CF$_3$CHOO. They include, among others, direct formation of CO$_2$ + CHF$_3$ (HFC-23), HF + CO$_2$ + CF$_2$, and fragmentation routes that are not evident from static reaction path calculations alone. The computed probability for formation of HFC-23 of 14 \% qualitatively agrees with a value of $(7.9^{+0.4}_{-0.2})$ \% from recent measurements, given the differences in the two approaches. Non-statistical dynamics is found for almost all decomposition pathways and the simulations show that excess energy can redirect reaction outcomes away from minimum-energy pathways. The results highlight the power of machine-learned PESs to elucidate multi-step reaction mechanisms of atmospherically relevant intermediates beyond traditional Master equation/electronic structure approaches to provide molecular-level understanding of the role of dynamics.

physics.chem-ph↗

arXiv2026

Full-Dimensional Reactive Potential Energy Surfaces for OCS$^+$ $\rightarrow$ CO+S$^+$ Dissociation: Ground and Excited States

Full-dimensional reactive potential energy surfaces (PESs) for the OCS$^+$ cation are constructed to describe S$^+$ loss in the electronic ground state and seven low-lying electronically excited states. High-level \textit{ab initio} reference energies were computed at the MRCI+Q/aug-cc-pVTZ level and were used to generate PESs employing reproducing kernel Hilbert space representations (RKHS). The PESs accurately reproduce the measured dissociation limits to CO(X$^1Σ^+$)+S$^+$ in different electronic states. The topology of the PESs reveals multiple linear and T-shaped minima, pronounced angular anisotropy, and state-crossing manifolds. Exploratory quasi-classical trajectory simulations on selected PESs confirm numerical stability and energy conservation, illustrating the suitability of the surfaces for dynamical applications. The present work represents the most comprehensive characterization to date of the lowest PESs of OCS$^+$ and provides a reliable foundation for future studies of the photodissociation of OCS$^+$ and the chem-ionization dynamics of OCS.

physics.chem-ph↗

arXiv2026

Full Reaction Pathway Dynamics for Atmospheric Decomposition Reactions: The Photodissociation of H$_2$COO

Branching ratios for fragmentation channels of important meta- and unstable species are essential for a molecular-level characterization of atmospheric chemistry. Here, the molecular product channels for the decomposition dynamics of the smallest Criegee intermediate, H$_2$COO, are quantitatively investigated. Using a high-quality, full-dimensional machine learned potential energy surface (CASPT2/aug-cc-pVTZ), the translational, rotational, and vibrational energy distributions of the CO$_2$+H$_2$, H$_2$O+CO, and HCO+OH fragmentation channels were analyzed to elucidate partitioning of the available energy. The CO$_2$ + H$_2$ product forms through two different pathways that bifurcate after formation of the OCH$_2$O intermediate. Along the direct pathway, CO$_2$ is preferentially vibrationally excited with H$_2$in its vibrational ground state, whereas for the indirect pathway going through formic acid, H$_2$ can populate levels with $v > 0$. For all product channels passing through energized formic acid, the lifetime distributions are described by stretched exponentials with $β$ ranging from 1.1 to 1.7. This is a clear signature of non-RRKM effects and suggests that the explicit molecular dynamics needs to be followed for a quantitative and realistic description of the photodissociation dynamics.

physics.chem-ph↗

arXiv2025

Structure and Spectroscopy of Criegee Intermediates in Gas- and Aqueous Environments

The dynamics and spectroscopy of the small (H$_2$COO) and large (CH$_3$CHOO) Criegee intermediates (CIs) in the gas phase, inside/on water droplets, on amorphous solid water (ASW) and in bulk water are investigated using validated energy functions. For both species, facile diffusion between surface and inside positions for water droplets are found whereas on amorphous solid water at low temperatures (50 K) no surface diffusion is observed on the multiple-nanosecond time scale. This is at variance with other species, such as CO or NO on ASW. The infrared spectroscopy of both CIs in contact with an aqueous environment leads to shifts of the spectral features on the order of a few to a few tens of cm$^{-1}$, depending on the vibrational mode considered. This is consistent with Stark-induced spectral shifts for small molecules in protein environments. However, the spectroscopy of both CIs in contact with water droplets does not depend on the positioning relative to the droplet (inside vs. surface).

physics.chem-ph↗

arXiv2025

End-to-End Photodissociation Dynamics of Energized H$_2$COO

The end-to-end dynamics of the smallest energized Criegee intermediate, H$_2$COO, was characterized for vibrational excitation close to and a few kcal/mol above the barrier for hydrogen transfer. From an aggregate of at least 5 $μ$s of molecular dynamics simulations using a neural network-representation of CASPT2/aug-cc-pVTZ reference data, the branching ratios into molecular products HCO+OH, CO$_2$+H$_2$, or H$_2$O+CO was quantitatively determined. Consistent with earlier calculations and recent experiments, decay into HCO+OH was found to be rare $(\sim 2 \%)$ whereas the other two molecular product channels are accessed with fractions of $\sim 30 \%$ and $\sim 20 \%$, respectively. On the 1 ns time scale, which was the length of an individual MD simulation, more than 40 \% of the systems remain in the reactant state due to partial intramolecular vibrational redistribution (IVR). Formation of CO$_2$+H$_2$ occurs through a bifurcating pathway, one of which passes through formic acid whereas the more probable route connects the di-radical OCH$_2$O with the product through a low-lying transition state. Notably, none of the intermediates along the pathway accumulate and their maximum concentration always remains well below 5 \%. This work demonstrates that atomistic simulations with global reactive machine-learned energy functions provide a quantitative understanding of the chemistry and reaction dynamics for atmospheric reactions in the gas phase.

physics.chem-ph↗

arXiv2014

The power-law reaction rate coefficient for barrierless reactions

The power-law reaction rate coefficient for the barrierless reactions is studied if the reactions take place in systems with power-law distributions, and a generalized rate formula for the barrierless reactions in Gorin model is derived. We show that due to barrierless, different from those for bimolecular and unimolcular reactions, the power-law rate coefficient for the barrierless reactions does not have the factor of power-law distribution function and thus it is not very strongly dependent on the nu-parameter. Four barrierless reactions are taken as the application examples to calculate the new rate coefficients, which with larger fitting nu-parameters can be exactly in agreement with the measurement values in experimental studies.

physics.chem-ph↗

arXiv2014

The power-law TST reaction rate coefficient with tunneling correction

We study the TST reaction rate for the systems with power-law distributions. We derive the expressions of the reaction rate coefficient with tunneling correction, which strongly depends on the power-law parameter. The numerical results show that a small deviation from one in the parameter can result in a significant change in the rate coefficient, but only cause a small change in the tunneling correction. Thus the tunneling correction is not sensitive to the power-law distributions. As an application example, we take the hydrogen reaction to calculate the power-law reaction rate coefficient with the tunneling correction, the results of which with the parameter slightly different from one are in good agreement with all the experimental studies in temperature range 200~1000K.

physics.chem-ph↗

arXiv2014

The rate coefficients of unimolecular reactions in the systems with power-law distributions

The rate coefficient formulae of unimolecular reactions are generalized to the systems with the power-law distributions based on nonextensive statistics, and the power-law rate coefficients are derived in the high and low pressure limits, respectively. The numerical analyses are made of the rate coefficients as functions of the nu-parameter, the threshold energy, the temperature and the number of degrees of freedom. We show that the new rate coefficients depend strongly on the nu-parameter different from one (thus from a Boltzmann-Gibbs distribution). Two unimolecular reactions are taken as application examples to calculate their power-law rate coefficients, which obtained with the nu-parameters slightly different from one can be exactly in agreement with all the experimental studies on these two reactions in the given temperature ranges.

physics.chem-ph↗

arXiv2014

The collision theory reaction rate coefficient for power-law distributions

The collision theory for power-law distributions and a generalized collision theory rate coefficient is studied when the reactions take place in nonequilibrium systems with power-law distributions. We obtain the power-law rate coefficient and by numerical analyses we show a very strong dependence of the rate coefficient on the power-law parameter. We find that the power-law collision theory can successfully overcome the two difficulties of Lindemann-Christiansen mechanism. We take three reactions as examples to calculate the pre-exponential factor and yield the values that can be exactly in agreement with those measured in the experimental studies.

cond-mat.stat-mech↗

arXiv2013

The power-law reaction rate coefficient for an elementary bimolecular reaction

The power-law TST reaction rate coefficient for an elementary bimolecular reaction is studied when the reaction takes place in a nonequilibrium system with power-law distributions. We derive a generalized TST rate coefficient, which not only depends on a power-law parameter but also on the reaction coordinate frequency of transition state. The numerical analyses show a very strong dependence of TST rate coefficient on the power-law parameter, and clearly indicate that a tiny deviation from unity in the parameter (thus from the Boltzmann-Gibbs distribution) would result in significant changes in the rate coefficient. We take an elementary F+H2 reaction as an application example to calculate the reaction rate coefficient, and yield the rate values being exactly agreement with the measurement values in all the experimental studies in temperature range 190~765K.

physics.chem-ph↗