Electronic Excitation Of Molecules Research Articles

Dual-Band Coupling of Phonon and Surface Plasmon Polaritons with Vibrational and Electronic Excitations in Molecules

Strong coupling (SC) between light and matter excitations bears intriguing potential for manipulating material properties. Typically, SC has been achieved between mid-infrared (mid-IR) light and molecular vibrations or between visible light and excitons. However, simultaneously achieving SC in both frequency bands remains unexplored. Here, we introduce polaritonic nanoresonators (formed by h-BN layers on Al ribbons) hosting surface plasmon polaritons (SPPs) at visible frequencies and phonon polaritons (PhPs) at mid-IR frequencies, which simultaneously couple to excitons and molecular vibrations in an adjacent layer of CoPc molecules, respectively. Employing near-field optical nanoscopy, we demonstrate the colocalization of near fields at both visible and mid-IR frequencies. Far-field transmission spectroscopy of the nanoresonator structure covered with a layer of CoPc molecules shows clear mode splittings in both frequency ranges, revealing simultaneous SPP-exciton and PhP-vibron coupling. Dual-band SC may offer potential for manipulating coupling between exciton and molecular vibration in future optoelectronics, nanophotonics, and quantum information applications.

GPU Acceleration of Large-Scale Full-Frequency GW Calculations.

Many-body perturbation theory is a powerful method to simulate electronic excitations in molecules and materials starting from the output of density functional theory calculations. By implementing the theory efficiently so as to run at scale on the latest leadership high-performance computing systems it is possible to extend the scope of GW calculations. We present a GPU acceleration study of the full-frequency GW method as implemented in the WEST code. Excellent performance is achieved through the use of (i) optimized GPU libraries, e.g., cuFFT and cuBLAS, (ii) a hierarchical parallelization strategy that minimizes CPU-CPU, CPU-GPU, and GPU-GPU data transfer operations, (iii) nonblocking MPI communications that overlap with GPU computations, and (iv) mixed precision in selected portions of the code. A series of performance benchmarks has been carried out on leadership high-performance computing systems, showing a substantial speedup of the GPU-accelerated version of WEST with respect to its CPU version. Good strong and weak scaling is demonstrated using up to 25 920 GPUs. Finally, we showcase the capability of the GPU version of WEST for large-scale, full-frequency GW calculations of realistic systems, e.g., a nanostructure, an interface, and a defect, comprising up to 10 368 valence electrons.

Electronic excitation of C6H6 by positron impact

Experiments on electronic excitation of molecules using positron as incident particle have shown much larger cross sections than in the electron scattering case. The comprehension of these inelastic processes represent a great challenge and only few studies on electronic excitation of molecules are discussed in the literature. For example, for the C6H6 molecule experimental and theoretical calculations are not in a very advanced state same for electron scattering case (Benzene represent a simplest aromatic hydrocarbon and very important chemical compound due to its role as a key precursor in process pharmaceutical). Recent experiments on electronic excitation of C6H6 (1B1u, and 1E1u electronic states) using electron as incident particle are available by Kato et al (J.Chem.Phys.134 134308(2011)). Motivated by their experiments we have taken up the task to investigate the same electronic excitation of C6H6 using positron as incident particle. For the first time, integral cross sections in e+ - C6H6 (1B1u, and 1E1u electronic states) using the scaling Born positron (SBP) approach are reported and in the absence of the experimental data and developments theoretical, comparisons are made with analogous electron scattering.Keywords: Born, positron, scaling

Base Promoted Synthesis of 2-((5-methoxynaphthalen-1-yl)methyl)-3-methyl-5-sec-amino-[1,1′-biphenyl]-4-carbonitrilederivatives: Photophysical, Solvatochromic and DFT studies

Newer, simple and efficient synthesis of 2-((5-methoxynaphthalen-1-yl)methyl)-3-methyl-5-sec-amino-[1,1′-biphenyl]-4-carbonitrile derivatives using ring transformation strategy. The synthesized products were characterized by spectral analysis, fluorescence emission and absorption measurements were studied in various solvents having varying polarity and dielectric constants to find out a change in dipole moment in ground state and excited state of derivatives. Lippert–Mataga plots were utilized to know the effect of solvent polarity on spectral behavior of molecules. The position of the Stokes shifts was found to be depends on specific solute–solvent interactions, playing a vital role in electronic excitation of molecules. A rational relationship is examined between fluorescence quantum efficiencies and calculated HOMO and LUMO energies by DFT calculations. The electrochemical investigation of derivatives was performed by cyclic voltammetry.

PySCF-NAO: An efficient and flexible implementation of linear response time-dependent density functional theory with numerical atomic orbitals

We present an algorithm and its implementation to calculate the propertiesof electronic excitations in molecules and clusters from first principles, using time-dependent density functional theory (TDDFT). The algorithm assumes the use of some localized functions as a basis set to represent the spatial degrees of freedom. It relies on an iterative computation of the induced density according to the Dyson-like equation for the linear response function. The current implementation is built upon so-called numerical atomic orbitals. It is suitable for a wide variety of density functional theory (DFT) software. In this work, we demonstrate TDDFT calculations starting from preceding DFT runs with SIESTA, GPAW and PySCF packages, while a coupling with the other DFT packages such as Fireball and OpenMX is planned. The mentioned packages are capable of performing ab initio molecular dynamics simulations, and the speed of our TDDFT implementation makes feasible to perform a configuration average of the optical absorption spectra. Our code is written mostly in Python language allowing for a quick and compact implementation of most numerical methods and data-managing tasks with the help of NumPy/SciPy libraries and Python intrinsic constructs. Part of the code is written in C and Fortran to achieve a competitive speed in particular sections of the algorithm. Many parts of the current algorithm and implementation are useful in other ab initio methods for electronic excited states, such as Hedin’s GW, Bethe–Salpeter equation and DFT with hybrid functionals. Corresponding proof-of-principles implementations are already part of the code. Program summaryProgram Title:PySCF-NAOProgram Files doi:http://dx.doi.org/10.17632/9wgp6255hn.1Licensing provisions: Apache License, Version 2.0Programming language: Python (2 or 3), Fortran90 and CSupplementary material: We provide the source code and input files to organize example and benchmark calculations discussed in the paper.Nature of the problem: The study of the interaction of photons and charged particles with matter depends upon understanding of electronic excitations in matter. A description of the electronic excitations within time-dependent density functional theory (TDDFT) is popular due to the combination of its reasonable accuracy and its relatively low computational cost. Despite the relative simplicity of TDDFT, its application becomes difficult for quantum systems containing several hundreds of atoms. Such microscopically small systems are relevant in organic electronics, plasmonics and surface science. Therefore, much work has been devoted to the development of adequate electronic structure methods. Moreover, thermal motion of atoms and the presence of solvents affects the properties of electronic excited states in a decisive manner. Modeling of the electronic excited states including the system’s dynamics within the Born–Oppenheimer approximation leads to even stronger efficiency requirements from the corresponding electronic structure methods.Solution method: We discretize the Kohn–Sham Hamiltonian using a basis of numerical atomic orbitals. The induced electronic density is determined in response to a dipolar external perturbation, according to linear response TDDFT, using an iterative algorithm. The method takes advantage of the sparsity generated by the finite support of the numerical atomic orbitals. This allows computing the dynamical polarizability of molecules and clusters containing up to several thousands of atoms.Additional comments including Restrictions and Unusual features: The current algorithm is formulated within the formalism of density response functions. Therefore, one needs to know explicitly an exchange–correlation kernel (second-derivative of energy with respect to variation of the density). The exchange–correlation kernel is analytically known for (semi-)local density functionals, but not for the hybrid density functionals. To date, we implemented only the local density approximation (LDA) for the exchange–correlation kernel to be used with the iterative TDDFT in PySCF-NAO. Spin-restricted formalism for finite systems is covered in the current implementation. Moreover, although our code is prepared to compute the electronic response of all-electron systems, our current implementation of an auxiliary product basis (density-fitting basis) is working best in combination with the use of pseudopotentials.

The Particle-Hole Map: A Computational Tool To Visualize Electronic Excitations.

We introduce the particle-hole map (PHM), which is a visualization tool to analyze electronic excitations in molecules in the time or frequency domain, to be used in conjunction with time-dependent density-functional theory (TDDFT) or other ab initio methods. The purpose of the PHM is to give detailed insight into electronic excitation processes that is not obtainable from local visualization methods, such as transition densities, density differences, or natural transition orbitals. The PHM is defined as a nonlocal function of two spatial variables and provides information about the origins, destinations, and connections of charge fluctuations during an excitation process; it is particularly valuable to analyze charge-transfer excitonic processes. In contrast with the transition density matrix, the PHM has a statistical interpretation involving joint probabilities of individual states and their transitions; it satisfies several sum rules and exact conditions, and it is easier to read and interpret. We discuss and illustrate the properties of the PHM and give several examples and applications to excitations in one-dimensional model systems, in a hydrogen chain, and in a benzothiadiazole (BT)-based molecule.

Vibrational kinetics of electronically-excited molecules in nitrogen discharge plasma

A model on formation of the vibrational distribution function of metastable electronically excited states in nitrogen discharge plasma is presented. By means of comparison with the experimental data the rate constants for the reaction have been clarified. It is assumed that the probability of population of states by collisional deactivation of N2(B 3Πg, v′) molecules is proportional to the Franck–Condon factors of correspondent transitions, i.e. that the quenching is essentially vertical. The results of calculations carried out under this assumption are in agreement with measured vibrational distribution at the end of pulsed high current discharge in nitrogen.Study of a variation of the population of vibrational level in the afterglow of streamer discharge in nitrogen at atmospheric pressure has been performed. It is shown that the dynamics of number density depends essentially not only on their loss in the pooling reaction product, but also on additional population due to the quenching of high vibrational levels. The fraction of dissociation channel in the process of deactivation by oxygen molecules have been calculated. It is shown that temporal evolution of the fraction of the dissociation channel is first of all associated with a variation of the relative population of the vibrational level . In air discharge plasma at high pressure, the population of state is relatively low, so the fraction of the dissociation channel in the deactivation of all electronically excited states of nitrogen by oxygen exceeds 90–95%.

Plasmons in Molecules

The classification of electronic excitations in molecules and molecular nanostructures plays an important role when tailoring materials with desired properties. One example of such a class of excitations are plasmons. Plasmons appear in solid-state physics, where they are characterized as density oscillations in an electron gas that are driven by the restoring forces associated with the electromagnetic field induced by the density oscillations themselves. Here, we investigate how this concept can be transferred to molecular systems by performing a step-by-step analysis, starting from three-dimensional bulk systems and ending with molecules. On the basis of this analysis, we propose to scale the electron–electron interaction in quantum-chemical response calculations in order to identify plasmons in molecules. This approach is illustrated for molecular chains and clusters. Our results show that the concept of plasmons is still applicable for extended molecular systems and demonstrate that the proposed scaling approach provides an easy way of characterizing electronic excitations.

On-Line Gas Measurements from PEM Fuel Cell Flow Fields by Electric Arc Emission Spectroscopy

This contribution presents an arc emission spectrometer for the investigation of exhaust gases of PEM fuel cell flow fields. The method bases on the electronic excitation of molecules by collision with charge carriers within an electric arc. The excited molecules undergo irradiant deactivations that lead to light emissions which are specific for each molecular species. The method enables the record of gas concentrations on a millisecond time scale. Detectable concentrations range from pure gases to concentrations of a few ppm. The gas detector was used to investigate the intrinsic cross over of gases through the MEA unit of a fuel cell. In particular, the passage of nitrogen from the cathode to the anode was studied under selected conditions of fuel cell operation. In addition, pre-stages of membrane degradation can be pursued.

Ionization of electron-excited molecules in a strong electric field

In this paper, we prove the direct effect of a strong electric field on the ionization process occurring in dense hightemperature mixtures of molecular gases containing electronexcited particles. Our proof is based on the analysis of systematic divergences of experimental and theoretical data on the value of the electric current onto an electric probe placed into these mixtures. For a wide number of mixtures of var� ious compositions, the electric current calculated on the basis of the conventional theory of electric probes always turns out to be several times lower than the measured current. Introducing into the conventional theory an additional ionization source stipulated by the strong electric field near the probe surface has resulted in satisfactory consistency between the calcu� lated data and the available experimental data. The electricprobe theory for dense chemically reacting plasmas at normal pressure was developed by the authors (together with G.S. Aravin, P.A. Vlasov, and Yu.K. Karasevich, Institute of Chemical Physics, Russian Academy of Sciences) in the 1980s (1-5). A review of these studies was presented in (6). The theory is based on experimental data on the unsteady chemi� calionization process in a shock tube beyond the reflected shock wave in various gas mixtures. In these measurements, a twoconductor microwave interfer� ometer was used, with one of the conductors being a cylindrical probe maintained at a constant negative electric potential. Upon significantly elevating the temperature in the region beyond the reflected shock wave in the gas mix� ture under study, unsteady chemical processes began to develop. Among them, chemicalionization reac� tions including the production of positive ions and electrons were present. In the experiment, the unsteady electric current onto a probe was measured. Then, in accordance with microwave measurements, the electronconcentration profiles were plotted as functions of time. These data have made it possible to find the effective timedependent ionization rate. This function obtained experimentally was used for numer� ically solving the probe problem. The conventional setting of the problem of immobile lowtemperature ( T ~ 2000-3500 K) ionized gas mixtures at pressures on the order of atmospheric pressure can be formu� lated in the following manner. The unsteady set of equations in a cylindrical coor� dinate system can be written out in the form

Symmetry-adapted cluster and symmetry-adapted cluster-configuration interaction method in the polarizable continuum model: Theory of the solvent effect on the electronic excitation of molecules in solution

In this paper we present the theory and implementation of the symmetry-adapted cluster (SAC) and symmetry-adapted cluster-configuration interaction (SAC-CI) method, including the solvent effect, using the polarizable continuum model (PCM). The PCM and SAC/SAC-CI were consistently combined in terms of the energy functional formalism. The excitation energies were calculated by means of the state-specific approach, the advantage of which over the linear-response approach has been shown. The single-point energy calculation and its analytical energy derivatives are presented and implemented, where the free-energy and its derivatives are evaluated because of the presence of solute-solvent interactions. We have applied this method to s-trans-acrolein and metylenecyclopropene of their electronic excitation in solution. The molecular geometries in the ground and excited states were optimized in vacuum and in solution, and both the vertical and adiabatic excitations were studied. The PCM-SAC/SAC-CI reproduced the known trend of the solvent effect on the vertical excitation energies but the shift values were underestimated. The excited state geometry in planar and nonplanar conformations was investigated. The importance of using state-specific methods was shown for the solvent effect on the optimized geometry in the excited state. The mechanism of the solvent effect is discussed in terms of the Mulliken charges and electronic dipole moment.

On the mechanism of angular-momentum transfer in electronic excitation of molecules by electron impact

Experiments on electron-impact vibronic excitation of molecular hydrogen in gas discharge plasmas and by electron beams reveal significant angular-momentum transfer and change in the ortho/para ratio. The observed effects are due to excitation via formation of negative intermediate ions. A model of such processes is proposed that provides a quantitative description of experimental results. The results of chi-square significance tests suggest that the dominant process is H2(X1Σg|) + e(p) → H2−(2Πu) → H2(d3Πu, I1Πg) + e′(l). The relative probabilities of ion decay by autodetachment of s, p, and other electrons are determined.

Light Switching of Molecules on Surfaces

Smart surfaces, surfaces that respond to an external stimulus in a defined manner, hold considerable potential as components in molecular-based devices, not least as discrete switching elements. Many stimuli can be used to switch surfaces between different states, including redox, light, pH, and ion triggers. The present review focuses on molecular switching through the electronic excitation of molecules on surfaces with light. In developing light-responsive surfaces, investigators face several challenges, not only in achieving high photostationary states and fully reversible switching, but also in dealing with fatigue resistance and the effect of immobilization itself on molecular properties. The immobilization of light-responsive molecules requires the design and synthesis of functional molecular components both to achieve light switching and to anchor the molecular entity onto a surface. This review discusses several demonstrative examples of photoswitchable molecular systems in which the photochemistry has been explored in the immobilized state under ambient conditions and especially on electroactive surfaces, including self-assembled monolayers, bilayers, and polymer films.

Decomposition of electron-excited molecules of molybdenum and tungsten phosphine hydride and phosphine nitrogen complexes

The kinetics and mechanism of photodehydrogenation of the phosphine hydride complexes MH4L4 (M = Mo, W; L are phosphine ligands) and the formation of coordinatively unsaturated species ML4 were studied by the absorbance of long-wavelength bands with λmax at 450–460 nm appeared in the absorption spectra of the photoproducts. The rate constants of the reactions of the coordinatively unsaturated M(DPPE)2 species (M = Mo, W; DPPE = Ph2PCH2CH2PPh2) with molecular nitrogen in benzene were determined (kW = 200 s−1, kMo = 8700 s−1).

Polarization effects on electronic excitation of molecules by low-energy electron impact: Study one−-furan scattering

The Schwinger multichannel method is applied to study the influence of polarization effects on the electronic excitation of the furan molecule by low-energy electron impact. We discuss the importance of inclusion of these effects through the comparison of theoretical results for the electronic excitation of the ${^{3}B}_{2}$ state of furan obtained with and without the proper treatment of the polarization of the target. The electron-furan scattering presents two prominent shape resonances in the ${^{2}A}_{2}$ and ${^{2}B}_{1}$ symmetries at around the electronic excitation threshold of the ${^{3}B}_{2}$ state (3.7 eV). At this low-energy, the inclusion of polarization effects in the calculation moves to lower energies the resonances positions obtained either in the close-coupling or in the static-exchange level of approximation. This phenomenon strongly influences the electronic excitation process. The present results show that a simple close-coupling calculation cannot be applied for molecular systems with low-energy electronic excitation thresholds around misplaced resonances.

The effect of nonequilibrium excitation on the ignition of hydrogen-oxygen mixtures

The part played by various radicals and excited particles in reducing the induction period is discussed when analyzing the ignition of combustible mixtures under highly nonequilibrium conditions. A review is made of experimental and theoretical investigations of the effect made by atoms of hydrogen and oxygen, electron-excited molecules of O2(a 1Δg), and vibrationally excited molecules of H2 and O2 on the times of induction and on the shift of the limits of ignition of hydrogen-oxygen mixtures. The addition of atoms of oxygen and hydrogen to a combustible mixture may cause a significant reduction of the times of induction and the lowering of the temperature limit of ignition of combustible mixtures. However, the latter effect is observed only in the vicinity of the limit of ignition. In the zone of relatively low initial temperatures, the shift of the ignition limit is largely associated with the self-heating of mixture owing to recombination of atomic particles being added. In so doing, the nonequilibrium pattern of the impact hardly shows up. Molecules of singlet oxygen (SO) may be involved in both chain initiation reactions and chain branching reactions. The rates of these processes increase significantly with the temperature of the mixture. However, the temperature increase further causes an increase in the rate of reaction of hydrogen quenching of singlet oxygen O2(a 1Δg). The acceleration of deactivation of O2(a 1Δg) with increasing temperature, which was previously ignored by researchers, causes an abrupt reduction of the effectiveness of the impact of SO on the times of induction. In the low-temperature region, where the quenching is relatively weak, the effect of O2(a 1Δg) on the ignition of H2 : O2 mixtures may be significant in the case of a sufficiently high (in excess of 10−2) level of relative concentrations of SO. Vibrationally excited molecules of H2(v) may likewise be involved in chain initiation and branching reactions. The available results lead one to assume the possibility of lowering the temperature limit of ignition of hydrogen-oxygen mixtures in the case of intensive excitation of vibrational degrees of freedom of H2(v) molecules. However, in the case of electric-discharge excitation (even in H2 : O2 mixtures, in which a significant fraction of discharge energy may be delivered to the vibrational subsystem of H2(v)), it is quite difficult to attain high levels of vibrational excitation (T v H2 ≥ 2000 K). In a stoichiometric mixture of H2:air, where the bulk of discharge energy is spent for vibrational excitation of nitrogen molecules, this problem becomes even more complicated.

Ion photostimulated desorption as a tool for investigating adsorption and electronicexcitation of molecules on semiconductor surfaces

Ion photostimulated desorption (PSD) is a specific surface sensitive process. This paperreviews ion PSD following core level excitation of molecules adsorbed on the Si(111)7 × 7 andSi(100) 2 × 1 surfaces. Several aspects of ion PSD will be discussed; (i) the use ofion PSD as a tool for investigating the adsorption configurations ofO2 onSi(111) 7 × 7, (ii) the relevance of ion PSD for probing the physisorption–chemisorption transition ofbenzene molecules on Si(111), (iii) the comparison between ion PSD of methanol adsorbedon Si(111) and photofragmentation of the methanol molecule in the gas phase, (iv) thecomparison between ion PSD of formic acid adsorbed on the Si(111) and Si(100) surfaces and(v) the chemical selectivity of ion PSD following core level excitation of NO dissociativelyadsorbed on Si(111). Finally, the interplay between ion dynamics and electronic relaxationfollowing core level excitation and Auger processes will be discussed in the case ofO+ photodesorptionfrom O2 adsorbedon Si(111) 7 × 7, as investigated via isotope and temperature effects on both the intensities and kineticenergies of the desorbed ions.

From Supramolecular Photochemistry to Self-Assembled Photoactive Architectures: The Emergence of Photochemical Nanosciences

The use of supramolecular interactions to control and direct excited-state reactivity is described, with emphasis on the author's contributions in this area. The systems are based on the multiple-point hydrogen-bonding molecular recognition motif present in the melamine–barbituric acid system, and are designed to operate in solution phase. Despite the weak nature of non-covalent interactions, control of high energy intermediates such as excited cinnamantes, stilbenes, or fullerenes is possible. These results demonstrate the particular sensitivity of short-lived electronically excited molecules to their environment, and particularly to the mutual orientation of the reactants.

Possibilities for controlling attosecond x-ray pulse generation during molecular ionization by femtosecond laser radiation

We discuss the role of different factors (molecular sizes and configuration, orientation of the molecular axis with respect to the electric field of a laser pulse, the type of the molecular orbital, etc.), which characterize molecules and their state, in the formation of the nonlinear response of a molecule ionized by a high-power femtosecond laser pulse. In numerical experiments within the framework of a two-dimensional model for the H + molecular ion, we study possibilities for controlling the process of nonlinear frequency conversion of femtosecond optical radiation into X-ray radiation of attosecond duration by means of preliminary vibrational or electronic excitation of molecules. We demonstrate the possibilities of using the attosecond pulse generation as a diagnostic tool for probing vibration-rotational dynamics of molecules.

Electronic excitation ofN2by positron impact

Recent experiments on electronic excitation of molecules by positron impact have shown much larger cross sections than in the electron scattering case. The challenge of understanding the origin of this enhancement, especially just above electronic excitation thresholds, motivates the search of theoretical explanations of the phenomenon as well as new experimental efforts to confirm the data. In an earlier theoretical effort, an application of the Schwinger multichannel method at two-state level of approximation for the X {sup 1}{sigma}{sub g}{sup +}{yields}B {sup 1}{sigma}{sub u}{sup +} electronic excitation of the H{sub 2} molecule, gave cross sections with smaller magnitude but reasonable qualitative agreement with the experimental data. The purpose of this work was to study the numerical stability of the previous calculation and to investigate the influence of open channels (energetically accessible electronic states--multichannel effects) and closed channels (energetically inaccessible electronic states--polarization effects) on this transition. Our results show minor multichannel effects and a significative enhancement of the X {sup 1}{sigma}{sub g}{sup +}{yields}B {sup 1}{sigma}{sub u}{sup +} cross section at the threshold due to polarization effects if compared to the usual static results.