Oscillator Strengths Of Electronic Transitions Research Articles

Electron-Impact Excitation and Dissociation of Heavy Rare Gas Heteronuclear Ions via Transitions to Charge Transfer States

Heteronuclear diatomic rare gas molecular cations feature excited electronic terms with charge transfer character located several eV above the ground term. The role of such terms in collisions involving heteronuclear ions is studied theoretically under conditions typical of the plasma-based sources of UV and IR radiation. Calculations were carried out for processes of dissociative excitation, dissociative recombination and electron impact bound–bound excitation in Ar/Xe and Kr/Xe plasmas using the recently developed semiclassical approach combined with the ab initio data for potential energy curves and oscillator strengths of electronic transitions. The approach consistently describes the contributions from the entire rovibrational manifold to the processes studied. The cross sections of the processes mentioned are calculated for wide ranges of gas temperatures and electron energies. We show that the processes considered are quite effective when they are accompanied by transitions to charge transfer terms. For the range of electron energies typical of active media of UV and IR radiation sources the cross sections exceed those reported for the processes usually considered to involve transitions between the ground and first excited electronic state. The excitation of charge transfer electronic terms can play an important role in the kinetics of rare gas mixture plasmas.

Design, Synthesis, Spectroscopic Characterization, Molecular Docking and Biological Evaluation of 9H-Carbazole Linked 4-Nitrophenol: A DFT Approach

The FT-IR and FT-Raman spectra of 3-(4-nitrophenoxy)-9,9a-dihydro-4aH-carbazole (9HCNP) molecule were recorded. The theoretical wavenumbers are in proper agreement with the observed results. The electronic properties, such as natural bond orbital study of the molecule, were confirmed using the inter/intra molecular interactions, molecular electrostatic potential, and Mulliken population analyses on the atomic charge distribution, performed by the Becke-3–Lee–Yang–Parr (CAM-B3LYP) method utilizing Gaussian 09 software. The oscillator strength of electronic transitions and energies of HOMO and LUMO molecular orbitals were carried out to evaluate the global chemical reactivity descriptors of 9HCNP. The molecular docking evaluation was carried out to evaluate the structure–activity relationship and the binding affinity of the molecule, which shows that the molecule can perform as an inhibitor of breast cancer-causing human topoisomerase-I and II-DNA receptors. The result of the conjugation was found to play a significant role in bioactivity and drug-likeliness properties, which can be supported by the molecular docking analysis of those compounds. In addition, the morphological changes in control and in vitro cytotoxicity of 9HCNP confirm the anticancer activity of 9HCNP against human breast cancer cell lines was assessed in 24-h tetrazolium-dye (MTT) cytotoxicity assays and also analyzed in terms of the role of reactive oxygen species (ROS) in cell life. Hence, the results show the in vitro anticancer activity of the molecule for the development of human breast cancer drugs.

Spectroscopic Behavior and Photophysical Parameters of 2-(Acetoxymethyl)-6-(1,2,4-triazinylaminodihydroquinazolinyl)tetrahydropyran Derivative in Different Solid Hosts

Optical and photophysical properties of 6-substituted-1,2,4-Triazine fluorescent derivative dye doped in silicate based sol–gel, homopolymer of methyl methacrylate (PMMA), and copolymer (MMA/diethylene glycol dimethacrylate) (DEGDMA) were investigated. The pores of different hosts and caging of the dye were found to effect on the parameters such as molar absorptivity, cross sections of singlet–singlet electronic absorption and emission spectra, excited state lifetime, quantum yield of fluorescence. The dipole moment of electronic transition, the length of attenuation and oscillator strength of electronic transition from So → S1 have been calculated. The dye was pumped with different powers using 3rd harmonic Nd: YAG laser of 355 nm and pulse duration 8 ns, with repetition rate 10 Hz. Good photo stability for dye was attained. After 55,000 pumping pulses of (10 mJ/pulse), the photo-stabilities were decreased to 53%, 48%, and 45% of the initial ASE of dye in sol gel, PMMA, and Co-polymer respectively. The dye in sol–gel matrix showed improvement of photo stability compared with those in organic polymeric matrices.

Substituent effects on the UV-visible spectrum and excited electronic states of dithiocarboxylates.

The absorption spectra of a series of dithiocarboxylates were investigated in the ultraviolet-visible region. Two questions that this study aimed to address were as follows: (1) What transitions give rise to the features in the electronic spectra? And (2) what are the long- and short-range substituent effects on the absorption spectra? A series of 11 dithiocarboxylates were prepared as organic soluble salts. Time-dependent density functional theory (TDDFT) was used to calculate excited state energies and oscillator strengths of electronic transitions. TDDFT at the CAM-B3LYP/def2-TZVPD level of theory predicts two low-energy n → π* transitions and two π → π* transitions at higher energy, consistent with the experimental spectra. This state ordering and density is in contrast to the better studied thiocarbonyls for which only two transitions within the singlet manifold appear in the UV-visible region. For derivatives of dithiobenzoate, the energy of the three lowest energy states are insensitive to changes to substituents para to the dithiocarboxylate group. In contrast, the energy of the highest ππ* state varies by 0.78eV. This work shows that the results of TDDFT calculations can be used to predict the electronic absorption spectra of dithiocarboxylates, providing a useful tool for designing dithiocarboxylate light absorbers.

Spectral Behavior and Photophysical Parameters of Dihydrophenanthro[9,10-e][1,2,4]Triazine Derivative Dyes in Sol-Gel and Methyl Methacrylate Polymer Matrices.

This paper deals with the optical and photophysical properties of dihydrophenanthro[9,10-e][1,2,4]triazine fluorescent dyes doped in Silicate based sol-gel and homo-poly methyl methacrylate (PMMA). Solid hosts were found to effect on the optical and photophysical parameters such as molar absorptivity, cross sections of singlet-singlet electronic absorption and emission spectra, excited state lifetime, quantum yield of fluorescence. The dipole moment of electronic transition, the length of attenuation and oscillator strength of electronic transition from So → S1 have been calculated. The dyes were pumped with different powers using 3rd harmonic Nd:YAG laser of 355nm and pulse duration 8ns, with a repetition rate at (10Hz). Good photostability for compounds 1 and 2 were attained. It was decreased to 49% & 54% and 46% & 40% of the initial ASE of dyes in sol gel and PMMA, respectively, after 55,000 pumping pulses at (10mJ/pulse). The dyes in sol-gel showed improved photostability compared with those in organic polymeric matrices.

Time-resolved cathodoluminescence investigations of AlN:Ge/GaN nanowire structures

Light emitting diodes represent a key technology that can be found in many areas of everydays life. Therefore, the improvement of the efficiency of such structures offers a high economic and ecological potential. One approach is electrostatic screening of the quantum-confined Stark effect (QCSE) in polar III-V heterostructures by n-type doping in order to increase the oscillator strength of electronic transitions in quantum structures. In this study, we analyzed the cathodoluminescene (CL) spectra of different functional parts of individual AlN/GaN nanowire superlattices and studied their decay characteristics with sub-nanosecond time resolution. This allows us to extract information about strain and electric fields in such heterostructures with an overall spatial resolution <100 nm. The samples, which were investigated in a temperature range from 10 to 300 K by using time-integrated cathodoluminescence spectroscopy (TICL) and time-resolved cathodoluminescence spectroscopy (TRCL) consist of GaN bottom and top layer and a 40-fold stack of GaN nanodiscs, embedded in AlN barriers that were doped with Ge. We show, that the QCSE is reduced with increasing doping concentration due to a screening of the internal electric fields inside GaN nanodiscs, resulting in a reduction of the carrier lifetimes and a blue shift of the emitted light. Due to the small diameter of the electron excitation beam CL offers the possibility to individually analyze the different functional parts of the nanowires.

Efficient strain-induced light emission in lonsdaleite germanium

Lonsdaleite germanium has a direct band gap, but it is not an efficient light emitter due to the vanishing oscillator strength of electronic transitions at the fundamental gap. Transitions involving the second lowest conduction band are instead at least three orders of magnitude stronger. The inversion of the two lowest conduction bands would therefore make hexagonal germanium ideal for optoelectronic applications. In this work, we investigate the possibility to achieve this band inversion by applying strain. To this end we perform ab initio calculations of the electronic band structure and optical properties of strained hexagonal germanium, using density functional theory with the modified Becke-Johnson exchange-correlation functional and including spin-orbit interaction. We consider hydrostatic pressure, uniaxial strain along the hexagonal c axis, as well as biaxial strain in planes perpendicular to and containing the hexagonal c axis to simulate the effect of a substrate. We find that the conduction-band inversion, and therefore the transition from a pseudo-direct to a direct band gap, is attainable for moderate tensile uniaxial strain parallel to the lonsdaleite c axis.

Optical and Judd‐Ofelt spectroscopic study of Er3+‐doped strontium gadolinium gallium garnet single‐crystal

AbstractEr3+‐doped strontium gadolinium gallium garnet (SrGdGa3O7) single‐crystal was grown by Czochralski method. The Er3+ concentration in the crystal was determined as 4.2 × 1021 ions/cm3 by inductively coupled plasma atomic absorption spectroscopy. Refractive index of the crystal was measured at the wavelengths of 633, 1311 and 1553 nm by prism coupling technique. The results show that the crystal is a positive uniaxial crystal with a birefringence of ~0.01, and the Sellmeier equation reported previously for the crystal doped with Nd3+ is also valid for the one doped with Er3+. Unpolarized absorption spectrum of the crystal was measured at room temperature and the Er3+ absorption cross‐section spectrum was calibrated from it. The Er3+ spectroscopic properties were studied by Judd‐Ofelt theory. Some fundamental spectroscopic parameters were obtained that include absorption coefficient and cross‐section spectral distributions, electronic transition oscillator strength, Judd‐Ofelt parameters, fluorescence branch ratio, transition probability, radiative and fluorescence lifetimes, and quantum efficiency.

MgH Rydberg series: Transition energies from electron propagator theory and oscillator strengths from the molecular quantum defect orbital method

Vertical excitation energies belonging to several Rydberg series of MgH have been inferred from 3+ electron-propagator calculations of the electron affinities of MgH+ and are in close agreement with experiment. Many electronically excited states with n > 3 are reported for the first time and new insight is given on the assignment of several Rydberg series. Valence and Rydberg excited states of MgH are distinguished respectively by high and low pole strengths corresponding to Dyson orbitals of electron attachment to the cation. By applying the Molecular Quantum Defect Orbital method, oscillator strengths for electronic transitions involving Rydberg states also have been determined.

CaH Rydberg series, oscillator strengths and photoionization cross sections from Molecular Quantum Defect and Dyson Orbital theories

In this work, electron-propagator methods are applied to the calculation of the ionization potential and vertical excitation energies for several Rydberg series of the CaH molecule. The present calculations cover more highly excited states than those previously reported. In particular, excitation energies for ns (n>5), np (n>5), nd (n>4) and nf Rydberg states are given. Oscillator strengths for electronic transitions involving Rydberg states of CaH, as well as photoionization cross sections for Rydberg channels, also have been determined by using the Molecular Quantum Defect Orbital approach. Good agreement has been found with the scarce comparative data that are available for oscillator strengths. To our knowledge, predictions of photoionization cross sections from the outermost orbital of CaH are made here for the first time. A Cooper minimum and mixed atomic orbital character in some of the Dyson orbitals are among the novel features of these present calculations.

Infrared and Ultraviolet Spectra of Diborane(6): B2H6 and B2D6.

We recorded absorption spectra of diborane(6), B2H6 and B2D6, dispersed in solid neon near 4 K in both mid-infrared and ultraviolet regions. For gaseous B2H6 from 105 to 300 nm, we report quantitative absolute cross sections; for solid B2H6 and for B2H6 dispersed in solid neon, we measured ultraviolet absorbance with relative intensities over a wide range. To assign the mid-infrared spectra to specific isotopic variants, we applied the abundance of (11)B and (10)B in natural proportions; we undertook quantum-chemical calculations of wavenumbers associated with anharmonic vibrational modes and the intensities of the harmonic vibrational modes. To aid an interpretation of the ultraviolet spectra, we calculated the energies of electronically excited singlet and triplet states and oscillator strengths for electronic transitions from the electronic ground state.

Charge Distribution and UV Absorption Spectra of Liquid Crystals with Structural Element [closo-B10H10]2-: A Theoretical Approach

The present work reports the UV absorption spectral characteristics, and charge distribution analysis of two liquid crystal molecules viz., 1-Dinitrogen-10-(4-pentyl-1-quinuclidinyl)-closo-decaborate (NCDB), and 1-Dinitrogen- 10-(4-pentyl-1-thiacyclohexyl)-closo-decaborate (SCDB). Mulliken and Loewdin population analysis have been performed to understand the charge distribution of the molecules. The electronic transitions, ultraviolet (UV) absorption wavelengths, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energies have been reported. Further, charge distribution, and UV stability of the molecules has been discussed in the light of partial atomic charges, absorption wavelength and electronic transition oscillator strength (f) respectively.

Electric field effects on the excited properties of Si2N2 molecule with special configuration：a density-functional study

In order to understand in depth the electroluminescence mechanism, the influences of the external electric field on the geometric and electronic structure in ground state, the molecular vibrational spectra of Si2N2 molecule with Cs special symmetry are studied by density functional theory with B3LYP exchange-correlation prescription at the aug-cc-pVTZ basis set level. Following each optimization, the vibrational frequencies are calculated and all optimized structures are stable. The results show that the molecular vibrational Stark effect, i.e., red-shift for the low-frequency modes and blue-shift for the high-frequency modes are observed with the increase of the applied field strength. The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the energy gap between HOMO and LUMO of Si2N2 molecule diminish with the increase of external field. A time-dependent density functional theory is used to investigate the excited properties of Si2N2 (Cs) molecule. The calculated absorption spectra of Si2N2 molecule with Cs symmetry are in agreement with the experiment values. The analysis reveals that the absorption spectrum wavelength increases in the visible region with a concomitant increase in the electronic transition oscillator strengths in the course of the increase of the external electric field strength. The results reveal that the excited properties of Si2N2 molecule can be easily tuned by the external electric field, which indicates that the silicon nitride is an interesting optoelectronic functional material. These investigations on the various properties of Si2N2 molecule with Cs symmetry under an external electric field are useful to understand the electroluminescence mechanism for silicon nitride used in molecular electronics.

Effects of dynamic disorder on exciton delocalization and photoinduced charge separation in DNA

The nature of electronically excited states in DNA is analyzed in detail using a combination of quantum mechanical (QM) semiempirical calculations and molecular dynamics (MD). For this purpose, we consider homogeneous π stacks extracted from the MD trajectory of a poly(A)·poly(T) oligomer. The environment is accounted for within the QM/MM scheme. The effects of structural fluctuations on exciton delocalization and photoinduced charge separation are explored using the quantitative analysis of the electron density in the excited states. We distinguish the effects generated by the vibronic interactions within nucleobases and by the environment of the π stack. While in ideal B-DNA stacks (A-T)n singlet excited states are spread over all intrastrand nucleobases, the average exciton length is ~0.75n, thermal fluctuations decrease considerably the extent of delocalization. The QM/MD model predicts that the excitons in (A-T)n stacks are spread over 3 bases (for n = 4 and 6, the average exciton length is found to be 2.6 ± 0.3 and 2.8 ± 0.3, respectively). We show that the main factor reducing the exciton length is the vibronic interactions within nucleobases whereas fluctuations of the π stack environment play a relatively minor role. The oscillator strength of electronic transitions from the ground state to charge-separated states Ak(+)Ak±1(-) and Tk(+)Tk±1(-) is found to be strong enough to populate directly these states by UV absorption at E = 5.0-5.3 eV. In contrast, the direct formation of interstrand charge transfer states Ai(+)Tj(-) is predicted to be unlikely.

Calculation of spectral shifts in UV–visible region and photoresponsive behaviour of fluorinated liquid crystals: Effect of solvent and substituent

The photoresponsive behaviour of fluorinated liquid crystals p-phenylene-4-methoxy benzoate-4-trifluoromethylbenzoate (FLUORO1), and 4-propyloxyphenyl-4-(4-trifluoromethylbenzoyloxy) benzoate (FLUORO2) has been systematically investigated using the CNDO/S + CI and INDO/S + CI methods. These methods have been employed to calculate/analyze the spectral shifts, and absorbance measurements in UV–visible region of the systems. The electronic transitions, absorption wavelength, HOMO (highest occupied molecular orbital), and LUMO (lowest unoccupied molecular orbital) energies have been calculated. Further, ultraviolet (UV) stability of the molecules has been discussed in the light of absorption wavelength and electronic transition oscillator strength (f). The effect of different solvent media and substituents on transition energies, oscillator strength, and other absorption parameters have also been reported. The present article provides valuable information regarding enhancing the UV stability of molecules by marinating their conductivity.

Integrated computational approaches for spectroscopic studies of molecular systems in the gas phase and in solution: pyrimidine as a test case

An integrated computational approach built on quantum mechanical (QM) methods, purposely tailored inter- and intra-molecular force fields and continuum solvent models combined with time-independent and time-dependent schemes to account for nuclear motion effects is applied to the spectroscopic investigation of pyrimidine in the gas phase as well as in aqueous and CCl4 solutions. Accurate post-Hartree–Fock methodologies are employed to compute molecular structure, harmonic vibrational frequencies, energies and oscillator strengths for electronic transitions in order to validate the accuracy of approaches rooted into density functional theory with emphasis also on hybrid QM/QM′ models. Within the time-independent approaches, IR spectra are computed including anharmonicities through perturbative corrections while UV–vis line-shapes are simulated accounting for the vibrational structure; in both cases, the environmental effects are described by continuum models. The effects of conformational flexibility, including solvent dynamics, are described through time-dependent models based on purposely DFT-tailored force fields applied to molecular dynamics simulations and on QM computations of spectroscopic properties. Such procedures are exploited to simulate IR and UV–vis spectra of pyrimidine in the gas phase and in solutions, leading in all cases to good agreement with experimental observations and allowing to dissect different effects underlying spectral phenomena.

Calculation of spectral shifts in UV–visible region and photo stability of thermotropic liquid crystals: Solvent and alkyl chain length effects

The calculation of spectral shifts in UV–Visible region, and photo stability of thermotropic liquid crystals p-n-Alkoxy cinnamic acid (nOCAC) with n=2, 4, 6, and 8 have been systematically investigated using the CNDO/S+CI, and INDO/S+CI methods. These methods have been employed to calculate/ analyze the geometric structures, absorbance measurements, spectral shifts in UV–Visible region, and photo stability of the nOCAC molecules. The electronic transitions, absorption wavelength, HOMO, and LUMO energies have been reported. Further, photo stability of the molecules has been discussed in the light of absorption wavelength, and electronic transition oscillator strength (f). The effect of different solvent media and alkyl chain length on these factors has also been discussed. It has been observed that the HOMO, LUMO, and energy gap are independent for the chosen solvents. In the present article, an attempt has been made for a new insight on photo stability of liquid crystals by maintaining their conductivity. Further, these absorption spectra will provide valuable data base for future computational studies, and the other experimental investigations.

A series of luminescent Re(I) complexes with electron-donor/acceptor moieties: Synthesis, characterization, and photoluminescence

In this paper, we synthesize three Re(I) complexes of Re(CO)3(PPO)Br, Re(CO)3(PTO)Br, and Re(CO)3(PBI)Br, where PPO=2-phenyl-5-(pyridin-2-yl)-1,3,4-oxadiazole, PTO=2-(pyridin-2-yl)-5-p-tolyl-1,3,4-oxadiazole, PBI=2-(pyridin-2-yl)-1H-benzo[d]imidazole. Their single crystals and photophysical properties are measured and discussed in detail. The correlation between ligand structure and corresponding PL characteristics of Re(I) complex has been investigated. It is found that a ligand with strong electron-donor can efficiently increase both absorption and emissive energy of Re(I) complex. In addition, electron-rich ligand can increase the electron density of the complex and thus enhance the oscillator strength of electronic transition, improving the photoluminescence performance.

Substituent and solvent effects on UV‐visible absorption spectra of liquid crystalline disubstituted biphenylcyclohexane derivatives – a computational approach

AbstractA computational approach has been carried out on liquid crystalline disubstituted biphenylcyclohexanes (BCHs) of general formula R‐C6H10‐C6H4‐C6H4‐X with R: C3H7; X: H (BCH30) and R: C5H11; X: CN (BCH5CN) using the CNDO/S + CI and INDO/S + CI methods. These methods have been employed to calculate and analyze the spectral shifts, and absorbance measurements in UV‐visible range of the systems. The electronic transitions, absorption wavelength, HOMO (Highest Occupied Molecular Orbital), and LUMO (Lowest Unoccupied Molecular Orbital) energies have been calculated. Further, ultraviolet (UV) stability of the molecules has been discussed in the light of absorption wavelength and electronic transition oscillator strength (f). The effect of different solvent media and substituents on transition energies, oscillator strength, and other absorption parameters has also been reported. The present article provides valuable information regarding enhancing the UV stability of molecules by marinating their conductivity. Further, these absorption spectra will provide valuable data base for future computational studies, and the other experimental investigations. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electronic Transition Oscillator Strength and UV Stability of Nematogenic Cyanobiphenyls—Role of Alkyl Chains

The electronic transitions in the ultraviolet (UV)/visible (VIS) range of nematogenic 4′-n-alkyl-4-cyanobiphenyl (nCB) with butyl (4CB) and hexyl (6CB) groups have been studied. The UV/VIS and circular dichroism (CD) spectra of nCB (n = 4, 6) molecules have been simulated using the TDDFT/B3LYP/6-31+G (d) method. Mulliken atomic charges for each molecule have been compared with Loewdin atomic charges to analyze the molecular charge distribution and phase stability. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies corresponding to the electronic transitions in the UV/VIS range have been reported. Excited states have been calculated via CI-singles (CIS) with semiempirical Hamiltonian ZINDO. Furthermore, two types of calculations have been performed for model systems containing single and double molecules of nCB. The dimer complexes during the different modes of molecular interactions have also been taken into consideration in order to investigate the most energetically stable configuration. These computations provide valuable information regarding the role and flexibility of end chains in a particular phase behavior and UV stability.