Molecular Dissociation Research Articles

Vibronic coherence and quantum beats of O2+ based on laser pump-probe dissociation dynamics

We report theoretical investigations of vibronic quantum beats (QBs) which can be observed in the molecular dissociation under the intense infrared (IR) laser pump-IR laser probe scheme. We show how the vibronic coherences can be probed by analyzing the interchannel QB signals obtained from the numerical solution of the time-dependent Schr\"odinger equation (TDSE) and the quantum Liouville equation in combination with the strong-field approximation for the treatment of coherences between multiple states of the target ion. The validities of our methods are first tested on a one-dimensional model of ${{\mathrm{H}}_{2}}^{+}$, for which exact solutions of the TDSE can be obtained. We then illustrate our method using an example of ${\mathrm{O}}_{2}$, for which various experiments have been reported recently. The case of an attosecond pump pulse is also considered. Our results indicate that the strong-field dissociation pump-probe experiments are capable of providing information on the vibronic coherences that complements other techniques such as attosecond transient absorption spectroscopy.

Probing Laser-Induced Plasma Generation in Liquid Water.

Understanding photoexcitation dynamics in liquid water is of crucial significance for both fundamental scientific exploration and technological applications. Despite the observations of photoinduced macroscopic phenomena, the initial atomistic movements and associated energy transfer pathways immediately following laser irradiation are hard to track due to the extreme complexity of laser-water interaction and its ultrafast time scale. We explore the femtosecond evolution of liquid water upon intense photoexcitation based on nonadiabatic quantum dynamics simulations. Separate ionic and electronic dynamics were explicitly monitored with tremendous details unveiled on an unprecedented microscopic level. Water was found to undergo the two-step heating processes. The strong-field effects and electronic excitations dominate the first-stage heating and pressurization. Subsequent relaxation of ionic and electronic subsystems further increases the ionic temperature but releases the large internal pressure. The water molecules are stretched during the laser pulses, and the electronic excitations result in the proton transfers after laser pulses. Intense laser pulses violently excite liquid water, giving rise to severe molecular dissociation and plasma generation during the laser pulses. The laser-induced water plasma is characterized by a high fraction of free protons (∼50%), nonequilibrium ionic and electronic distributions, and a metallic electronic density of states.

Pnictogen effects on the electronic interactions in the Lewis pair complexes Ph3EB(C6F5)3 (E = P, As, Sb)

Lewis pair complexes bearing a BR3 acid and a R’3E base (E = P, As, Sb; R, R’ – organic ligands) play key roles in modern chemistry and technology, their properties being strongly relevant to catalysis and material science. The donor acceptor complexes Ph3EB(C6F5)3 attract special interest due to the elongated E-B bond that shifts the molecular parameters towards those of a frustrated Lewis pair. In depth theoretical and experimental studies of the Ph3EB(C6F5)3 systems presented in this work provide new insights into the nature of intramolecular electronic interactions in these molecules. A missing X-ray structure of Ph3AsB(C6F5)3 is described. Analyses of molecular structures, orbitals, dissociation energies and electron density distributions reveal crucial influence of non-covalent interactions on the Ph3EB(C6F5)3 stability.

Phase Diagram of a Deep Potential Water Model.

Using the Deep Potential methodology, we construct a model that reproduces accurately the potential energy surface of the SCAN approximation of density functional theory for water, from low temperature and pressure to about 2400K and 50GPa, excluding the vapor stability region. The computational efficiency of the model makes it possible to predict its phase diagram using molecular dynamics. Satisfactory overall agreement with experimental results is obtained. The fluid phases, molecular and ionic, and all the stable ice polymorphs, ordered and disordered, are predicted correctly, with the exception of ice III and XV that are stable in experiments, but metastable in the model. The evolution of the atomic dynamics upon heating, as ice VII transforms first into ice VII^{''} and then into an ionic fluid, reveals that molecular dissociation and breaking of the ice rules coexist with strong covalent fluctuations, explaining why only partial ionization was inferred in experiments.

Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient

The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P–T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3–3.0 GPa/540–1500 K, yields a negative Clapeyron slope of -0.003 GPa K–1 (on average). The intersection of the P–T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: Na2O,K2O,H2O). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = –1.97 ± 0.35; –1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.

High hydrogen uptake by a metal-graphene-microporous carbon network

High-surface area carbon nanomaterials are promising candidate as reversible physisorption materials for hydrogen storage in personal transportation vehicles at moderate temperatures and pressures. Metal-incorporated graphitic microporous carbon is considered to be an excellent H2 storage material due to high molecular hydrogen uptake at the micropores coupled with considerable H2 adsorption at the graphitic network. A simple, cost-effective sputtering technique is adopted to fabricate metal-incorporated graphitic microporous carbon film and differential resistance measurements are carried out in H2 ambient to observe the high hydrogen uptake within the samples. Ultrathin amorphous carbon film is irradiated with metal nanoparticles which converts it into graphitic carbon, whereas sputtering plasma acts as dry-etchant to activate it into microporous carbon. Average number of graphene layer formation is observed to be dependent on sputtering parameters (current/voltage/time), which manifest bi-layer to multi-layer graphene sheets. With increase in the graphene layers, hydrogen adsorption also increases due to a four-fold effect - higher molecular hydrogen uptake at the graphene layers, more active sites into the micropores, promotion of molecular dissociation into atomic hydrogen by the metal nanoparticles, followed by adsorption at the active surface sites and the formation of hydrogenated carbon via destruction of the π-bonds.

Electron irradiation and thermal chemistry studies of interstellar and planetary ice analogues at the ICA astrochemistry facility

The modelling of molecular excitation and dissociation processes relevant to astrochemistry requires the validation of theories by comparison with data generated from laboratory experimentation. The newly commissioned Ice Chamber for Astrophysics-Astrochemistry (ICA) allows for the study of astrophysical ice analogues and their evolution when subjected to energetic processing, thus simulating the processes and alterations interstellar icy grain mantles and icy outer Solar System bodies undergo. ICA is an ultra-high vacuum compatible chamber containing a series of IR-transparent substrates upon which the ice analogues may be deposited at temperatures of down to 20 K. Processing of the ices may be performed in one of three ways: (i) ion impacts with projectiles delivered by a 2 MV Tandetron-type accelerator, (ii) electron irradiation from a gun fitted directly to the chamber, and (iii) thermal processing across a temperature range of 20–300 K. The physico-chemical evolution of the ices is studied in situ using FTIR absorbance spectroscopy and quadrupole mass spectrometry. In this paper, we present an overview of the ICA facility with a focus on characterising the electron beams used for electron impact studies, as well as reporting the preliminary results obtained during electron irradiation and thermal processing of selected ices.Graphic

Computer Simulations of the Dissociation Mechanism of Gleevec from Abl Kinase with Milestoning.

Gleevec (a.k.a., imatinib) is an important anticancer (e.g., chronic myeloid leukemia) chemotherapeutic drug due to its inhibitory interaction with the Abl kinase. Here, we use atomically detailed simulations within the Milestoning framework to study the molecular dissociation mechanism of Gleevec from Abl kinase. We compute the dissociation free energy profile, the mean first passage time for unbinding, and explore the transition state ensemble of conformations. The milestones form a multidimensional network with average connectivity of about 2.93, which is significantly higher than the connectivity for a one-dimensional reaction coordinate. The free energy barrier for Gleevec dissociation is estimated to be ∼10 kcal/mol, and the exit time is ∼55 ms. We examined the transition state conformations using both, the committor and transition function. We show that near the transition state the highly conserved salt bridge K217 and E286 is transiently broken. Together with the calculated free energy profile, these calculations can advance the understanding of the molecular interaction mechanisms between Gleevec and Abl kinase and play a role in future drug design and optimization studies.

Enhancing the formation of ionic defects to study the ice Ih/XI transition with molecular dynamics simulations

Ice Ih, the common form of ice in the biosphere, contains proton disorder. Its proton-ordered counterpart, ice XI, is thermodynamically stable below 72 K. However, the formation of ice XI is kinetically hindered, and experimentally it is obtained by doping with KOH. Doping creates ionic defects that promote the migration of protons and the associated change in proton configuration. In this article, we mimic the effect of doping with a bias potential that enhances the formation of ionic defects in molecular dynamics simulations. The recombination of the ions thus formed proceeds through fast migration of the hydroxide along hydrogen bond loops, providing a physical and expedite way to change the proton configuration. A key ingredient of this approach is a machine learning potential trained with density functional theory data and capable of modelling molecular dissociation. We exemplify the usefulness of this idea by studying the order-disorder transition using an appropriate order parameter that distinguishes the proton environments in ice Ih and XI. We calculate the changes in free energy, enthalpy, and entropy associated with the transition. Our estimated entropy agrees with experiment within the error bars of the calculation.

Parallel electrodes gliding plasma: Working principles and application in dry reforming of methane

A parallel electrodes gliding plasma (PEGP) is developed and applied for the dry reformation of methane. It consists of a pair of parallel wire-plate electrodes that is fed by repetitive nanosecond electrical pulses and placed into a divergent acoustic waveguide. The waveguide conducts the plasma-produced acoustic shock waves forward, causing sparking at the low pressure moving nodes of the waves. Consequently, PEGP provides a high plasma mobility within the volume of the parallel electrodes where constant reduced field (E/N), optimized for molecular dissociation, is applied. This is in contrast to the case of the conventional glide plasma, where E/N covers a range of values. Besides, the energy of the mobile plasma is mostly dissipated in the layers of fresh reactants gas, instead of overheating or dissociating the products. Consequently, higher conversion rates and energy efficiencies are obtained. The performances of PEGP and different pulsed plasmas, are examined and compared in this study. Conversion rates of almost 50% for both reactants (CO2, CH4) with ECE exceeding 70%, at temperature of about 100 °C, are provided by PEGP. The cold PEGP has comparable conversion performance with the high temperature thermal plasmas while performs much better compared to the other reported cold plasmas.

Study of the Rate-Determining Step of Rh Catalyzed CO2 Reduction: Insight on the Hydrogen Assisted Molecular Dissociation

In the context of climate change mitigation, CO2 methanation is an important option for the production of synthetic carbon-neutral fuels and for atmospheric CO2 recycling. While being highly exothermic, this reaction is kinetically unfavorable, requiring a catalyst to be efficiently activated. Recently Rh nanoparticles gained attention as effective photocatalyst, but the rate-determining step of this reaction on Rh surface has not been characterized yet. In this work, Density Functional Theory and Nudged Elastic Band calculations were performed to study the Rh-catalyzed rate-determining step of the CO2 methanation, which concerns the hydrogen assisted cleavage of the CO* molecule and subsequent formation of CH* and O* (* marks adsorbed species), passing through the CHO* key intermediate. The configurations of the various adsorbates on the Rh (100) surface were investigated and the reaction mechanism was studied exploiting different exchange-correlation functionals (PBE, RPBE) and the PBE+U technique. The methanation rate-determining step consists of two subprocesses which subsequently generate and dissociate the CHO* species. The energetics and the dynamics of such processes are extensively studied and described. Interestingly, PBE and PBE+U calculated activation barriers are in good agreement with the available experimental data, while RPBE largely overestimate the CHO* dissociation barrier.

The study of oxygen reduction reaction on Ge‐doped MoS 2 monolayer based on first principle

Rational design of electrocatalyst for air cathode with high oxygen reduction reaction (ORR) performance is highly demanded. However, current low performance of electrocatalyst remains a big challenge. Herein, the possibility of germanium (Ge)-doped molybdenum disulfide (MoS2) monolayer as a novel electrocatalyst for ORR in alkaline solution was studied based on first principle calculation. The results indicate that oxygen is strongly adsorbed on the surface of Ge-doped MoS2 monolayer. Moreover, a high barrier should be overcome during H2O molecular dissociation and the barrier of dissociation decreases with the number of doped Ge atoms increasing. Based on the calculation of transition state and Gibbs free energy, the triple Ge atoms-doped MoS2 monolayer shows the best activity toward ORR with the lowest rate-determine step barrier of 0.23 eV. Our study may provide a meaningful method for the design of novel MoS2-based electrocatalyst for ORR.

Molecular oxygen dissociation on tin carbide monolayers with gold adatoms

In this work, the interactions between oxygen molecule O2 with pristine and gold-decorated tin carbide (SnC) monolayers were investigated by using density functional calculations. The results indicate that O2 is adsorbed, with an energy of 0.95 eV, on the pristine SnC nanosheet in a bond-like configuration with each oxygen close to a carbon and a tin atom, respectively. There is a large electronic charge transfer from the SnC monolayer to the O2 molecule, which enlarges the molecule internal bond, indicating an activation towards a peroxo-like state. Also, a gold adatom can strongly bind to the SnC monolayer over a C atom. Furthermore, when O2 interacts with this Au adatom, it is spontaneously dissociated with an energy gain of 1.84 eV and where the final adsorption configuration consists of each oxygen on the top of different tin atoms but sharing the gold one.

Application of spatially hybrid fluid–kinetic neutral model on JET L-mode plasmas

Abstract We present a spatially hybrid fluid–kinetic neutral model that consists of a fluid model for the hydrogen atoms in the plasma grid region coupled to a kinetic model for atoms sampled at the plasma–void interfaces and a fully kinetic model for the hydrogen molecules. The atoms resulting from molecular dissociation are either treated kinetically (approach 1) or are incorporated in the fluid model (approach 2). For a low-density JET L-mode case, the hybrid method reduces the maximum fluid–kinetic discrepancies for the divertor strike-point electron densities and electron temperatures from approximately 150% to approximately 20% for approach 1 and to approximately 40% for approach 2. Although the simulations with purely fluid neutral model become more accurate for increasing upstream plasma density, we still observe a significant improvement by using the hybrid approach. When consuming the same CPU time in averaging the electron strike-point densities and temperatures over multiple iterations as for the simulations with fully kinetic neutrals, hybrid approach 1 reduces the statistical error with on average a factor 2.5. Hybrid approach 2 further increases this factor to approximately 3.3, at the expense of accuracy.

From Kohn-Sham to Many-Electron Energies via Step Structures in the Exchange-Correlation Potential.

Accurately describing excited states within Kohn–Sham (KS) density functional theory (DFT), particularly those which induce ionization and charge transfer, remains a great challenge. Common exchange-correlation (xc) approximations are unreliable for excited states owing, in part, to the absence of a derivative discontinuity in the xc energy (Δ), which relates a many-electron energy difference to the corresponding KS energy difference. We demonstrate, analytically and numerically, how the relationship between KS and many-electron energies leads to the step structures observed in the exact xc potential in four scenarios: electron addition, molecular dissociation, excitation of a finite system, and charge transfer. We further show that steps in the potential can be obtained also with common xc approximations, as simple as the LDA, when addressed from the ensemble perspective. The article therefore highlights how capturing the relationship between KS and many-electron energies with advanced xc approximations is crucial for accurately calculating excitations, as well as the ground-state density and energy of systems which consist of distinct subsystems.

Molecular dissociation and proton transfer in aqueous methane solution under an electric field.

Methane-water mixtures are ubiquitous in our solar system and they have been the subject of a wide variety of experimental, theoretical, and computational studies aimed at understanding their behaviour under disparate thermodynamic scenarios, up to extreme planetary ice conditions of pressures and temperatures [Lee and Scandolo, Nat. Commun., 2011, 2, 185]. Although it is well known that electric fields, by interacting with condensed matter, can produce a range of catalytic effects which can be similar to those observed when material systems are pressurised, to the best of our knowledge, no quantum-based computational investigations of methane-water mixtures under an electric field have been reported so far. Here we present a study relying upon state-of-the-art ab initio molecular dynamics simulations where a liquid aqueous methane solution is exposed to strong oriented static and homogeneous electric fields. It turns out that a series of field-induced effects on the dipoles, polarisation, and the electronic structure of both methane and water molecules are recorded. Moreover, upon increasing the field strength, increasing fractions of water molecules are not only re-oriented towards the field direction, but are also dissociated by the field, leading to the release of oxonium and hydroxyde ions in the mixture. However, in contrast to what is observed upon pressurisation (∼50 GPa), where the presence of the water counterions triggers methane ionisation and other reactions, methane molecules preserve their integrity up to the strongest field explored (i.e., 0.50 V Å-1). Interestingly, neither the field-induced molecular dissociation of neat water (i.e., 0.30 V Å-1) nor the proton conductivity typical of pure aqueous samples at these field regimes (i.e., 1.3 S cm-1) are affected by the presence of hydrophobic interactions, at least in a methane-water mixture containing a molar fraction of 40% methane.

Effects of the Voltage and Pressure on the Carburizing of Martensitic Stainless Steel in Pulsed DC Glow Discharge

Abstract In the present work, the influence of the pulse voltage and pressure on the treatment glow discharge characteristics and consequently on the surface properties obtained for low temperature plasma carburized AISI 420 martensitic stainless steel was investigated. Two distinct sets of samples were carburized at 450 °C, for 8 h, one aiming to study the applied pulse voltage effects, which was varied for 500, 600 and 700 V, at a fixed pressure of 400 Pa, and the other aiming to study the pressure effects, which was varied for 200, 400, and 800 Pa, at a fixed pulse voltage of 700 V. Treated samples were characterized by means of confocal laser scanning microscopy (CLSM), X-ray diffraction (XRD) analysis, microhardness and roughness measurements. The glow discharge (plasma) was characterized by optical emission spectroscopy (OES) and current measurements. Results show that the edge effect, surface roughness, hardness and outer layer growth kinetics are dependent on the studied plasma parameters. OES analyses showed that the pulse voltage parameter does not promote significant changes on the plasma chemistry, but confirmed that the molecular H2 gas dissociation rate tends to be significantly affected by the pressure parameter giving important support for the results obtained here.

Pipeline durability and integrity issues at hydrogen transport via natural gas distribution network

Hydrogen will play a significant role in a decarbonised energy system in the coming decades. Extensive infrastructure exists for distribution of natural gas and it is an obvious step to assess it to be used for supply hydrogen or mixtures with natural gas. The durability and integrity issues in existing natural gas distribution steel pipelines associated with hydrogen embrittlement of steel at transporting hydrogen or mixtures natural gas with hydrogen are considered. Two mechanisms of steel hydrogenation from pipe internal surface are taken into account, namely, as a result of electrochemical corrosion due to presence of moisture in transported gas, and molecular hydrogen dissociation. Dissipated microdamaging in the bulk of the pipe wall is distinguished as the main factor of manifestation of hydrogen embrittlement of steel and its durability loss, increasing a risk of structural integrity violation. A sensitivity of some physico-mechanical properties of pipeline steels to hydrogen degradation is analysed. Some technological solutions are considered which should accompany experimental research on the assessment of the pipe serviceability at supply hydrogen or mixtures natural gas with hydrogen, including long-term exposing of pre-loaded specimens in hydrogen or hydrogen-natural gas mixture.

Ab initio dynamics simulation of laser-induced photodissociation of phenol.

We theoretically investigated the photodissociation dynamics of phenol molecules steered by a sequence of temporally shaped femtosecond laser pulses with high intensity and ultrashort duration, via the real-time Time-Dependent Density Functional Theory (rt-TDDFT) combined with a Molecular Dynamics (MD) simulation. The principal findings of this research are that the phenol photodissociation can take place in 50 fs; the bonds broke sequentially; the degree of phenol molecular dissociation has a strong linear correlation with the intensity. For an incident laser being 800 nm-40 fs (wavelength-pulse duration), the threshold intensity is 7 × 1014 W cm-2 and the products are hydrogen from OH1 (phenolic hydroxyl group) and C6H5O-fragments. More fragments will be found at stronger intensity, shorter wavelengths, and longer pulse duration. More accurately, we estimated the critical values of bond cleavage of an isolated phenol molecule are 1.779 Å for O-H1 and 2.184 Å for C-Hs via Electron localization function (ELF) analysis. The photodissociation of the phenol molecule was triggered via the excitation of electrons and the dissociation process of phenol here is in good agreement with the characteristics of field-assisted dissociation (FAD) theory. Orthogonal tests with an L9 (34) matrix and threshold intensity decrease tests were conducted to confirm the mechanism. Our research gives an insight into the photodissociation experiment of phenol and provides a simple yet effective way to understand the photochemical experiments of more complex organic pollutants with toxicity.

Modelling the non-local thermodynamic equilibrium spectra of silylene (SiH2).

This paper sets out a robust methodology for modelling spectra of polyatomic molecules produced in reactive or dissociative environments, with vibrational populations outside local thermal equilibrium (LTE). The methodology is based on accurate, extensive ro-vibrational line lists containing transitions with high vibrational excitations and relies on the detailed ro-vibrational assignments. The developed methodology is applied to model non-LTE IR and visible spectra of silylene (SiH2) produced in a decomposition of disilane (Si2H6), a reaction of technological importance. Two approaches for non-LTE vibrational populations of the product SiH2 are introduced: a simplistic 1D approach based on the Harmonic approximation and a full 3D model incorporating accurate vibrational wavefunctions of SiH2 computed variationally with the TROVE (Theoretical ROVibrational Energy) program. We show how their non-LTE spectral signatures can be used to trace different reaction channels of molecular dissociations.