Dissociation Dynamics Research Articles

Jahn-Teller Effect on CF3I Photodissociation Dynamics.

The Jahn-Teller (JT) effect, as a spontaneous symmetry-breaking mechanism arising from the coupling between electronic and nuclear degrees of freedom, is a widespread phenomenon in molecular and condensed matter systems. Here, we investigate the influence of the JT effect on the photodissociation dynamics of CF3I molecules. Based on ab initio calculation, we obtain the three-dimensional potential energy surfaces for 3Q0+ and 1Q1 states and establish a diabatic Hamiltonian model to study the wavepacket dynamics in the CF3I photodissociation process. Using the wave function of the final state after dissociation, we calculate the rotational density matrix of the CF3 fragment and analyze its rotational excitation under the JT effect, as well as its partial coherence property and selection rules. Our work paves the way to the experimental observation and quantification of the JT effect in molecular dissociation dynamics beyond the classical ball-and-stick model.

A multimass velocity-map and covariance-map imaging study of the dissociative electron ionisation dynamics of carbonyl sulphide (OCS)

Abstract We present the results of a velocity-map imaging study into the dissociative electron ionisation dynamics of OCS at electron energies of 50, 75, and 100 eV, supported by new EOM-CCSD and CASSCF potential energy surface calculations for the OCS+ cation and OCS2+ dication. We report partial ionisation cross-sections and scattering distributions for the observed ionic fragments, and interpret these in terms of the dissociation dynamics of the singly and doubly charged OCS parent ions formed in the electron-molecule collisions. For many of the dissociation channels, consideration of the calculated potential energy surfaces supplemented by comparison with data from photoionisation studies provides insight into the electronic potential energy surfaces involved. For fragmentation products arising from dissociation of the dication, time-of-flight and recoil-frame covariance maps provide mechanistic insight into the stepwise dissociation mechanisms leading to these products and support previous conclusions by other authors.

Isolated spin polarized hydrogen atoms as targets for laser-induced polarized electron acceleration

High density Spin Polarized Hydrogen (SPH) atoms, which can be prepared using UV dissociation of hydro-halide molecules, can be attractive as potential targets for laser ionization/acceleration schemes aiming to create high energy and high current polarized electron beams. However, for these SPH targets to be of practical use, they have to be spatially isolated from the halide atoms which accompany hydrogen in the parent hydro-halide molecule. We show how the UV dissociation dynamics of hydro-halides and the dissociation geometry and timing can be combined to prepare a variety of isolated SPH targets aimed to accommodate laser acceleration schemes.

Photodissociation Spectroscopy and Photofragment Imaging of the Mg+(Benzene) Complex

Photodissociation Spectroscopy and Photofragment Imaging of the Mg⁺(Benzene) Complex

Vibrational ladder climbing dissociation of LiH molecules excited by non-chirped pulses

Vibrational ladder climbing (VLC) is one of the advanced methods for achieving molecular dissociation, usually requires the use of chirped laser pulses to adapt to the change of energy difference between molecular vibrational levels. In this work, a scheme is proposed for using non-chirped pulses to couple the vibrational rotational levels of excited state to realize VLC dissociation of LiH molecules in the excited state. The first pulse induces population excitation to the excited state A1Σ+, while the second pulse is used to drive the excited state vibrational levels population up step by step until dissociation occurs. The non-chirped pulse VLC dissociation dynamics is investigated using the time-dependent quantum wave packet theory method. The “”climb steps” process for excited state molecules at low full width at half maximum (FWHM) of the second laser pulse is demonstrated. The frequency has an obvious influence on the distribution of excited state vibrational rotational levels and further affects the relaxation process of the undissociated part and the angular distribution of dissociation products. It is also explained that the anomalous decrease of dissociation probability is caused by vibrational levels oscillation.

Different Photodissociation Mechanisms in Fe(CO)5 and Cr(CO)6 Evidenced with Femtosecond Valence Photoelectron Spectroscopy and Excited-State Molecular Dynamics Simulations.

Measured and calculated time-resolved photoelectron spectra and excited-state molecular dynamics simulations of photoexcited gas-phase molecules Fe(CO)5 and Cr(CO)6 are presented. Samples were excited with 266 nm pump pulses and probed with 23 eV photons from a femtosecond high-order harmonic generation source. Photoelectron intensities are seen to blue-shift as a function of time from binding energies characteristic of bound electronic excited states via dissociated-state energies toward the energies of the dissociated species for both Fe(CO)5 and Cr(CO)6, but differences are apparent. The excited-state and dissociation dynamics are found to be faster in Cr(CO)6 because the repopulation from bound excited to dissociative excited states is faster. This may be due to stronger coupling between bound and dissociative states in Cr(CO)6, a notion supported by the observation that the manifolds of bound and dissociative states overlap in a narrow energy range in this system.

1H and 13C NMR and FTIR Spectroscopic Analysis of Formic Acid Dissociation Dynamics in Water.

The formation and transport of ionic charges in formic acid-water (HCOOH-H2O) mixtures with initial water mole fractions ranging from XH2Oi = 0 to 1 were investigated using 13C and 1H NMR, FTIR spectroscopy, viscosity, conductivity, and pH measurements. The maximum molar concentration of ions (H3O+ and HCOO-), along with the relative differences between theoretical and experimental densities, spin-lattice relaxation times (T1), activation energies (Ea), viscosity (η), and conductivity (σ), were identified within the range of XH2Oi ≈ 0.5-0.7. These results indicate that pure formic acid (FA) solutions predominantly consist of cyclic dimers at room temperature. As the water mole fraction increases up to 0.6, a structural shift occurs from cyclic dimers to a mixture of linear and cyclic dimers, driven by the formation of strong hydrogen bonds. Beyond a water mole fraction of 0.6, the structure transitions to linear dimers, with FA molecules behaving as free entities in the water. Furthermore, the acidity was found to increase approximately 2-fold with every 0.1 increment in water mole fraction. These findings are critical for understanding the kinetics of formic acid anions in body fluids, the structure of the hydrogen bonding network, and ionization energies.

Cc¯ and bb¯ suppression in the glasma

This study investigates the evolution and dissociation dynamics of cc¯ and bb¯ pairs within the preequilibrium, gluon-dominated stage of high energy nuclear collisions. An attractive potential made of a perturbative Coulomb-like term and of a confining term is used to simulate the attractive strong force in the pairs. Besides, we implement the interaction of the pairs with the evolving glasma fields by virtue of the Wong equations. The interaction with the classical color fields dominates the dynamics, causing an increase in pair separation and subsequent dissociation. The observed finite probability of dissociation for these states reveals the intricate interplay between quantum chromodynamics (QCD) dynamics and the suppression of cc¯ and bb¯ states during the preequilibrium stage. The research highlights differences between cc¯ and bb¯ pairs, revealing the role of quark flavor in the dissociation process. Dissociation spectra analysis indicates a peak shift toward higher momentum, reflecting a slight energy gain by the pairs. This investigation provides valuable insights into the complex dynamics of cc¯ and bb¯ pairs in the glasma, which may help in better interpretation of experimental results on further integration with subsequent phases of the created matter. Published by the American Physical Society 2024

High-entropy perovskite (Pr1/6Nd1/6Sm1/6Ba1/6Sr1/6)6/7(Mn1/6Co)6/7O3-δ as a highly active and CO2 durable cathode for solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are environmentally friendly energy conversion devices that convert the chemical energy in fuels directly into electrical power. The development of high-catalytic activity and durability cathode materials is crucial for the commercialization of SOFCs. Herein, a novel A-site deficient high-entropy perovskite oxide (Pr1/6Nd1/6Sm1/6Ba1/6Sr1/6)6/7(Mn1/6Co)6/7O3-δ (PNSBSMC) was designed. The strong disorder effect associated with entropy increase in high-entropy perovskite oxide is anticipated to accelerate oxygen reduction reaction (ORR) kinetics. PNSBSMC demonstrates excellent catalytic activity for oxygen reduction, primarily due to the entropy-driven enhancement that promotes oxygen adsorption, dissociation, and charge transfer dynamics. The increase in entropy not only significantly mitigates the thermal expansion of PNSBSMC but also provides additional pathways for ionic/electronic transport. At 800 °C, the PNSBSMC-based single cell achieved a remarkable peak power density of 1.41 W cm−2, representing an 88.0 % improvement over the single-phase PBC. Additionally, PNSBSMC demonstrates superior performance stability and CO2 tolerance. These findings suggest that high-entropy PNSBSMC perovskite holds significant potential as an effective cathode for SOFCs. This study offers new insights for the future development of high-stability, high-activity SOFC cathode materials.

Observation of sequential three-body dissociation of camphor molecule—a native frame approach

Abstract The three-body dissociation dynamics of the dicationic camphor molecule (C10H16O2+) resulting from Auger decay are investigated using soft x-ray synchrotron radiation. A photoelectron-photoion-photoion coincidence method, a combination of a velocity map imaging spectrometer and a time-of-flight spectrometer is employed to measure the 3D momenta of ions detected in coincidence. The ion mass spectra and the ion–ion coincidence map at photon energies of 287.9 eV (below the C 1s ionization potential) and 292.4 eV (above the C 1s ionization potential for skeletal carbon) reveal that fragmentation depends on the final dicationic state rather than the initial excitation. Using the native frame method, three new fragmentation channels are discussed; (1) CH2CO+ + C7H 11 + + CH3, (2) CH 3 + + C7H 11 + + CH2CO, and (3) C2H 5 + + C6H 9 + + CH2CO. The dominating nature of sequential decay with deferred charge separation is clearly evidenced in all three channels. The results are discussed based on the experimental angular distributions and momenta distributions, corroborated by geometry optimization of the ground, monocationic, and dicationic camphor molecule.

Few-femtosecond time-resolved study of the UV-induced dissociative dynamics of iodomethane

Ultraviolet (UV) light that penetrates our atmosphere initiates various photochemical and photobiological processes. However, the absence of extremely short UV pulses has so far hindered our ability to fully capture the mechanisms at the very early stages of such processes. This is important because the concerted motion of electrons and nuclei in the first few femtoseconds often determines molecular reactivity. Here we investigate the dissociative dynamics of iodomethane following UV photoexcitation, utilizing mass spectrometry with a 5 fs time resolution. The short duration of the UV pump pulse (4.2 fs) allows the ultrafast dynamics to be investigated in the absence of any external field, from well before any significant vibrational displacement occurs until dissociation has taken place. The experimental results combined with semi-classical trajectory calculations provide the identification of the main dissociation channels and indirectly reveal the signature of a conical intersection in the time-dependent yield of the iodine ion. Furthermore, we demonstrate that the UV-induced breakage of the C-I bond can be prevented when the molecule is ionized by the probe pulse within 5 fs after the UV excitation, showcasing an ultrafast stabilization scheme against dissociation.

Clay mineral mediated dynamics of CO2 hydrate formation and dissociation: Experimental insights for carbon sequestration

Marine sediment-based CO2 sequestration through hydrate formation is a promising approach for long-term CO2 storage. Despite the prevalence of clay minerals in marine sediments, their impact on CO2 hydrate dynamics remains disputed and necessitates validation. This study systematically investigated the effect of clay minerals, specifically Ca-montmorillonite (Ca-mmt) and illite, on CO2 hydrates nucleation, growth, and dissociation in clay suspensions (0–10 wt% concentrations). Clay minerals significantly promoted CO2 hydrate nucleation by providng additional nucleation sites and influencing water distribution through the surface electric field. Ca-mmt exhibited a more pronounced nucleation effect due to stronger hydration ability of Ca2+ increasing nucleation sites and local CO2 concentration. Both clay minerals enhanced CO2 hydrate growth, with concentration-dependent effects that differ between clay types. Ca-mmt exhibited a higher growth rate at lower concentrations (≤1 wt%), but a lower rate at higher concentrations (>1 wt%) due to the stronger swelling and aggregation ability upon water contact. Morphological analysis revealed the clay minerals’ impact on fluid behavior and mass transfer during hydrate growth. During dissociation, clay minerals impeded the process, prolonging overall dynamics. This study highlights the impact of physicochemical disparities between Ca-mmt and illite on CO2 hydrate dynamics, underscoring the importance of these interactions for CO2 sequestration.

High-resolution anionic velocity map imaging apparatus for dissociative electron attachment dynamics study.

Dissociative electron attachment (DEA) of a molecular target XY, e- + XY → XY- → X + Y-, is an important process in plasma, atmosphere, interstellar space, and ionizing radiation. DEA dynamics, i.e., the formation and fragmentation of an electron-molecule resonant complex, can be unveiled by measuring the product Y- with the velocity map imaging (VMI) technique. However, it is still challenging to achieve a high-resolution VMI measurement. Recently, we developed a high-resolution DEA apparatus that combined the VMI technique with a trochoidal electron monochromator. The energy spread (around 500meV) of thermally emitted incident electrons can be reduced remarkably, but the monochromatized electrons become diffuse again in energy when being pulsed with an electric pulse applied on the grid electrode because the electron beam must be pulsed in the VMI measurement. Now this long-standing technical contradiction is settled by introducing a parallel resistance-capacitor circuit to the electrode of an electron gun. A delay-line detector is used for the VMI measurement, which allows the three-dimensional velocity or momentum images of multiple anionic yields to be recorded efficiently. Our high-resolution VMI apparatus works at a pulse frequency of 5kHz and an energy spread of about 120-150meV, and its credible performances are exhibited by the measurements of the DEA processes of CO(a3Π) and NO2.

Imaging study of O3 photodissociation in the Huggins band.

We report a velocity-mapped ion imaging study of the photodissociation of O3 in the Huggins band. The O(3PJ) images show evidence for three electronic channels producing O2(X3Σg-), O2(a1∆g), and O2(b1Σg+) state fragments, with the latter two arising from the spin-forbidden photodissociation of O3. Forward convolution simulations of the derived total translational energy distributions permit extraction of the vibrational state distribution for each O2 electronic state. All these distributions peak near v = 0 and decrease monotonically with the vibrational state. The wavelength-dependent branching of the three electronic channels has been determined and is approximately constant over the wavelength region studied (322-328nm). We have observed that the O2 electronic state branching ratios depend on the coincident O(3PJ) spin-orbit state, and the O2(b1Σg+) state is particularly sensitive. These results are qualitatively consistent with previous calculations on the coupling of the initially excited state to dissociative states by Rosenwaks and Grebenshchikov [J. Phys. Chem. A. 114, 9809-9819 (2010)]. The spatial anisotropy of the three dissociation channels has been determined through analysis of the O(3P0) angular distributions. The results are consistent with recent calculations but differ from previous experimental reports. The experimental results provide detailed information on the dissociation dynamics and should motivate new calculations.

Enhanced Intramolecular Hole Transfer in Block Copolymer Enables >15% and Operational Stable Single-Material-Organic Solar Cells.

Recent studies on narrow bandgap all-conjugated block copolymer (BCP) single-material-organic solar cells (SMOSCs) have made unprecedented progress in power conversion efficiency (PCE); however, it still lacks understanding of the structure-property relationship in these highly mixed materials. Herein, the impact of different synthetic protocols (direct synthesis (d-BCP) versus sequential synthesis (s-BCP)) is first investigated on the relevant photovoltaic properties. Targeting the same BCP, namely PBDB-T-b-PYIT, it is found that the change in polymerization reaction leads to quite different optical and transport properties. The d-BCP outputs a record-high PCE of 15.02% for SMOSCs as well as enhanced operation stability under simulated 1-sun illumination, which is significantly higher than that of s-BCP (10.33%) and even close to its bulk heterojunction (BHJ) counterparts. Detailed transient absorption spectroscopy reveals ultrafast dynamics of charge transfer (CT) and exciton dissociation in BCP. In together with morphology characterization, it is revealed that the d-BCP has more phase pure composition, enhanced molecular ordering, and higher intramolecular CT efficiency relative to those of s-BCP. These findings gain insight into both the structure and carrier dynamic of BCP and demonstrate the possibility of achieving high-efficiency and stable SMOSCs.

Calculations of Dissociation Dynamics of CH3OH on a Global Potential Energy Surface Reveal the Mechanism for the Formation of HCOH; Roaming Plays a Role.

The experimental observation of hydroxymethylene, HCOH, following excitation of methanol at 193 nm, was reported recently (Hockey, E. K.; McLane, N.; Martí, C.; Duckett, L.; Osborn, D. L.; Dodson, L. G. Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation. J. Am. Chem. Soc. 2024, 146, 14416-14421, 10.1021/jacs.4c03090). This stimulated us to investigate a dynamic mechanism for its formation using a global potential energy surface for the ground electronic state (S0) of methanol. We show via quasi-classical trajectory calculations that hydroxymethylene is indeed formed under the reasonable assumption that the initially excited state undergoes rapid internal conversion to S0. From the trajectories, fractional yields of the six major product channels are determined as a function of excitation energy, and the rate of production of each is determined from the fractions and rate of methanol disappearance. Roaming is observed in trajectories leading to the OH and CH3 products as well as in those leading to CH2OH + H and non-reactive trajectories. A "frustrated" roaming accounts for roughly 20% of the HCOH production.

Molecular insights into methane hydrate dissociation: Role of methane nanobubble formation.

Understanding the underlying physics of natural gas hydrate dissociation is necessary for efficient CH4 extraction and in the exploration of potential additives in the chemical injection method. Silica being "sand" is already present inside the reservoir, making the silica nanoparticle a potential green additive. Here, molecular dynamics (MD) simulations have been performed to investigate the dissociation of the CH4 hydrate in the presence and absence of ∼1, ∼2, and ∼3nm diameter hydrophilic silica nanoparticles at 100bar and 310K. We find that the formation of a CH4 nanobubble has a strong influence on the dissociation rate. After the initial hydrate dissociation, the rate of dissociation slows down till the formation of a CH4 nanobubble. We find the critical concentration and size limit to form the CH4 nanobubble to be ∼0.04 molefraction of CH4 and ∼40 to 50 CH4 molecules, respectively. The solubility of CH4 and the chemical potential of H2O and CH4 are determined via Gibbs ensemble Monte Carlo simulations. The liquid phase chemical potential of both H2O and CH4 in the presence and absence of the nanoparticle is nearly the same, indicating that the effect of this additive will not be significant. While the formation of the hydration shell around the nanoparticle via hydrogen bonding confirms the strength of interactions between the water molecules and the nanoparticle in our MD simulations, the contact of the nanoparticle with the interface is infrequent, leading to no explicit effect of the nanoparticle on the dynamics of methane hydrate dissociation.

Dissoziative Elektronenanlagerungsdynamik eines vielversprechenden Krebsmedikaments zeigt sein Potential als Radiosensitizer

Abstract2‐Bromo‐1‐(3,3‐dinitroazetidin‐1‐yl)ethan‐1‐on (RRx‐001) ist ein Chemotherapeutikum für sauerstoffarme Tumore und zeigte bereits Synergieeffekte in Kombination mit Strahlentherapie. Die Wechselwirkung des Moleküls mit niederenergetischen Sekundärelektronen, die in großer Zahl in der physikalisch‐chemischen Phase der Bestrahlung gebildet werden, könnten für diesen Synergieeffekt verantwortlich sein. Die vorliegende Studie konzentriert sich auf die ersten Schritte in der Wechselwirkung von RRx‐001 mit niederenergetischen Elektronen, in denen das temporäre negative Ion gebildet wird und rasch fragmentiert. Die Kombination von Resultaten zweier Versuchsaufbauten erlaubt es uns, den Zerfall des RRx‐001 Anions auf verschiedenen Zeitskalen zu entflechten. Die alleinige Anwesenheit des eingefangenen Elektrons führt zu einer raschen Abspaltung von NO2 und HNO2, während NO2− und Br− Anionen direkt beziehungsweise über andere Zwischenprodukte gebildet werden. Auf Basis quantenchemischer Rechnungen schlagen wir einen bidirektionalen Wechsel zwischen π*(NO2) und σ*(C−Br) Zuständen vor, welcher die experimentellen Spektren erklärt. Die beobachtete schnelle Dynamik lässt auch Schlussfolgerungen für die Chemie des Anions in der kondensierten Phase zu.

Mean field approximation of a surface-reaction growth model with dissociation

We utilize mean field approximation to derive a set of coupled non-linear differential equations for a surface-reaction growth model, incorporating dissociative adsorption processes. These equations account for inaccessible sites caused by the height differences in the growing film, which inhibit dissociative adsorption. Numerical integration of these equations highlights the substantial effect of dissociation rates on growth kinetics, showing excellent agreement with prior kinetic Monte Carlo simulations, offering crucial insights into the interaction between dissociative adsorption and surface growth dynamics.

Potential energy of heavy quarkonium in flavor-dependent systems from a holographic model

Within the framework of the Einstein-Maxwell-dilaton model, which incorporates information on the equation of state and baryon number susceptibility from lattice results, we have conducted a comprehensive analysis of the potential energy, running coupling, and dissociation time for heavy-quark–antiquark pairs using gauge/gravity duality. This study encompasses various systems, including pure gluon systems, 2-flavor systems, 2+1-flavor systems, and 2+1+1-flavor systems under finite temperature and chemical potential. The results reveal that the linear component of the potential energy diminishes as the flavor increases. It is also found that our results are extremely close to the recent lattice results for 2+1-flavors at finite temperature. Moreover, we have thoroughly investigated the dissociation distance and effective running coupling constant of quark-antiquark pairs to gain a comprehensive understanding of their behavior across various flavors. Finally, we have examined real-time dynamics of quark dissociation. The findings indicate that the dissociation time of quark-antiquark pairs is dependent on temperature, chemical potential, and flavor of the systems. Published by the American Physical Society 2024