Electronic Transition Dipole Research Articles

Candidates for direct laser cooling of diatomic molecules with the simplest Σ1−Σ1 electronic system

We propose to utilize the $^{1}\mathrm{\ensuremath{\Sigma}}{\ensuremath{-}}^{1}\mathrm{\ensuremath{\Sigma}}$ electronic transition system for direct laser cooling of heteronuclear diatomic molecules. AgH, as well as its deuterium isotopolog AgD, is used as an example to illustrate the cooling schemes. Potential-energy curves and relevant molecular parameters of both AgH and AgD, including the spin-orbit constants and the electronic transition dipole moments, are determined in internally contracted multiconfiguration-reference configuration interaction calculations. The highly diagonal Franck-Condon matrices of the $A{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}\ensuremath{-}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ transitions predicted by the calculations suggest the existence of quasi-closed-cycle transitions, which renders these molecules suitable for direct laser cooling. By solving rate equations numerically, we demonstrated that both AgH and AgD molecules can be cooled from 25 K to 2 mK temperature in approximately 20 ms. Our investigation elucidates and supports the hypothesis that molecules in the simplest $^{1}\mathrm{\ensuremath{\Sigma}}{\ensuremath{-}}^{1}\mathrm{\ensuremath{\Sigma}}$ system can serve as favorable candidates for direct laser cooling.

Synthesis, Characterization and Comparative Luminescence Studies of Rare-Earth-Doped Gd2O3 Nanoparticles

A facile direct precipitation method was used for the synthesis of luminescence nanomaterial. Gd2O3 doped with rare earth element Eu3+ is synthesized by polyol route. The synthesized nanoparticles show their characteristic red emission. The nanoparticles are characterized by x-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and photoluminescence (PL) study. The synthesized nanoparticles are spherical particles with ~ 30 nm size. The photoluminescence studies show the characteristic Eu3+ red emission. The PL study shows the intensity of the magnetic dipole transition ( $$ ^{5} D_{0} \to \,^{7} F_{1} $$ ) at 592 nm compared to that of the electronic dipole transition ( $$ ^{5} D_{0} \to \,^{7} F_{2} $$ ) at 615 nm. The nanomaterials can show significant application in various display devices and biomedical applications for tracking.

On-the-Fly Ab Initio Semiclassical Evaluation of Absorption Spectra of Polyatomic Molecules beyond the Condon Approximation.

To evaluate vibronic spectra beyond the Condon approximation, we extend the on-the-fly ab initio thawed Gaussian approximation by considering the Herzberg-Teller contribution due to the dependence of the electronic transition dipole moment on nuclear coordinates. The extended thawed Gaussian approximation is tested on electronic absorption spectra of the phenyl radical and benzene; calculated spectra reproduce experimental data and are much more accurate than standard global harmonic approaches, confirming the significance of anharmonicity. Moreover, the extended method provides a tool to quantify the Herzberg-Teller contribution; we show that in the phenyl radical, anharmonicity outweighs the Herzberg-Teller contribution, whereas in benzene, the Herzberg-Teller contribution is essential because the transition is electronically forbidden and the Condon approximation yields a zero spectrum. Surprisingly, both adiabatic harmonic spectra outperform those of the vertical harmonic model, which describes the Franck-Condon region better. Finally, we provide a simple recipe for orientationally averaging spectra, valid beyond the Condon approximation, and a relation among the transition dipole, its gradient, and nonadiabatic coupling vectors.

The highly excited-state manifold of guanine: calibration for nonlinear electronic spectroscopy simulations

A computational protocol based on the complete and restricted active space self-consistent field (CASSCF/RASSCF) methods and their second-order perturbation theory extensions (CASPT2/RASPT2) is employed to benchmark the highly excited-state manifold of the DNA/RNA canonical purine nucleobase guanine in vacuo. Several RASPT2 schemes are tested, displaying a steady convergence of electronic transition energies and dipole moments upon active space enlargement toward the reference values. The outcome allows calibrating and optimizing computational efforts by considering cheaper and more approximate RAS schemes that could enable the characterization of the excited-state manifolds of multi-chromophoric systems, such as DNA/RNA nucleobase dimers or multimers. Simulations of two-dimensional electronic spectra show similar trends to those observed on the other purine nucleobase adenine, deviating from this and other pyrimidine nucleobases in featuring its main excited-state absorption signal, embodied by sizable double HOMO to LUMO excitation contributions, in the UV probing window.

Two-dimensional electronic spectroscopy as a tool for tracking molecular conformations in DNA/RNA aggregates.

A computational strategy to simulate two-dimensional electronic spectra (2DES) is introduced, which allows us to analyse ground state dynamics and to sample and measure different conformations attained by flexible molecular systems in solution. An explicit mixed quantum mechanics/molecular mechanics (QM/MM) approach is employed for the evaluation of the necessary electronic excited state energies and transition dipole moments. The method is applied towards a study of the highly flexible water-solvated adenine-adenine monophosphate (ApA), a system featuring two interacting adenine moieties that display various intermolecular arrangements, known to deeply affect their photochemical outcome. Molecular dynamics simulations and cluster analysis have been used to select the molecular conformations, reducing the complexity of the flexible ApA conformational space. By using our sum-over-states (SOS) approach to obtain the 2DES spectra for each of these selected conformations, we can discern spectral changes and relate them to specific nuclear arrangements: close lying π-stacked bases exhibit a splitting of their respective 1La signal traces; T-stacked bases exhibit the appearance of charge transfer states in the low-energy Vis probing window while displaying no 1La splitting, being particularly favoured when promoting amino to 5-ring interactions; unstacked and distant adenine moieties exhibit signals similar to those of the adenine monomer, as is expected for non-interacting nucleobases. 2DES maps reveal the spectral fingerprints associated with specific molecular conformations, and are thus a promising option to enable their quantitative spectroscopic detection beyond standard 1D pump-probe techniques. This is expected to aid the understanding of how nucleobase aggregation controls and modulates the photostability and photo-damage of extended DNA/RNA systems.

Long-range behavior of the transition dipole moments of heteronuclear dimers XY (X, Y = Li, Na, K, Rb) based on ab initio calculations.

The ab initio electronic transition dipole moments (ETDMs) of heteronuclear dimers XY (X, Y = Li, Na, K, Rb) were calculated between the ground and excited states converging to the lowest three dissociation limits. The spin-allowed ETDMs were evaluated in a wide range of interatomic distances, R, by means of the quasi-relativistic electronic wave functions obtained by the multi-reference configuration interaction method. The inner-shell electrons (2 electrons for Li and Na atoms, and 10 and 28 for K and Rb, respectively) were described using the non-empirical shape-consistent effective core potentials. The l-independent core polarization potentials of each atom were used to take core-polarization and core-valence correlation effects into account. The long-range behavior of both singlet-singlet X1Σ+-(2,3)1Σ+;(1,2)1Π and triplet-triplet a3Σ+-(2,3)3Σ+;(1,2)3Π transition moments is perfectly fitted at large R-distance by the asymptotic formula of X. Chu and A. Dalgarno, Phys. Rev. A: At., Mol., Opt. Phys., 2002, 66, 024701: , where the coefficient β is equal to 2 and -1 for the Σ-Σ and Σ-Π transitions, respectively. The n2S-n2P transition moments, dA, and dynamic polarizabilites, αB, of the alkali atoms in the n2S state extracted from the present molecular calculations coincide with their empirical and ab initio counterparts to within a few percent.

Precise determination of the orientation of the transition dipole moment in a Bodipy derivative by analysis of the magnetophotoselection effect.

Magnetophotoselection effects, observed for the lowest triplet state of a boron-dipyrromethene (Bodipy) derivative by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy, have been exploited to determine with a good precision of about 5° the 3D orientation of the singlet-singlet transition dipole moment (TDM) in the molecular frame. We have shown that the TR-EPR spectrum lineshape, obtained using a polarized laser pulse, is different from that obtained by using depolarized light. We first developed a new model for the simulation of the TR-EPR spectra obtained with polarized light excitation; the model explicitly takes into account the orientation of the TDM in the molecular frame. We then applied the method for the determination of the TDM orientation, directly from glassy frozen isotropic solutions of Bodipy. The experimentally determined direction has been compared with that obtained from quantum mechanical calculations.

Nonperturbative response functions: A tool for the interpretation of four-wave-mixing signals beyond third order.

Considering an electronic two-level system coupled to vibrational degrees of freedom and driven by short and intense non-overlapping laser pulses, we introduce the concept of nonperturbative response functions. These response functions are expressed in terms of effective electronic transition dipole moments which depend on the strength of the field-matter coupling and on the pulse durations. It is shown that the nonlinear polarization representing four-wave-mixing signals can elegantly be expressed in terms of these nonperturbative response functions to all orders in the field-matter coupling. The nonperturbative response functions generalize the interpretative power of the familiar third-order response functions to four-wave-mixing experiments with intense laser pulses.

Measurement of the Na2 51Σg+→A1Σu+ and 61Σg+→A1Σu+ transition dipole moments using optical-optical double resonance and Autler-Townes spectroscopy.

Accurate knowledge of transition dipole moment matrix elements is crucial since important parameters associated with the interaction of light with matter, such as emission and absorption line intensities, lifetimes, and Einstein coefficients, depend on these matrix elements. We report here an experimental study of the Na2 51Σg+↔A1Σu+ and 61Σg+↔A1Σu+ electronic transition dipole moments and their dependence on internuclear distance. We have measured absolute transition dipole matrix elements for ro-vibrational transitions of the Na2 51Σg+↔A1Σu+ and 61Σg+↔A1Σu+ electronic transitions using Autler-Townes and optical-optical double resonance spectroscopy, and we compare the results to ab initio theoretical values [A. Sanli et al., J. Chem. Phys. 143, 104304 (2015)].

Photoluminescence properties of a double perovskite tungstate based red-emitting phosphor NaLaMgWO6:Sm3+

In this work, the conventional solid-state method was applied to synthesize a series of red-emitting NaLaMgWO6:Sm3+ phosphors. The crystal structure, phase purity, morphology, particle size distribution as well as elemental composition of the as-prepared phosphors were investigated carefully with the aid of XRD, SEM, EDS, FT-IR analyses, indicating the high-purity and micron-sized NaLaMgWO6:Sm3+ phosphors with monoclinic structure were prepared successfully. The spectroscopic properties of Sm3+ in NaLaMgWO6 host including UV–vis diffuse reflection spectrum, photoluminescence excitation and emission spectra, decay curves, chromaticity coordinates and internal quantum efficiency were investigated in detail. Upon excitation with UV (290nm) and n-UV (406nm), NaLaMgWO6:Sm3+ phosphor presented red emission corresponding to the 4G5/2→6HJ (J = 5/2, 7/2, 9/2, and 11/2) transitions of Sm3+, in which the hypersensitive electronic dipole transition 4G5/2→6H9/2 (645nm) was with the strongest emission intensity because Sm3+ ions were located at a lattice site with anti-inversion symmetry. The optimal concentration of Sm3+ was different for the given excitation wavelength such as 290nm and 406nm, which was interpreted by the extra effect of the energy transfer from W6+-O2- group to Sm3+. The decay lifetime for 4G5/2→6H9/2 transition of Sm3+ was very short (< 1ms) and decreased with the increasing Sm3+ concentration. The present investigation indicates that NaLaMgWO6:Sm3+ phosphor could be a potential red component for application in w-LEDs.

Signatures of vibronic coupling in two-dimensional electronic-vibrational and vibrational-electronic spectroscopies.

Two-Dimensional Electronic-Vibrational (2D EV) spectroscopy and Two-Dimensional Vibrational-Electronic (2D VE) spectroscopy are new coherent four-wave mixing spectroscopies that utilize both electronically resonant and vibrationally resonant field-matter interactions to elucidate couplings between electronic and vibrational degrees of freedom. A system Hamiltonian is developed here to lay a foundation for interpreting the 2D EV and 2D VE signals that arise from a vibronically coupled molecular system in the condensed phase. A molecular system consisting of one anharmonic vibration and two electronic states is modeled. Equilibrium displacement of the vibrational coordinate and vibrational frequency shifts upon excitation to the first electronic excited state are included in our Hamiltonian through linear and quadratic vibronic coupling terms. We explicitly consider the nuclear dependence of the electronic transition dipole moment and demonstrate that these spectroscopies are sensitive to non-Condon effects. A series of simulations of 2D EV and 2D VE spectra obtained by varying parameters of the system, system-bath, and interaction Hamiltonians demonstrate that one of the following conditions must be met to observe signals: (1) non-zero linear and/or quadratic vibronic coupling in the electronic excited state, (2) vibrational-coordinate dependence of the electronic transition dipole moment, or (3) electronic-state-dependent vibrational dephasing dynamics. We explore how these vibronic interactions are manifested in the positions, amplitudes, and line shapes of the peaks in 2D EV and 2D VE spectroscopies.

Line-shape theory of the X3Σg-→a1Δg,b1Σg+ transitions in O2-O2 collision-induced absorption.

We derive the theory of collision-induced absorption for electronic transitions in the approximation of an isotropic interaction potential. We apply this theory to the spin-forbidden X3Σg-→a1Δg and X3Σg-→b1Σg+ transitions in O2-O2, which are relevant for calibration in atmospheric studies. We consider two mechanisms for breaking the spin symmetry, either by the intermolecular exchange interaction between paramagnetic collision partners or by the intramolecular spin-orbit coupling. The calculations for the exchange-based mechanism employ the diabatic potential energy surfaces and transition dipole moment surfaces reported in Paper I [T. Karman et al., J. Chem. Phys. 147, 084306 (2017)]. We show that the line shape of the theoretical absorption spectra is insensitive to the large uncertainty in the electronic transition dipole moment surfaces. We also perform calculations using a simple model of the alternative mechanism involving intramolecular spin-orbit coupling, which leads to absorption intensities which are well below the experimental results. The relative intensity of this spin-orbit-based mechanism may impact the relative contribution to the absorption by collisions with diamagnetic collision partners, such as the atmospherically relevant N2 molecule. We furthermore show that both the line shape and temperature dependence are signatures of the underlying transition mechanism.

Fluorescence Polarization Measurements to Probe Alignment of a Bithiophene Dye in One-Dimensional Channels of Self-Assembled Phenylethynylene Bis-Urea Macrocycle Crystals

This paper describes the use of polarized fluorescence microscopy to directly probe guest molecule orientation in bis-urea macrocycle crystals. These macrocycles assemble to afford one-dimensional (1D) microchannels ∼9 A in diameter that have previously been shown to exhibit normal Fickian diffusion and induce selective reactivity among the confined guest molecules. In the present work, we take advantage of the quasi-1D morphology of fiber-like microcrystals with the extended dimension corresponding to the channel axis to measure excitation and emission polarization values for a bithiophene guest. Guest fluorescence is shown to be polarized along the fiber axis with emission polarization values up to 0.729, indicating a high degree of orientational order within the 1D channels. The observed behavior is consistent with calculated results for the guest orientation and electronic transition dipole moment. The results indicate the value of functional fluorescent probes as a measure of guest confinement in low...

The influence of Breit interaction and E1-M2 quantum interference on polarization following inner-shell electron-impact excitation of Li-like ions

SynopsisThe total and magnetic sublevel cross sections of electron-impact excitation from the ground state 1s22s 2S1/2 to the inner-shell excited states 1s2s2p (J=3/2) are calculated based on a relativistic distorted-wave method for highly charged Li-like ions. The important influence of the Breit interactions and quantum interferences between electronic dipole (E1) and magnetic quadrupole (M2) transition on polarization of emitted X-ray are systematically studied.

A time-correlation function approach to nuclear dynamical effects in X-ray spectroscopy.

Modern X-ray spectroscopy has proven itself as a robust tool for probing the electronic structure of atoms in complex environments. Despite working on energy scales that are much larger than those corresponding to nuclear motions, taking nuclear dynamics and the associated nuclear correlations into account may be of importance for X-ray spectroscopy. Recently, we have developed an efficient protocol to account for nuclear dynamics in X-ray absorption and resonant inelastic X-ray scattering spectra [Karsten et al., J. Phys. Chem. Lett. 8, 992 (2017)], based on ground state molecular dynamics accompanied with state-of-the-art calculations of electronic excitation energies and transition dipoles. Here, we present an alternative derivation of the formalism and elaborate on the developed simulation protocol using gas phase and bulk water as examples. The specific spectroscopic features stemming from the nuclear motions are analyzed and traced down to the dynamics of electronic energy gaps and transition dipole correlation functions. The observed tendencies are explained on the basis of a simple harmonic model, and the involved approximations are discussed. The method represents a step forward over the conventional approaches that treat the system in full complexity and provides a reasonable starting point for further improvements.

Luminescence properties of a novel yellow-emitting phosphor NaLaMgWO6:Dy3+

A series of NaLaMgWO6:Dy3+ phosphors were prepared by a conventional solid-state reaction method at high temperature. The phase structure, morphology, particle size distribution and concentration dependent luminescence properties of the phosphors were investigated for the first time. Upon excitation with UV (290nm) and n-UV (388nm), NaLaMgWO6:Dy3+ phosphor exhibited typical Dy3+ emission corresponding to the 4F9/2→6HJ (J = 15/2, 13/2, 11/2) transitions of Dy3+, in which the hypersensitive electronic dipole transition 4F9/2→6H13/2 (575nm) was with strongest emission intensity leading to the yellow emission with the CIE chromaticity coordinates (0.441, 0.475). The optimal concentration of Dy3+ was found to be 8%. The luminescence decay lifetime for 4G5/2→6H13/2 transition of Dy3+ was of millisecond level and decreased with the increase in Dy3+ ion concentration. The obtained results indicated that NaLaMgWO6:Dy3+ phosphor could be a potential yellow emitting phosphor for possible application in w-LEDs.

On the spectral properties of methyl and methoxy derivatives of 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoxalines: Experiment and DFT/TDDFT calculations

Paper reports the synthesis and spectroscopic studies of two novel 1,3-diphenyl pyrazoloquinoxaline (PQX) derivatives with 6-substituted methyl (MePQX) or methoxy (MeOPQX) side groups. The optical absorption and fluorescence emission spectra are recorded in solvents of different polarity. Other photophysical constants, such as the fluorescence lifetime and quantum yield, radiationless and radiative rate constants, electronic transition dipole moments, give complete characterization of MePQX and MeOPQX dyes as materials for potential luminescence or electroluminescence applications. Measured optical absorption and fluorescence emission spectra are compared with the results of quantum-chemical analysis using density functional theory (DFT/TDDFT) methods based on hybrid and long range corrected (LRC) exchange-correlation (xc) functionals in combination with solvation self consistent reaction field model. Comparing to conventional hybrid xc-functionals, the DFT/TDDFT calculations using LRC xc-functionals yield considerably more accurate description of optical absorption and fluorescence emission spectra. The best description of the absorption-emission circle provides the model assuming that optical absorption takes place from preferably flat or weakly twisted molecular conformations in the ground state, as particularly is suggested by the geometrical DFT optimization, whereas the fluorescence emission would be expected from more twisted molecular conformations in the excited state.

Electronic transition dipole moments from orthogonality constrained Hartree–Fock wavefunctions

In our recent paper (Glushkov V N and Assfeld X 2016 Theor. Chem. Acc. 135), we have presented some applications of the so-called orthogonality constrained Hartree–Fock (OCHF) theory to calculations of excited state energies of the same symmetry, including highly and core excited states. In this paper we report an attempt to apply this methodology to practical calculations of electronic dipole transition moments focusing on transitions between states having the same spatial symmetry as the ground state. Basic tenets of this method are briefly reviewed and its specific features for ab initio calculations of electronic transitions are discussed. The effect of non-orthogonal HF wave functions on transition dipole moments is studied and analyzed. OCHF electronic transition moments calculated for the HeH molecule are presented as a function of the internuclear separation for five 2Σ+ states (X 2Σ+, A2Σ+, C2Σ+, D2Σ+and F 2Σ+). The results of the calculations are, in general, in good agreement with those obtained by multi-reference configuration interaction methods which take correlation effects into account and show a favorable balance between accuracy and computational efficiency.

Charge Localization after Ultrafast Photoexcitation of a Rigid Perylene Perylenediimide Dyad Visualized by Transient Stark Effect.

The intramolecular charge-transfer (CT) dynamics of a rigid and strongly conjugated perylenediimide-bridge-perylene dyad (PDIPe) has been investigated in dichloromethane using ultrafast transient electronic absorption spectroscopy and quantum chemical calculations. The strong electronic coupling between the dyad units gives rise to a CT band. Its photoexcitation forms a delocalized CT state with well-preserved ion bands despite the strong coupling. In the dyad, the electronic transition dipole moment of the electron donor perylene is aligned along the axis of the electric field vector with respect to the CT species. This alignment makes the donor sensitive to the Stark effect and thus charge density fluctuations in the CT state. Charge localization on the picosecond time scale manifests as a time-dependent Stark shift in the visible region. Quantum chemical calculations reveal a twist around the acetylene bridging unit to be the responsible mechanism generating a partial to an almost complete CT state. An estimate of the electric field strength in the CT state yields approximately 25 MV/cm, which increases to around 31 MV/cm during charge localization. Furthermore, the calculations illustrate the complexity of electronic structure in this strongly delocalized superchromophore and reflect the complications in the interpretation of transient absorption results when compared to steady-state approaches such as spectroelectrochemistry and model chromophore experiments such as photoinduced bimolecular charge transfer.

Efficient Chemical Synthesis of Functional Rare Earth Orthoborates Nanosheets and Their Photoluminescence Studies.

In contrast to routine methods, La x Y1−x BO3:Eu3+ and La y Lu1−y BO3:Eu3+ were efficiently prepared by a supersonic microwave co-assistance method within 60 minutes at 90 °C. Orderly aggregated nanosheets were formed in high yields by preferential crystal growth processes. More importantly, it has never been reported that the magnetic-dipole transition with orange-color emissions (5D0 → 7F1) could be finely tuned based on the concentration variation of lanthanum elements. The electronic dipole transition (5D0 → 7F2) increases gradually when La/Y and La/Lu ratios are 4/1 and 7/3 respectively.