Heteronuclear Molecules Research Articles

Split electrons in partition density functional theory

Partition density functional theory is a density embedding method that partitions a molecule into fragments by minimizing the sum of fragment energies subject to a local density constraint and a global electron-number constraint. To perform this minimization, we study a two-stage procedure in which the sum of fragment energies is lowered when electrons flow from fragments of lower electronegativity to fragments of higher electronegativity. The global minimum is reached when all electronegativities are equal. The non-integer fragment populations are dealt with in two different ways: (1) An ensemble approach (ENS) that involves averaging over calculations with different numbers of electrons (always integers) and (2) a simpler approach that involves fractionally occupying orbitals (FOO). We compare and contrast these two approaches and examine their performance in some of the simplest systems where one can transparently apply both, including simple models of heteronuclear diatomic molecules and actual diatomic molecules with two and four electrons. We find that, although both ENS and FOO methods lead to the same total energy and density, the ENS fragment densities are less distorted than those of FOO when compared to their isolated counterparts, and they tend to retain integer numbers of electrons. We establish the conditions under which the ENS populations can become fractional and observe that, even in those cases, the total charge transferred is always lower in ENS than in FOO. Similarly, the FOO fragment dipole moments provide an upper bound to the ENS dipoles. We explain why and discuss the implications.

A multiple scattering theoretical approach to time delay in high energy core-level photoemission of heteronuclear diatomic molecules

We present analytical expressions of momentum-resolved core-level photoemission time delay in a molecular frame of a heteronuclear diatomic molecule upon photoionization by a linearly polarized soft x-ray attosecond pulse. For this purpose, we start to derive a general expression of photoemission time delay based on the first order time dependent perturbation theory within the one electron and single channel model in the fixed-in-space system (atoms, molecules and crystals) and apply it to the core-level photoemission within the electric dipole approximation. By using multiple scattering theory and applying series expansion, plane wave and muffin-tin approximations, the core-level photoemission time delay is divided into three components, , and , which are atomic photoemission time delay, delays caused by the propagation of photoelectron among the surrounding atoms and the scattering of photoelectron by them, respectively. We applied a single scattering approximation to and obtained for a linearly polarized soft x-ray field with polarization vector parallel to the molecular axis for a heteronuclear diatomic molecule, where is the angle of measured photoelectron from the molecular axis. The core-level photoemission time delay is approximated well with this simplified expression in the high energy regime ( a.u.−1, where is the amplitude of photoelectron momentum ), and the validity of this estimated result is confirmed by comparing it with multiple scattering calculations for C 1s core-level photoemission time delay of CO molecules. shows a characteristic dependence on θ, it becomes zero at θ = 0°, exhibits extended x-ray absorption fine structure type oscillation with period at θ = 180°, where R is the bondlength, and gives just the travelling time of photoelectron from the absorbing atom to the neighbouring atom at θ = 90°. We confirmed these features also in the numerical results performed by multiple scattering calculations.

Achieving high molecular alignment and orientation for CH_3F through manipulation of rotational states with varying optical and THz laser pulse parameters

Increasing interest in the fields of high-harmonics generation, laser-induced chemical reactions, and molecular imaging of gaseous targets demands high molecular “alignment” and “orientation” (A&O). In this work, we examine the critical role of different pulse parameters on the field-free A&O dynamics of the CH_3F molecule, and identify experimentally feasible optical and THz range laser parameters that ensure maximal A&O for such molecules. Herein, apart from rotational temperature, we investigate effects of varying pulse parameters such as, pulse duration, intensity, frequency, and carrier envelop phase (CEP). By analyzing the interplay between laser pulse parameters and the resulting rotational population distribution, the origin of specific A&O dynamics was addressed. We could identify two qualitatively different A&O behaviors and revealed their connection with the pulse parameters and the population of excited rotational states. We report here the highest alignment of langle {mathrm{cos}^2theta }rangle = 0.843 and orientation of langle {mathrm{cos}(theta )}rangle = 0.886 for CH_3F molecule at 2 K using a single pulse. Our study should be useful to understand different aspects of laser-induced unidirectional rotation in heteronuclear molecules, and in understanding routes to tune/enhance A&O in laboratory conditions for advanced applications.

Real-time calibration of nuclear velocities in the dissociation of heteronuclear molecules in strong laser fields

We design a strategy for the real-time calibration of nuclear velocities in ultrafast molecular reactions in strong laser fields by simulating the four-dimensional time-dependent Schr\"odinger equation. Taking $\mathrm{H}{\mathrm{D}}^{+}$ as the prototype, an attosecond pump pulse initiates the molecular dissociation, and the other time-delayed attosecond probe pulse, polarized in the plane perpendicular to the molecular axis, ionizes the dissociating $\mathrm{H}{\mathrm{D}}^{+}$. During the dissociation, two nuclei with different masses acquire different velocities, which imprint on the photoelectron kicked off by the probe pulse. The expected photoelectron momentum along the molecular axis can be used to precisely calibrate instantaneous nuclear velocities at ionization. This strategy works widely in general heteronuclear molecules and paves the way for making high-definition molecular movies.

Direct evidence of the dominant role of multiphoton permanent-dipole transitions in strong-field dissociation of NO2+

We study laser-induced dissociation of a metastable ${\mathrm{NO}}^{2+}$ ion-beam target into ${\mathrm{N}}^{+}+{\mathrm{O}}^{+}$, focusing on the prominent contribution by molecules breaking parallel to the polarization at high peak laser intensity $(\ensuremath{\sim}{10}^{15}$ $\mathrm{W}/{\mathrm{cm}}^{2})$. Our experimental results and time-dependent Schr\"odinger equation calculations show that, contrary to commonly held intuition that electronic transitions always prevail, the dominant process underlying this highly aligned dissociation is a multiphoton permanent-dipole transition involving only the electronic ground state and leading to its vibrational continuum. Strong-field permanent-dipole transitions should thus be considered generally, as they may play a significant role in other heteronuclear molecules. Moreover, their role should only grow in importance for longer wavelengths, a trending direction in ultrafast laser studies.

Molecular Bonding in an Orbital-Free-Related Density Functional Theory.

A density functional theory based on polymer self-consistent field theory is applied to systems of two atoms in order to show that this approach is capable of predicted molecular bonding. Periodic table elements from hydrogen up to neon are examined and homonuclear diatomic molecules are found to form for H2, N2, O2, and F2, in agreement with known results. The heteronuclear molecules CO and HF, which are known to exist under ambient conditions, are also found to be stable. Bond lengths for most of these molecules agree with experimental results to within less than 8%, with the exception of O2 and F2 which deviate more significantly. The bonding energy for H2 is given and is within 16% of the known value, but fundamental vibrational frequencies do not agree well with experiment. The main approximations of the theory are very simple and include a Fermi-Amaldi correction to the electron-electron interaction to account for self-interactions and a basic expression for the Pauli potential to account for the exclusion principle. The self-consistent equations are solved in terms of basis functions that encode the cylindrical symmetry of diatomic molecules. Since orbitals are not used, the approach is related to orbital-free density functional theory.

A heteronuclear bimetallic organic molecule enabling targeted synthesis of an efficient Pt1Fe1 intermetallic compound for oxygen reduction reaction

A controllable pyrolysis of a Pt–Fe heteronuclear bimetallic organic molecule was reported for the synthesis of highly ordered and small-sized Pt1Fe1-IMC/C catalysts.

Quantum gases in optical boxes

Quantum atomic and molecular gases are flexible systems for studies of fundamental many-body physics. They have traditionally been produced in harmonic electromagnetic traps and thus had inhomogeneous densities, but recent advances in light shaping for optical trapping of neutral particles have led to the development of flat-bottomed optical box traps, allowing the creation of homogeneous samples. Box trapping simplifies the interpretation of experimental results, provides more direct connections with theory and, in some cases, allows qualitatively new, hitherto impossible experiments. It has now been achieved for both Bose and Fermi atomic gases in various dimensionalities, and also for gases of heteronuclear molecules. Here we review these developments and the consequent breakthroughs in the study of both equilibrium and non-equilibrium phenomena such as superfluidity, turbulence and the dynamics of phase transitions. Optical box traps create a potential landscape for quantum gases that is close to the homogeneous theoretical ideal. This Review of box trapping methods highlights the breakthroughs in experimental many-body physics that have followed their development.

Readdressing molecular dissociation within the Kohn–Sham formalism of density-functional theory: simple models and a different point of view

Within the Kohn–Sham (KS) formalism of density-functional theory (DFT), most available exchange-correlation (XC) density-functionals are known to yield incorrect molecular dissociation even when dealing with simple molecules as H. It is known that the KS potential must develop a peak and a step in the region between the isolated atoms, in order to prevent electrons to hop from one atom to another, and hence avoiding fractionally charged dissociated atoms. Considering one-dimensional models of two-electron homo and heteronuclear molecules, we here (i) readdress the limit of dissociated molecules, with a different point of view on the formation of the peaks and steps in the KS potential and (ii) investigate a simple approach to treat not necessarily dissociated molecules, by employing single-particle orbitals (not KS orbitals). Our results indicate that an accurate description of dissociation can be achieved.

Stokes and Antistokes Raman Scattering in Heteronuclear and Homonuclear Molecules

Stokes and Antistokes Raman Scattering in Heteronuclear and Homonuclear Molecules

Theoretical aspects of radium-containing molecules amenable to assembly from laser-cooled atoms for new physics searches

We explore the possibilities for a next-generation electron-electric-dipole-moment experiment using ultracold heteronuclear diatomic molecules assembled from a combination of radium and another laser-coolable atom. In particular, we calculate their ground state structure and their sensitivity to parity- and time-reversal () violating physics arising from flavor-diagonal charge-parity () violation. Among these species, the largest -violating molecular interaction constants—associated for example with the electron electric dipole moment—are obtained for the combination of radium (Ra) and silver (Ag) atoms. A mechanism for explaining this finding is proposed. We go on to discuss the prospects for an electron EDM search using ultracold, assembled, optically trapped RaAg molecules, and argue that this system is particularly promising for rapid future progress in the search for new sources of violation.

Formation of ultracold heteronuclear polyatomic molecule: multi-path scheme and interference effect

We investigate the creation of stable heteronuclear polyatomic molecules based on the three-body and higher order Efimov state via a generalized stimulated Raman adiabatic passage scheme. Within the mean-field approximation, we establish the multi-path conversion models and derive the dark state solutions. The multi-path atom–polymer conversion dynamics and interference effect are studied. We show that the multi-path constructive interference permits higher conversion efficiency, resulting in a state that is closer to the ideal dark state solutions. The effects on the conversion efficiency of the single-photon detuning, the strength of the Rabi pulse, the spontaneous emission from intermediate states and the interaction between the particles are also discussed. Our results not only include the previous ultracold molecule formation techniques, such as the single-path polyatomic molecule, double-path pentamer and triple-path trimer, but also open a possible route for the multi-path formation of more complex polyatomic molecules.

Optical Radiation from the Sputtered Species under Excitation of Ternary Mixtures of Noble Gases by the 6Li(n,α)3H Nuclear Reaction Products

The present paper examines the luminescence of ternary Ar-Kr-Xe and Ne-Ar-Kr mixtures of noble gases in the spectral range from 300 to 970 nm, excited by the 6Li(n,α)3H nuclear reaction products in the core of a nuclear reactor. A thin layer of lithium applied on the walls of the experimental device, stabilized in the matrix of the capillary-porous structure, serves as a source of gas excitation. During in-pile tests, conducted at the IVG.1M research reactor, thermal neutrons interact via the 6Li(n,α)3H reaction, and the emergent alpha particles with a kinetic energy of 2.05 MeV and tritium ions with a kinetic energy of 2.73 MeV excite gaseous medium. The study was carried out in a wide temperature range. The temperature dependence of the intensity of the emission of the atoms of noble gases and alkali metals, heteronuclear ionic molecules of noble gases were studied. The obtained values of the activation energy of the emission process 1.58 eV for lithium and 0.72 eV for potassium agree well with the known values of evaporation energy. Excitation of alkali metals atoms occurs consequently of the Penning process of alkali metals atoms on noble gas atoms in the 1s-states and further ion-molecular reactions.

FCI calculations of the adiabatic electronic structure of KRb highlighting the K−Rb+ ionic limit effect

A non-relativistic description of the electronic structure for the heteronuclear molecule KRb was developed through the Full Configuration Interaction (FCI) and pseudo-potential method. Below the ionic limit K−Rb+, the ground and all excited singlet and triplet states of different symmetries Σ+, Π and Δ (in the Hund's case (a): 2s + 1Λ±) were intensively investigated. Several adiabatic results including potential energy curves (PECs), permanent and transition electric dipole moments (PEDM, TEDM) were analyzed in short and long-range of internuclear distances. Tables of these properties as functions of separation are presented. The spectroscopic constants with the vibrational levels were derived using the “Numerov” algorithm. These molecular parameters were compared with those published in the literature, feature a good agreement. In the adiabatic representation, strong couplings between successive 1Σ+ states were reached by series of avoided crossing, yielding to the appearance of the ionic character and the good illustration of charge transfer. This description of the KRb molecule was helpful in the design of the prospect photo-association experiments and the obtaining of the quantum-degenerate gas of the polar molecule.

Two-center interference and stereo Wigner time delay in photoionization of asymmetric molecules

We present numerical simulations of the time delay in photoemission of asymmetric diatomic molecules using the technique of reconstruction of attosecond beating by interference of two-photon transitions (RABITT) by solving the time-dependent Schr\"odinger equation. Our results show an obvious time delay between photoelectrons emitted to the left and right and this relative time delay oscillates as the photoelectron energy changes. More interestingly, the amplitude of this oscillation increases when the asymmetry degree of diatomic molecules decreases. With the method of the selected continuum wave functions, we calculate the Wigner time delay in photoionization. The obtained stereo Wigner time delay also oscillates with photoelectron energy. This oscillation is traced back to two-center interferences and it could explain the relative time delay in the RABITT measurement. Furthermore, our results indicate that the continuum-continuum time delay in photoemission of heteronuclear molecules is asymmetric.

Improving the SAFT-γ Mie equation of state to account for functional group interactions in a structural (s-SAFT-γ Mie) framework: Linear and branched alkanes.

The group contribution SAFT-γ Mie EoS is based on the statistical associating fluid theory for fused heteronuclear molecules. While the chain term of the model has been modified to account for the new functional group-specific parameters, it does not impose a bonding order to these functional groups, only considering intergroup interactions in the monomer reference fluid. This leaves the model unable to account for the different physical properties of structural isomers and implicitly introducing modeling bias to species where the molecular structure mimics those used in the parameter regression. In this work, a simple but physically meaningful modification to the chain term in SAFT-γ Mie is proposed that accounts for the number of intergroup bonds, thereby encoding structural information in the model, without introducing an additional regressed parameter. The resulting structural SAFT-γ Mie (s-SAFT-γ Mie) requires reparameterization of the group parameters, which we present for linear and branched alkanes (CH3, CH2, CH, and C groups) here. Following an identical parameterization procedure to the original model, validation showed that the modification actually improves prediction accuracy for linear alkanes while addressing the original inability of the framework to distinguish between structural isomers. The good predictive performance seen in this work, for both pure component and mixture properties, lays a good foundation for expansion to other functional groups in future work.

Strong-field ionization of heteronuclear diatomic molecules using an orthogonally polarized two-color laser field

We apply the improved molecular strong-field approximation to investigate high-order above-threshold ionization (HATI) of heteronuclear diatomic molecules by an orthogonally polarized two-color (OTC) laser field. The OTC field consists of two linearly polarized components with frequencies $r\ensuremath{\omega}$ and $s\ensuremath{\omega}$, where $r$ and $s$ are integers, and $\ensuremath{\omega}$ is the fundamental frequency. The molecule is aligned in the OTC laser field polarization plane. We show that the photoelectron momentum distribution obeys one reflection symmetry which is valid for arbitrary values of the relative phase between the OTC field components in the case when $r+s$ is odd. For molecules oriented along the polarization axis ${z}_{L}$ of the $r\ensuremath{\omega}$ component (${\ensuremath{\theta}}_{L}={0}^{\ensuremath{\circ}}$) and $r$ even and $s$ odd, the HATI spectrum exhibits the reflection symmetry with respect to the ${z}_{L}$ axis. When the molecular orientation is along the ${x}_{L}$ axis, which is perpendicular to the polarization axis of the $r\ensuremath{\omega}$ component (${\ensuremath{\theta}}_{L}={90}^{\ensuremath{\circ}}$), the spectrum exhibits the reflection symmetry with respect to the ${x}_{L}$ axis for $r$ odd and $s$ even. In addition, we analyze the asymmetry in the photoelectron spectra of heteronuclear molecules by comparing them with the photoelectron spectra obtained ionizing a homonuclear diatomic molecule. We also explore the influence of the shift of the relative phase by ${180}^{\ensuremath{\circ}}$ on the HATI spectra. We explain some characteristics of the obtained HATI spectra using a generalization of the classical two-dimensional simple man's model which includes ionization probabilities calculated using the imaginary-time method. Finally, we analyze the interference minima for different heteronuclear diatomic molecules and for particular values of the emission angle, laser-field parameters, and internuclear distance. These minima are well fitted with the curve obtained using the derived condition for the two-center destructive interference minima.

Energy loss of cluster ions in different concentration and temperature of plasma

The effects of the energy loss interference of homo and hetero nuclear di-cluster ions on the plasma in both Classic and Quantum Models are used to study of response dielectric function. The present work involves the interest ranges for Inertial Conference Fusion (ICF), Z-Pinch and Plasmas of Tokomak. The approximation of the Quantum Random Phase (RPA) is used, and the individual and collective Bohm-Pines model is calculated and the contribution of each mode will be calculated. For incident cluster ions, we present measurement and comparison of the stopping power for (homo nuclear di-cluster (H-H), (He-He)) and hetero-nuclear di-cluster ions like (He-H) in different plasma concentration and temperature of (ICF, Z-Pinch and TOKOMAK). The result appear the dependence of the interference term I(r) on homo and hetero-nuclear di cluster distance r 12 and the di-cluster velocity. All equations have been programmed to present the work using FORTRAN-90 and the program has been written for numerical calculations.

A general form of Macheret–Fridman classical impulsive dissociation model for nonequilibrium flows

The rate of dissociation behind a strong shock in thermochemical nonequilibrium depends on the vibrational excitation of the molecules, hence the rates become a function of translational-rotational and vibrational temperatures. The Macheret–Fridman (MF) model provides analytical expressions for nonequilibrium dissociation rates assuming the collision of molecules to be in the impulsive limit. However, the original form of the model was limited to the dissociation of homonuclear molecules. In this work, we present a general form of the Macheret–Fridman classical impulsive model by considering the dissociation of a heteronuclear molecule and present macroscopic rates applicable for modeling dissociation in computational fluid dynamics (CFD). The nonequilibrium dissociation rates from the MF-CFD model compared well with the available quasiclassical trajectory (QCT) data for some important reactions in the air. Additionally, we also present a comparison of the average vibrational energy removed in a dissociation reaction predicted by the MF-CFD model with QCT data for several reactions in air and propose some improvements to the model. The developed MF-CFD model was used to investigate various nonequilibrium flow problems and the results were compared with available experimental data. In general, the results from the MF-CFD model are promising and the model shows a possibility of becoming the standard tool for investigating nonequilibrium flows in CFD.

Accurate semiempirical potential energy curves for the a3Σ+-state of NaCs, KCs, and RbCs.

A five parameter semiempirical Tang-Toennies type model is used to describe the potential curves of the a3Σ+-state of the heteronuclear polar molecules NaCs, KCs, and RbCs. These molecules are of current interest in experiments at ultra-cold conditions to explore the effects of the strong dipole-dipole forces on the collective many-body quantum behavior. New quantum phenomena are also anticipated in systems consisting of atomic species with different fermion/boson statistics. The model parameters are obtained by simultaneously fitting all five of the parameters to the extensive LIF-Fourier transform spectroscopy published by Tiemann and collaborators [e.g., Docenko et al. J. Phys. B: At., Mol. Opt. Phys. 39, S929-S943 (2006)], who also report best fit potential curves. Although the new potentials are in good agreement with the earlier potentials, they have the advantage that they are continuous over the entire range of internuclear distances and have the correct long-range behavior. The scattering lengths for all isotope combinations show good agreement with dedicated experiments where available. The new potentials are also in excellent agreement with combining rules based on the potentials of the homonuclear systems.