Ordering Of Excited States Research Articles

Excited State Properties of Point Defects in Semiconductors and Insulators Investigated with Time-Dependent Density Functional Theory.

We present a formulation of spin-conserving and spin-flip hybrid time-dependent density functional theory (TDDFT), including the calculation of analytical forces, which allows for efficient calculations of excited state properties of solid-state systems with hundreds to thousands of atoms. We discuss an implementation on both GPU- and CPU-based architectures along with several acceleration techniques. We then apply our formulation to the study of several point defects in semiconductors and insulators, specifically the negatively charged nitrogen-vacancy and neutral silicon-vacancy centers in diamond, the neutral divacancy center in 4H silicon carbide, and the neutral oxygen-vacancy center in magnesium oxide. Our results highlight the importance of taking into account structural relaxations in excited states in order to interpret and predict optical absorption and emission mechanisms in spin defects.

Cyclometalated iridium-coumarin ratiometric oxygen sensors: improved signal resolution and tunable dynamic ranges.

In this work we introduce a new series of ratiometric oxygen sensors based on phosphorescent cyclometalated iridium centers partnered with organic coumarin fluorophores. Three different cyclometalating ligands and two different pyridyl-containing coumarin types were used to prepare six target complexes with tunable excited-state energies. Three of the complexes display dual emission, with fluorescence arising from the coumarin ligand, and phosphorescence from either the cyclometalated iridium center or the coumarin. These dual-emitting complexes function as ratiometric oxygen sensors, with the phosphorescence quenched under O2 while fluorescence is unaffected. The use of blue-fluorescent coumarins results in good signal resolution between fluorescence and phosphorescence. Moreover, the sensitivity and dynamic range, measured with Stern–Volmer analysis, can be tuned two orders of magnitude by virtue of our ability to synthetically control the triplet excited-state ordering. The complex with cyclometalated iridium 3MLCT phosphorescence operates under hyperoxic conditions, whereas the two complexes with coumarin-centered phosphorescence are sensitive to very low levels of O2 and function as hypoxic sensors.

Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering.

The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

Theoretical insights into the degradation of tyrosol stimulated by hydroxyl and sulfate radicals in wastewater and ecotoxicity evaluation

The photochemical degradation of tyrosol (TY) in wastewater was investigated by theoretical method. The highly oxidative radicals (⋅OH and SO4•-) were employed to study the TY transformation behaviors and kinetic properties. The ⋅OH-addition and H-abstraction reaction mechanisms were predominant for ⋅OH and SO4•--initiated reactions, respectively. The rate constants of TY + ⋅OH and TY + SO4•- systems were determined as 9.15 × 109 M−1 s−1 and 3.14 × 1010 M−1 s−1 at 298 K. The generated SO4•- from advanced oxidation processes (AOPs) showed good performance for TY degradation in liquid phase. The direct photolysis was also considered in the excited state in order to reveal the photo-excited reaction mechanism. Due to high endothermicity and energy barrier, direct photolysis is difficult to proceed spontaneously compared to indirect photochemical processes initiated by strong oxidative radicals. Half-life values of TY with ⋅OH reactions demonstrated that the high concentration of free radicals would significantly shorten the exposure time of TY in water environment. The results of the ecotoxicity to aquatic organisms suggested TY degradation was a mainly less harmful process. However, hydroxylation product P7 (hydroquinone) was a potential developmental toxicant with greater aquatic toxicity than TY, which was an exception to deserve more concern in environment.

Exploring connections between statistical mechanics and Green's functions for realistic systems: Temperature dependent electronic entropy and internal energy from a self-consistent second-order Green's function.

Including finite-temperature effects from the electronic degrees of freedom in electronic structure calculations of semiconductors and metals is desired; however, in practice it remains exceedingly difficult when using zero-temperature methods, since these methods require an explicit evaluation of multiple excited states in order to account for any finite-temperature effects. Using a Matsubara Green's function formalism remains a viable alternative, since in this formalism it is easier to include thermal effects and to connect the dynamic quantities such as the self-energy with static thermodynamic quantities such as the Helmholtz energy, entropy, and internal energy. However, despite the promising properties of this formalism, little is known about the multiple solutions of the non-linear equations present in the self-consistent Matsubara formalism and only a few cases involving a full Coulomb Hamiltonian were investigated in the past. Here, to shed some light onto the iterative nature of the Green's function solutions, we self-consistently evaluate the thermodynamic quantities for a one-dimensional (1D) hydrogen solid at various interatomic separations and temperatures using the self-energy approximated to second-order (GF2). At many points in the phase diagram of this system, multiple phases such as a metal and an insulator exist, and we are able to determine the most stable phase from the analysis of Helmholtz energies. Additionally, we show the evolution of the spectrum of 1D boron nitride to demonstrate that GF2 is capable of qualitatively describing the temperature effects influencing the size of the band gap.

Solvent-mediated internal conversion in diphenoxyethane-(H₂O)n clusters, n = 2-4.

1,2-diphenoxyethane (DPOE) is a flexible bichromophore whose excited states come in close-lying pairs whose splitting and vibronic coupling can be modulated by solvent. Building on the ground state infrared spectroscopy of DPOE-(H2O)n clusters with n = 2-4 from the adjoining paper [Walsh et al., J. Chem. Phys. 142, 154303 (2015)], the present work focuses on the vibronic and excited state infrared spectroscopies of the clusters. The type and degree of asymmetry of the water cluster binding to DPOE is reflected in the variation in the magnitude of the S1/S2 splitting with cluster size. Excited state resonant ion-dip infrared spectroscopy was performed at the electronic origins of the first two excited states in order to explore how the water clusters' OH stretch spectra report on the nature of the two excited states, and the interaction of the S2 state with nearby S1 vibronic levels mediated by the water clusters. The data set, when taken as a whole, provides a state-to-state view of internal conversion and the role of solvent in mediating conversion of electronic excitation between two chromophores, providing a molecular-scale view of Kasha's rule.

Blue-Green Iridium(III) Emitter and Comprehensive Photophysical Elucidation of Heteroleptic Cyclometalated Iridium(III) Complexes

Synthesis and photophysical properties of the highly emissive complex [Ir(Fppy)2(dmb)](+) are reported along with those of additional heteroleptic cyclometalated Ir(III) complexes, [Ir(ppy)2(NN)](PF6): FppyH = 2-(2,4-difluorophenyl)pyridine; ppyH = 2-phenylpyridine; NN = 4,4'-dimethyl-2,2'-bipyridine (dmb), 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (Ph2phen). TD-DFT calculations and Franck-Condon emission spectral band shape analyses show that the broad and structureless emission from [Ir(Fppy)2(dmb)](+) in acetonitrile at 298 K mainly arises from a triplet metal-to-ligand charge-transfer excited state, (3)MLCTIr(ppy)→NN. The emission maximum varies systematically with variations in electron-donating or -withdrawing substituents on both the NN and the Xppy ligands, and emission efficiencies are high, with an impressive ϕ ≈ 1 for [Ir(Fppy)2(dmb)](+). At 77 K in propionitrile/butyronitrile (4/5, v/v), emission from [Ir(Fppy)2(dmb)](+) is narrow and highly structured consistent with a triplet ligand-centered transition ((3)LCNN) and an inversion in excited-state ordering between the (3)MLCTIr(ppy)→NN and (3)LCNN states. In a semirigid film of the poly(ethyleneglycol)dimethacrylate with nine ethylene glycol spacers, PEG-DMA550, emission from [Ir(Fppy)2(dmb)](+) is MLCT-based. The thermal sensitivity of the photophysical properties of this excited state points to a possible application as a temperature sensor in addition to its more known use in light-emitting devices.

Theoretical investigation of thermally and photochemically induced haptotropic rearrangements of chromium ligands on naphthalene systems

The description of chemical reactions by means of quantum mechanical methods is an important task and gets even more challenging if excited states have to be considered. This work focuses on the haptotropic rearrangements of chromium atoms bearing three coligands which migrate on a naphthalene-like system. The reactions are either thermally or photochemically controllable and thus the systems are candidates for molecular switches. We propose a detailed reaction scheme for the investigated system. Furthermore, we provide a detailed analysis of the important steps of the reaction cycle. In comparison to previous publications, the scope of this work also involves the quantum mechanical treatment of excited states in order to describe occurring photon absorption processes in a proper way. Linear response time-dependent density functional theory calculations were carried out to describe the molecules’ responses to the external electromagnetic perturbations.

Optical Emission Spectroscopy Measurement of Processing Plasmas

The present paper reviews fundamentals of optical emission spectroscopy (OES) of plasmas and, in particular, its applications to processing plasmas. Collisional radiative model is described to understand the excitation kinetics and population distributions of excited states in order to examine the electron temperature and density. It is shown that corona equilibrium is often adopted as justifiable assumption of excitation kinetics for general processing plasmas. Two OES methods to understand molecular gas discharge plasmas are also reviewed. One of them is to determine vibrational or rotational temperatures of molecular gas discharge plasmas by OES measurement. The other is the actinometry measurement method to examine neutral radical density.

Singular Perturbations and Lindblad-Kossakowski Differential Equations

We consider an ensemble of quantum systems described by a density matrix, solution of a Lindblad-Kossakowski differential equation. We focus on the special case where the decoherence is only due to a highly unstable excited state and where the spontaneously emitted photons are measured by a photo-detector. We propose a systematic method to eliminate the fast and asymptotically stable dynamics associated with the excited state in order to obtain another differential equation for the slow part. We show that this slow differential equation is still of Lindblad-Kossakowski type, that the decoherence terms and the measured output depend explicitly on the amplitudes of quasi-resonant applied field, i.e., the control. Beside a rigorous proof of the slow/fast (adiabatic) reduction based on singular perturbation theory, we also provide a physical interpretation of the result in the context of coherence population trapping via dark states and decoherence-free subspaces. Numerical simulations illustrate the accuracy of the proposed approximation for a 5-level systems.

High-order coupled cluster method calculations of spontaneous symmetry breaking in the spin-half one-dimensional J_{1}-J_{2} model

In this article we present new formalism for high-order coupled cluster method (CCM) calculations for generalized ground-state expectation values and the excited states of quantum magnetic systems with spin quantum number s ≥ 1/2. We use high-order CCM to demonstrate spontaneous symmetry breaking in the spin-half J1-J2 model for the linear chain using the coupled cluster method (CCM). We show that we are able to reproduce exactly the dimerized ground (ket) state at the Majumdar-Ghosh point (J2/J1= 1/2) using a Neel model state. We show that the onset of dimerized phase is indicated by a bifurcation of the nearest-neighbour ket- and bra-state correlation coefficients for the nearest-neighbour Neel model state. We show that ground-state energies are in good agreement with the results of exact diagonalizations of finite-length chains across this entire regime (i. e., J1>0 and J2 ≤ 1/2). The effects of the bifurcation point are also observed for the sublattice magnetization for the nearest-neighbour model state. Finally, we use the new formalism for the excited state in order to obtain the excitation energy as a function of J2/J1 for the nearest-neighbour model state by solving up to the LSUB14 level of approximation. We obtain an extrapolated value for the excited-state energy gap of -0.0036 at J2/J1=0.0 and of 0.2310 at J2/J1=0.5. We show that an excitation energy gap opens up at J2/J1 ≈ 0.24, although the gap only becomes large at J2/J1≈ 0.4.

Shape-Persistent Macrocycles Functionalised with Coumarin Dyes: Acid-Controlled Energy- and Electron-Transfer Processes

We have investigated the spectroscopic properties (absorption spectra, emission spectra, emission lifetimes) of three triads in CH(2)Cl(2): C2-M-C2, C343-M-C343, and C2-M-C343, in which M is a shape-persistent macrocyclic hexagonal backbone composed of two 2,2'-bipyridine (bpy) units embedded in opposing sides, and C2 and C343 are coumarin 2 and coumarin 343, respectively. All the components are strongly fluorescent species (Phi=0.90, 0.79, and 0.93 for M, C2, and C343, respectively, as established by investigating suitable model compounds). In each triad excitation of M leads to almost quantitative energy transfer to the lowest coumarin-localised excited state. Upon addition of acid, the two bpy units of the M component undergo independent protonation leading to monoprotonated (e.g., C2-MH(+)-C2) and diprotonated (e.g., C2-M2 H(+)-C2) species. Further addition of acid leads to protonation of the coumarin component so that each triad is involved in four protonation equilibria. Protonation causes strong (and reversible, upon addition of base) changes in the absorption and fluorescence properties of the triads because of inversion of the excited-state order and/or the occurrence of electron-transfer quenching processes.

Δself-consistent field method to obtain potential energy surfaces of excited molecules on surfaces

We present a modification of the $\ensuremath{\Delta}$ self-consistent field ($\ensuremath{\Delta}\text{SCF}$) method of calculating energies of excited states in order to make it applicable to resonance calculations of molecules adsorbed on metal surfaces, where the molecular orbitals are highly hybridized. The $\ensuremath{\Delta}\text{SCF}$ approximation is a density-functional method closely resembling standard density-functional theory (DFT), the only difference being that in $\ensuremath{\Delta}\text{SCF}$ one or more electrons are placed in higher lying Kohn-Sham orbitals instead of placing all electrons in the lowest possible orbitals as one does when calculating the ground-state energy within standard DFT. We extend the $\ensuremath{\Delta}\text{SCF}$ method by allowing excited electrons to occupy orbitals which are linear combinations of Kohn-Sham orbitals. With this extra freedom it is possible to place charge locally on adsorbed molecules in the calculations, such that resonance energies can be estimated, which is not possible in traditional $\ensuremath{\Delta}\text{SCF}$ because of very delocalized Kohn-Sham orbitals. The method is applied to ${\text{N}}_{2}$, CO, and NO adsorbed on different metallic surfaces and compared to ordinary $\ensuremath{\Delta}\text{SCF}$ without our modification, spatially constrained DFT, and inverse-photoemission spectroscopy measurements. This comparison shows that the modified $\ensuremath{\Delta}\text{SCF}$ method gives results in close agreement with experiment, significantly closer than the comparable methods. For ${\text{N}}_{2}$ adsorbed on ruthenium (0001) we map out a two-dimensional part of the potential energy surfaces in the ground state and the $2\ensuremath{\pi}$ resonance. From this we conclude that an electron hitting the resonance can induce molecular motion, optimally with 1.5 eV transferred to atomic movement. Finally we present some performance test of the $\ensuremath{\Delta}\text{SCF}$ approach on gas-phase ${\text{N}}_{2}$ and CO in order to compare the results to higher accuracy methods. Here we find that excitation energies are approximated with accuracy close to that of time-dependent density-functional theory. Especially we see very good agreement in the minimum shift of the potential energy surfaces in the excited state compared to the ground state.

Nonlinear optical spectroscopy of excited states in polyfluorene

We used a variety of nonlinear optical (NLO) spectroscopies to study the singlet excited-state order and primary photoexcitations in polyfluorene, an important blue-emitting $\ensuremath{\pi}$-conjugated polymer. The polarized NLO spectroscopies include ultrafast pump-probe photomodulation in a broad spectral range of $0.2--2.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, two-photon absorption in the range of $3.2--4.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and electroabsorption covering the spectral range of $2.8--5.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. For completeness, we also measured the linear absorption and photoluminescence spectra. We found that the primary photoexcitations in polyfluorene are singlet excitons with $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ lifetime that have a characteristic photomodulation spectrum comprising of two photoinduced absorption (PA) bands, ${\mathrm{PA}}_{1}$ at $0.55\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and ${\mathrm{PA}}_{2}$ at $1.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and a strong stimulated emission band that peaked at $\ensuremath{\sim}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The two-photon absorption and electroabsorption spectra identify the exciton PA bands with optical transitions between the lowest-lying odd symmetry $1{B}_{u}$ exciton at $3.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and two strongly coupled even symmetry states, namely, $m{A}_{g}$ at $3.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $k{A}_{g}$ at $4.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The excited state manifold also contains a strongly coupled odd symmetry exciton $n{B}_{u}$ at $4.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which identifies the continuum band threshold. A polarization memory of $\ensuremath{\sim}2.1$, typical to films of $\ensuremath{\pi}$-conjugated polymers, characterizes all three NLO spectra, reflecting the highly anisotropic third-order NLO coefficient, ${\ensuremath{\chi}}^{(3)}$ of the polymer chains. The four essential states, namely, $1{A}_{g}$, $1{B}_{u}$, $m{A}_{g}$, and $n{B}_{u}$, were used to satisfactorily fit the ${\ensuremath{\chi}}^{(3)}$ optical spectra using the summation over states model. The combination of the three NLO spectra and the model fit conclusively show that the band model, typical in inorganic semiconductors, cannot properly describe the poly(9,9-dioctylfluorene) polymer. On the contrary, a strongly bound exciton with intrachain binding energy of $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ dominates the linear and NLO spectra of this polymer.

Sum-over-States Calculation of the Specific Rotations of Some Substituted Oxiranes, Chloropropionitrile, Ethane, and Norbornenone

A sum-over-states approach has been applied to the calculation of the specific rotations of several substituted oxiranes, 2-chloropropionitrile, and 30 degrees-rotated ethane. In each case, the first few excited states proved to have only a relatively small effect on the calculated specific rotation. It was necessary to use a very large number of excited states in order to achieve convergence with the results of the more direct linear response method. However, the latter does not give information on which excited states are important in determining the specific rotation. Norbornenone is unique in that its greatly enhanced specific rotation as compared to norbornanone is associated with the low-energy n-pi* transition. The C=C bond orbitals interact with the C=O in the LUMO, and a density difference plot for going from the ground state to the first excited state clearly shows the perturbation of the C=C.

Solvation of Coumarin 153 in Supercritical Fluoroform

We present a study of local density augmentation around an attractive solute (i.e., giving rise to more attractive interaction with the solvent than solvent-solvent interactions) in supercritical fluoroform. This work is based on molecular dynamics simulations of coumarin 153 in supercritical fluoroform at densities both above and below the critical density, ranging from dilute gas-like to liquid-like, at a reduced temperature (T/T(c)) of 1.03. We focused on studying the structure of the solvation shell and the variation of the solute electronic absorption and emission shifts with density. Quantum calculations at the density functional theory (DFT) level were run on the solute in the ground state, and time-dependent DFT calculations were performed in the solute excited state in order to determine the solute-solvent potential parameters. The results obtained for the Stokes shift are in agreement with the experimental measurements. To evaluate local density augmentation from simulations, we used two different definitions, one based on the solvation number and the other derived from solvatochromic shifts. In the former case, the agreement with experimental results is good, while, in the latter case, better agreement is achieved by perturbatively including the induced-dipole contribution to the solvation energy.

TD-DFT Computational Insight into the Origin of Wavelength-Dependent E/Z Photoisomerization of Urocanic Acid

DFT and TD-DFT calculations in the gas phase and water have been performed for both anionic and zwitterionic forms of urocanic acid (UA) to gain an insight into the origin of its wavelength-dependent E/Z photoisomerization. Theoretical examination of the rotational flexibility of the carboxylic acid group by the B3LYP/6-31+G(d,p) method in conjunction with the COSMO solvent model supports the earlier suggestion (Danielsson, J.; Soderhall, J. A.; Laaksonen, A. Mol. Phys. 2002, 100, 1873) that in aqueous solution the carboxylic acid group may rotate. Our TD-B3LYP results on vertical excitation energies for the different CCCOO- rotamers of zwitterionic UA indicate that deviation from planarity may lead to an alteration of the excited-state order. We suggest two possible singlet/triplet conical intersections (S2/T5 and S1/T3) located around 306 and 314 nm. A mechanistic discussion and simplified Jablonski-type diagrams are presented for three spectral regions where UA exhibits different photobehavior: (i) before S2/T5 (λ < 306 nm), (ii) between two, S2/T5 and S1/T3, intersections (306 nm < λ < 314 nm), and (iii) after S1/T3 (UV-A irradiation at λ > 314 nm). E/Z photoisomerization occurs in the narrow spectral region ii on the nπ* singlet excited surface. The comparison of molecular orbitals and an effective electronic potential analysis of anionic UA forms suggest that the excited state of parent N-anionic UA undergoes proton photoattachment, whereas T-anionic UA does not change its protonation state upon becoming excited. It is suggested that the formation of zwitterionic species from excited N-anionic UA is responsible for a similar photochemical behavior at different pH values (5.0 and 7.0) under the photostationary irradiation. The presence of the second tautomeric component can be one of the reasons for the slight differences for zwitterionic and anionic UA solutions observed experimentally in the transient absorption dynamics and fluorescence spectra.

Alkali Ion-Controlled Excited-State Ordering of Acetophenones Included in Zeolites: Emission, Solid-State NMR, and Computational Studies

The nature of the lowest triplet excited state of acetophenones included in zeolites has been inferred through steady-state and time-resolved emission spectra. Acetophenone shows cation-dependent state switching. Within NaLiY and NaY zeolites, the emitting state is identified to have ππ* character, whereas in NaRbY and NaCsY, two emissions characteristic of nπ* and ππ* were observed. In contrast, 4‘-methoxyacetophenone does not show cation-dependent state switching; in all alkali cation-exchanged zeolites, the lowest triplet is identified to have ππ* character. The results are attributed to a specific cation−acetophenone interaction. Static, MAS, and CP-MAS spectra of 13C-enriched acetophenone included in MY zeolites confirm the presence of such an interaction. The data reveals that the extent of interaction, as reflected by the molecular mobility, depends on the cation. Small cations such as Li+ and Na+ interact strongly whereas large cations such as Rb+ and Cs+ interact weakly with acetophenone. Consistent with these trends, small cations are found to switch the lowest triplet to ππ* character, whereas the large cations leave the nπ* and ππ* triplet states of acetophenone close to each other. Computational studies provide strong support for these interpretations. B3LYP/6-31G* calculations were carried out on acetophenone and 4‘-methoxyacetophenone as well as their Li+ and Na+ complexes. Geometries with cations bound to the carbonyl, phenyl, and methoxy groups were examined. The most-stable structures involve a cation−carbonyl interaction, which stabilizes the n orbital and, in turn, destabilizes the nπ* triplet state. Excited-state energetics were quantified using TDDFT/6-31+G* calculations. Consistent with experimental observations, acetophenone and 4‘-methoxyacetophenone are predicted to have nπ* and ππ* as their lowest triplet states, respectively. Complexation with Li+ or Na+ is predicted to lead to a ππ* triplet as the lowest excited state for both compounds. The present study, combining steady-state and time-resolved emission spectra, solid state NMR, and computations, demonstrates the occurrence of cation-dependent state switching in acetophenones and offers an internally consistent explanation of the effect in terms of specific cation−carbonyl interaction.

Half-life and internal conversion electron measurements in low-lying levels of ^{125,127}Ba

Decay spectroscopy using on-line mass separators has the advantage of allowing the study of level properties in the low energy region, including band head information, since g transitions between low-spin states can be measured more intensively under lower background conditions than can those with in-beam spectroscopy measurements. Neutron-deficient nuclides of 125,127 Ba have been studied by means of in-beam spectroscopy, and the level structures for high-spin states were successfully interpreted within the framework of the IBFM model @1‐5#. Decay studies of these nuclides have rarely been reported. Therefore, the last evaluations in Refs. @6, 7# were mainly based on in-beam studies. In 1991, the possibility of static octupole deformation, similar to that of the A5145 and 225 regions proposed, was proposed for the A5130 region by Cottle @8#. The presence of static octupole deformation is represented by parity doublets, which have equal spins and opposite parity levels of rotational bands with alternating bands. The E1 transitions between parity doublets are characterized by a two to four orders of magnitude enhancement compared to those of more normal cases. The 127‐130 Ba isotopes were successively studied by in-beam conversion electron measurements to investigate for parity doublets, and no evidence of them was observed @9,10#. These studies, however, focused only on the conversion electron measurements; the transition probabilities were not measured. It is necessary to measure not only the conversion electrons, but also the half-lives of the excited states in order to explain this feature. The aim of this investigation was to study the level properties of 125,127 Ba in the low energy region, focusing on a search for E1 transitions, and to study their properties by measuring the half-lives of low-lying excited states. The half-lives and conversion electrons were measured for the first time through the decays of 125,127 La, by means of a delayed coincidence technique and a cooled Si~Li! detector, respectively. The expected enhanced E1 transitions were not observed. Level properties based on the Nilsson model are proposed.

Resonant and nonresonant hyper–Rayleigh scattering of charge-transfer chromophores

The first molecular hyperpolarizabilities (β) of a series of charge-transfer nonlinear optical (NLO) chromophores are measured with the hyper-Rayleigh scattering (HRS) technique using two excitation wavelengths at 1064 and 1907 nm. The 1907 nm wavelength is the longest excitation wavelength used for the HRS experiment. For some of these chromophores, β values in excess of 1000×10−30 esu at 1907 nm are obtained, and due to two-photon enhancement, even greater β values are found with the 1064 nm excitation. Chromophores with such large hyperpolarizability are expected to have potential applications in practical electro-optical devices. The dispersion of β is analyzed using a two-vibronic-state model developed previously in our laboratory. The study shows that it is necessary to consider the vibronic structure of the chromophore in the excited state in order to account for the behavior of the first molecular hyperpolarizability of the charge-transfer NLO chromophores.