Dissociation Of Diatomic Molecules Research Articles

Level Adiabatic Model for the Dissociation of Diatomic Molecules

Level Adiabatic Model for the Dissociation of Diatomic Molecules

Dissociation of HF molecule in position and momentum representation by an optimally controlled polychromatic field: study in the dual space using simulated annealing

Complementarity of position and momentum representation is at the heart of quantum mechanics. Dissociation of diatomic molecule can be investigated and controlled in conventional coordinate space as well as in momentum space. Our objective in this article is to have the control over the dissociation process irrespective of the representation. We have applied Simulated Annealing (SA) technique for the optimisation of the field, where the effective field strength is kept sufficiently low. The optimisation is carried out in both momentum and position representation on with completely different objective function but leading to similar result.

Coherent atomic orbital polarization probes the geometric phase in photodissociation of polyatomic molecules

AbstractQuantum interference between multiple pathways in molecular photodissociation often results in angular momentum polarization of atomic products and this can give deep insight into fundamental physical processes. For dissociation of diatomic molecules, the resulting orbital polarization is fully understood and consistent with quantum mechanical theory. For polyatomic molecules, however, coherent photofragment orbital polarization is frequently observed but so far has eluded theoretical explanation, and physical insight is lacking. Here, we present a model of these effects for ozone photodissociation that reveals the importance of a novel manifestation of the geometric phase. We show this geometric phase effect permits the existence of coherent polarization in cases where it would otherwise vanish, and cancels it in some cases where it might otherwise exist. The model accounts for measurements in ozone that have hitherto defied explanation, and represents a step toward a deeper understanding of coherent electronic excitation in polyatomic molecules and a new role of the geometric phase.Key Points Coherent photofragment atomic orbital polarization reveals matter wave interference in dissociation along multiple paths. In diatomic molecules, this is well‐understood, but in polyatomic molecules, large effects are seen but these have defied a rigorous explanation. A model is developed describing these phenomena in the UV dissociation of ozone that accounts for a number of conflicting observations and reveals a new manifestation of the geometric phase in molecular physics.

Theory of polarization-averaged core-level molecular-frame photoelectron angular distributions: II. Extracting the x-ray-induced fragmentation dynamics of carbon monoxide dication from forward and backward intensities

Recent developments in high-repetition-rate x-ray free electron lasers (XFELs) such as the European XFEL and the LSCS-II, combined with coincidence measurements using the COLTRIMS-reaction microscope, are now opening a door to realize the long-standing dream of creating molecular movies of photo-induced chemical reactions in gas-phase molecules. In this paper, we propose a new theoretical method to experimentally visualize the dissociation of diatomic molecules via time-resolved polarization-averaged molecular-frame photoelectron angular distribution (PA-MFPAD) measurements using the COLTRIMS-reaction microscope and the two-color XFEL pump–probe set-up. We used first- and second-order scattering theory within the muffin-tin approximation, which is valid for a sufficiently high kinetic energy of photoelectrons, typically above 100 eV, and for long bond lengths. This leads to a simple extended x-ray absorption fine structure (EXAFS)-type formula for the forward and backward scattering peaks in the PA-MFPAD structure. This formula relies only on three semi-empirical parameters obtainable from the time-resolved measurements. It can be used as a ‘bond length ruler’ on experimental results. The accuracy and applicability of the new ruler equation are numerically examined against the PA-MFPADs of CO2+ calculated with full-potential multiple scattering theory as a function of the C–O bond length reported in the preceding work (Ota et al J. Phys. B: At. Mol. Opt.). The bond lengths retrieved from the PA-MFPADs via our EXAFS-like formula coincide within an accuracy of 0.1 Å with the original C–O bond lengths used in the reference ab initio PA-MFPADs. We expect time-resolved PA-MFPADs to become a new attractive tool to make molecular movies visualizing intramolecular reactions.

Automatic Selection of Active Orbitals from Generalized Valence Bond Orbitals.

The accurate multireference (MR) calculation of a strongly correlated chemical system usually relies on a correct preselection of a small number of active orbitals from numerous molecular orbitals. Currently, the active orbitals are generally determined by using a trial-and-error method. Such a preselection by chemical intuition and personal experience may be tedious or unreliable, especially for large complicated systems, and accordingly, the construction of active space becomes a bottleneck for large-scale MR calculations. In this work, we propose to automatically select the active orbitals according to the natural orbital occupation numbers by performing black box generalized valence bond calculations. We demonstrate the accuracy of this method through testing calculations of the ground states in various systems, ranging from bond dissociation of diatomic molecules (N2, C2, Cr2) to conjugated molecules (pentacene, hexacene, and heptacene) as well as a binuclear transition-metal complex [Mn2O2(H2O)2(terpy)2]3+ (terpy = 2,2':6,2″-terpyridine) with active spaces up to (30e, 30o) and comparing with the complete active space self-consistent field (CASSCF), density matrix renormalization group (DMRG)-CASSCF references, and other recently proposed inexpensive strategies for constructing active spaces. The results indicate that our method is among the most successful ones within our comparison, providing high-quality initial active orbitals very close to the finally optimized (DMRG-)CASSCF orbitals. The method proposed here is expected to greatly benefit the practical implementation of large active space ground-state MR calculations, for example, large-scale DMRG calculations.

Development of an impulsive model of dissociation in direct simulation Monte Carlo

A previously proposed classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo (DSMC), the Macheret-Fridman for direct simulation Monte Carlo (MF-DSMC) model [Luo et al., “Classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo,” Phys. Rev. Fluids 3, 113401 (2018)], is extended in this work. To improve the prediction of state-specific rates at high vibrational energy, the anharmonic vibrational phase angle distribution function is first incorporated into the model. Then, to improve the prediction of thermal equilibrium dissociation rates, the general concept of calculating total collision cross sections with the MF-DSMC model is discussed and the framework of implementing a collision model based on exponential potential is constructed. The improved model is validated by comparisons with quasiclassical trajectory calculations, empirical estimations, and experimental measurements. In general, better agreement compared with the original version of the model is obtained. The improved model is also evaluated by simulating O2 reacting shock experiment.

Classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo

Classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo

Dissociation of Potassium Iodide at a Graphite Surface within the Framework of an Impulsive Model

An impulsive model of dissociation of diatomic molecules with an ionic bond (for instance, alkali halide molecules) at a graphite surface is proposed. Within the framework of this model, a single encounter of an ion with the surface can consist of several elastic hits of the ion against graphite pseudoparticles of various masses. These hits instantly follow each other and are described in general in the same way as in Logan’s and Stickney’s well-known “hard-cube model” of scattering of atoms from a solid surface. The calculation results for dissociation of KI molecules with a translational energy from 4 to 14 eV are presented.

Unambiguous Signature of the Berry Phase in Intense Laser Dissociation of Diatomic Molecules.

We report strong evidence of Berry phase effects in intense laser dissociation of D2+ molecules, manifested as Aharonov-Bohm-like oscillations in the photofragment angular distribution (PAD). Our calculations show that this interference pattern strongly depends on the parity of the diatom initial rotational state, (-1) j. Indeed, the PAD local maxima (minima) observed in one case ( j odd) correspond to local minima (maxima) in the other case ( j even). Using simple topological arguments, we clearly show that such interference conversion is a direct signature of the Berry phase. The sole effect of the latter on the rovibrational wave function is a sign change of the relative phase between two interfering components, which wind in opposite senses around a light-induced conical intersection (LICI). Therefore, encirclement of the LICI leads to constructive ( j odd) or destructive ( j even) self-interference of the initial nuclear wavepacket in the dissociative limit. To corroborate our theoretical findings, we suggest an experiment of strong-field indirect dissociation of D2+ molecules, comparing the PAD of the ortho and para molecular species in directions nearly perpendicular to the laser polarization axis.

Symmetry breaking by quantum coherence in single electron attachment

Quantum coherence-induced effects in atomic and molecular systems are the basis of several proposals for laser-based control of chemical reactions. So far, these rely on coherent photon beams inducing coherent reaction pathways that may interfere with one another, to achieve the desired outcome. This concept has been successfully exploited for removing the inversion symmetry in the dissociation of homonuclear diatomic molecules, but it remains to be seen if such quantum coherent effects can also be generated by the interaction of incoherent electrons with such molecules. Here we show that resonant electron attachment to H2 and the subsequent dissociation into H (n = 2) + H− is asymmetric about the inter-nuclear axis, whereas the asymmetry in D2 is far less pronounced. We explain this observation as due to attachment of a single electron resulting in a coherent superposition of two resonances of opposite parity. In addition to exemplifying a new quantum coherent process, our observation of coherent quantum dynamics involves the active participation of all three electrons and two nuclei, which could provide new tools for studying electron correlations as a means to control chemical processes, and demonstrates the role of coherent effects in electron-induced chemistry. Resonant electron attachment and subsequent dissociation of diatomic molecules is shown to exhibit spatial asymmetry as a consequence of coherent excitation and subsequent interference between reaction pathways.

Observing electron localization in a dissociating H2+ molecule in real time

Dissociation of diatomic molecules with odd number of electrons always causes the unpaired electron to localize on one of the two resulting atomic fragments. In the simplest diatomic molecule H2+ dissociation yields a hydrogen atom and a proton with the sole electron ending up on one of the two nuclei. That is equivalent to breaking of a chemical bond—the most fundamental chemical process. Here we observe such electron localization in real time by performing a pump–probe experiment. We demonstrate that in H2+ electron localization is complete in just 15 fs when the molecule’s internuclear distance reaches 8 atomic units. The measurement is supported by a theoretical simulation based on numerical solution of the time-dependent Schrödinger equation. This observation advances our understanding of detailed dynamics of molecular dissociation.

Linearized Coupled Cluster Correction on the Antisymmetric Product of 1-Reference Orbital Geminals

We present a linearized coupled cluster (LCC) correction based on a reference state of the antisymmetric product of 1-reference orbital geminals (AP1roG). In our LCC ansatz, the cluster operator is restricted to double or to single and double excitations, as in standard single-reference CC theory. The performance of the AP1roG-LCC models is tested for the dissociation of diatomic molecules in their lowest-lying singlet state (C2, F2, and BN), the symmetric dissociation of the H50 hydrogen chain, and spectroscopic constants of the uranyl cation (UO2 2+). Our study indicates that an LCC correction based on an AP1roG reference function is more robust and reliable than corrections based on perturbation theory, yielding spectroscopic constants that are in very good agreement with theoretical reference data.

Isotope-selective dissociation of diatomic molecules by terahertz optical pulses

We propose a method for isotope-selective dissociation of diatomic molecules in the gas phase by using two kinds of terahertz-pulse fields. The first field consists of a train of pulses, composing a frequency comb, which excites only the selected isotope into a highly rotationally-excited state. The second field dissociates the excited molecules by further rovibrational excitations. We numerically demonstrate the applicability of the proposed scheme by molecular wave-packet computations using the lithium chlorides $^{7}\mathrm{Li}^{35}\mathrm{Cl}$ and $^{7}\mathrm{Li}^{37}\mathrm{Cl}$. Nearly 20% of the $^{7}\mathrm{Li}^{37}\mathrm{Cl}$ in the lowest rovibrational state is dissociated in a single interaction of the designed terahertz fields, while the dissociation probability is negligible in $^{7}\mathrm{Li}^{35}\mathrm{Cl}$. This scheme is expected to be applicable to the molecular ensemble whose rotational states spread in energy.

Electron Impact Excitation and Ionization of Sulfur, Selenium, and Tellurium Vapors

Excitation and ionization processes of sulfur, selenium, and tellurium vapors by a low-energy (1–50 eV) electron impact have been investigated. The emission spectra are studied in the wavelength range 200–590 nm. The optical excitation functions for the most intense atomic and ionic spectral lines, as well as for molecular bands and emissions, are measured. The energy dependences of the total cross sections for the formation of positive and negative sulfur and selenium ions by the electron impact are also measured. The ionization energies of sulfur and selenium are determined, and the origin of the features observed in the measured curves is identified. It is found that, in the interval of temperatures, at which the experiments were carried out, the vapors of the researched objects, besides polyatomic molecules, also contain diatomic molecules of the studied elements at high concentrations. It is demonstrated that the atoms in excited states are mainly produced due to the dissociation of diatomic molecules.

Revisiting Lewis dot structure weightings: a pair density perspective.

A method based on a real space partitioning to measure the importance of Lewis structures is proposed in this work. A matrix containing diverse QTAIM atomic and diatomic properties endowed with significance within a Lewis structure framework is expanded in terms of what we call Lewis-structure matrices. Each of these matrices flawlessly describes an individual resonance structure and its associated linear expansion coefficient (Q-ALE coefficient) indicates the importance or convenience of the given Lewis structure. These coefficients were inspected looking at their evolution in a series of usual chemical issues. Among all the results, we find of interest that σ resonance structures in systems with π electrons are more important than normally expected, which justifies why the qualitative predictions arising from the application of the resonance model and the quantitative results based on QTAIM properties are sometimes discrepant. Likewise, we observe that the variation of the dielectric constant of the medium affects the π resonance to a greater extent than it does the σ one. Other interesting results in this manuscript are connected to homolytic dissociation of diatomic molecules, periodic trends in hydrogen compounds, and polarization of aromatic systems as a consequence of their interaction with electric fields and with diverse ions.

Assessing the accuracy of new geminal-based approaches.

We present a systematic theoretical study on the dissociation of diatomic molecules and their spectroscopic constants using our recently presented geminal-based wave function ansätze. Specifically, the performance of the antisymmetric product of rank two geminals (APr2G), the antisymmetric product of 1-reference-orbital geminals (AP1roG) and its orbital-optimized variant (OO-AP1roG) are assessed against standard quantum chemistry methods. Our study indicates that these new geminal-based approaches provide a cheap, robust, and accurate alternative for the description of bond-breaking processes in closed-shell systems requiring only mean-field-like computational cost. In particular, the spectroscopic constants obtained from OO-AP1roG are in very good agreement with reference theoretical and experimental data.

Molecular dissociation and vibrational excitation on a metal hot filament surface

The dissociation of diatomic molecules (H2, N2, O2) on the surface of a metal hot filament (HF) has been studied theoretically, within the framework of a unified two-step reaction mechanism, and via joint experimental-modelling studies of the gas temperature and selected species concentration distributions in the vicinity of the HF. This approach allows description of the variations in atomic concentrations with distance (d, measured from the HF) as functions of HF temperature and gas pressure. Complications associated with the extension of such analyses to interpretations of prior experimental measurements for larger, heteronuclear molecules (CH4, nitrogen oxides) are discussed. The present analysis also provides a rationale for the d-dependent non-equilibrium [N2(v = 1)]/[N2(v = 0)] vibrational population ratios measured in tungsten HF-activated N2 gas samples.

Out-of-equilibrium catalysis of chemical reactions by electronic tunnel currents

We present an escape rate theory for current-induced chemical reactions. We use Keldysh nonequilibrium Green's functions to derive a Langevin equation for the reaction coordinate. Due to the out of equilibrium electronic degrees of freedom, the friction, noise, and effective temperature in the Langevin equation depend locally on the reaction coordinate. As an example, we consider the dissociation of diatomic molecules induced by the electronic current from a scanning tunnelling microscope tip. In the resonant tunnelling regime, the molecular dissociation involves two processes which are intricately interconnected: a modification of the potential energy barrier and heating of the molecule. The decrease of the molecular barrier (i.e., the current induced catalytic reduction of the barrier) accompanied by the appearance of the effective, reaction-coordinate-dependent temperature is an alternative mechanism for current-induced chemical reactions, which is distinctly different from the usual paradigm of pumping vibrational degrees of freedom.

A multiconfigurational hybrid density-functional theory

We propose a multiconfigurational hybrid density-functional theory which rigorously combines a multiconfiguration self-consistent-field calculation with a density-functional approximation based on a linear decomposition of the electron-electron interaction. This gives a straightforward extension of the usual hybrid approximations by essentially adding a fraction λ of exact static correlation in addition to the fraction λ of exact exchange. Test calculations on the cycloaddition reactions of ozone with ethylene or acetylene and the dissociation of diatomic molecules with the Perdew-Burke-Ernzerhof and Becke-Lee-Yang-Parr density functionals show that a good value of λ is 0.25, as in the usual hybrid approximations. The results suggest that the proposed multiconfigurational hybrid approximations can improve over usual density-functional calculations for situations with strong static correlation effects.

Generation and control of spin-orbit entanglement in atomic pairs

We propose a method for producing entangled spin and orbital states in pairs of atoms resulting from the dissociation of diatomic molecules. The method employs the interference between a one-photon and a two-photon process to break the backward-forward symmetry of the dissociation processes. The spin entanglement of distant atom pairs makes use of excited spin-orbit-coupled (molecular) intermediate states. When the dissociating molecules are confined to a one-dimensional optical lattice, the method can be used to generate long sequences of (spin- and orbit-) entangled pairs of atoms.