Confined helium: Excited singlet and triplet states

Energies for the first four singlet and triplet S states of a helium atom confined at the center of an impenetrable sphere are reported. All calculations used explicitly correlated Hylleraas basis sets. The first triplet state is shown to lie below the first excited singlet state only when the confinement radius is greater than 0.988_a0$0.988a_0$. A simple configuration interaction calculation was performed in parallel with Hylleraas calculation. The one-electron atomic orbitals of the configuration treatment provide insight into the physical concepts behind the numerical results of the Hylleraas treatment. This was particularly helpful in understanding the level crossing and avoided crossings observed with changing confinement radius.     

The lowest singlet and triplet excited states of pyrazine

The phosphoresence, fluorescence and absorption spectra of pyrazine-h₄ and -d₄ have been obtained at 4°K in a benzene matrix. For comparison, those of the isotopically mixed crystal pyrazine-h₄ in d₄ were also taken. All these spectra show extremely sharp and well-resolved lines and reveal detailed vibronic structure.The coincidence of origins of absorption and emission shows that the lowest singlet state is an allowed transition, properly designated as ${}^1\text{B}{}_{\mathrm{3u}}\text{←}{}^1\text{A}{}_{\mathrm{1g}}$. The forbidden component ¹B_2g, predicted by both “exciton” and MO theories to be below the allowed component, must lie higher. Its exact location still remains uncertain.The phosphorescence spectrum, when compared with the singlet-triplet excitation spectrum, indicates that the lowest triplet state is also symmetry allowed, showing a strong 0-0 band and the coincidence of origins of absorption and emission. In accordance with previous work, the triplet state is designated as ³B_3u.The vibronic structure of the phosphorescence spectrum is very complicated. The first work on the analysis of this spectrum concluded that a simple progression of ν_6a exists. Under the high resolution attainable in the present experiments, the supposed ν_6a progression proves to have a composite structure, starting from the second member of the progression. Not only is the ν_9a hydrogen-bending mode present as shown by the appearance of the CD bending mode in the d₄ spectrum, but a band assigned as 2ν_6b was also identified. The latter's anomalous intensity in the phosphorescence spectrum is interpreted as due to the Fermi resonance with 2ν_6a and ν_9a bands. To help resolve the present controversy over the origin of the phosphoresence spectrum observed in crystalline pyrazine, detailed vibrational analyses of the emission spectra were made. The fluorescence spectrum has essentially the same vibronic structure as the phosphorescence spectrum.     

Mixing of singlet and triplet exciton states in highly excited CdS

Fluorescence spectra due to the free excitons have been studied in CdS at 4.2 K under various excitation levels. It has been found that the triplet-exciton emission intensity relative to the singlet line is enhanced remarkably with the excitation power density. This effect is reasonably explained by the mixing of the singlet with the triplet through the many body interaction. The density of optically generated excitons is determined from the magnetic field dependence of the triplet emission intensity.     

Excited states and laser properties of tricarbocyanine dyes

Photoisomer absorption and emission were obtained in various tricarbocyanine dyes by using a laser photolysis technique. Triplet spectra were also recorded in the wavelength range of the laser emission. The results allow the discussion of the behaviour of these compounds as saturable absorbers.     

On the Josephson effect between superconductors in singlet and triplet spin-pairing states

An expression is derived for the Josephson current between two weakly coupled superconductors of which one or both have pairs in a spin-triplet state. It is shown that there can be no Josephson effect up to second order in the transition matrix elements between a superconductor with spin-triplet pairs and one with spin-singlet pairs if the coupling between the two superconductors can be described with a spin-conserving tunnel hamiltonian. This is shown to offer a possibility to investigate experimentally whether a particular superconductor has spin-triplet pairs by coupling it weakly to a well-known spin-singlet pairing superconductor.     

Magnetic field effects on the hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol as studied by a picosecond laser flash photolysis technique

The hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol at 265 K has been studied with a newly developed picosecond laser flash photolysis apparatus under magnetic fields of 0–1.7 T. The decay rate constant of the radical pair generated was found to increase from 3.42 × 10⁹ s^?1 to 4.15 × 10⁹ s^?1 with increasing the magnetic field from 0 to 1.7 T. The observed magnetic field effects can be explained by the Δg mechanism. Using the simple kinetics model with the Δg mechanism, the rate constant of the escape process from the pair (k _esc) and two rate constants for the T-S spin conversion process (k _T-S) at 0 and 1.7 T were found to be 1.97 × 10⁹ s^?1, 1.45 × 10⁹ s^?1, and 2.12 × 10⁹ s^?1, respectively. From the magnetic field dependence on k _T-S, the difference in the g values of the 4-methoxybenzophenone ketyl and phenylthiyl radicals was estimated to be 0.0087.     

Picosecond dynamics of photoinduced electron transfer involving singlet and triplet states in nitroporphyrin molecules

The dynamics of relaxation processes in meso-ortho-nitrophenyl-substituted octaethylporphyrins OEP-Ph(o-NO₂) and PdOEP-Ph(o-NO₂) occurring with the participation of the S ₁ and T ₁ states is studied in polar (dimethylformamide) and nonpolar (toluene) media at 295 and 77 K using pico-and nanosecond laser kinetic spectroscopy. It is shown that, at 295 K, the steric interactions between β-alkyl substituents in porphyrins and ortho-nitro groups of meso-phenyl in these compounds create optimal conditions for overlapping of molecular orbitals of the donor and the acceptor, which ensures an efficient photoinduced electron transfer (PET). It is ascertained that, in free-base OEP-Ph(o-NO₂), the PET occurs only via the S ₁ state of porphyrin in time intervals of 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing intersystem crossing S ₁ ? T ₁ is unlikely. In the case of the metal complex PdOEP-Ph(o-NO₂), the PET involves both S ₁ and T ₁ states. In the PdOEP-Ph(o-NO₂) molecule, direct PET from the T ₁ state to the charge-transfer state also occurs in the picosecond range (20 and 46 ps for dimethylformamide and toluene, respectively, at 295 K), with the rate constants being 3–5 times smaller in comparison with the corresponding values found for the PET occurring via the S ₁ state of this compound. For both compounds, the experimentally observed long-lived component in the decay kinetics of the induced T ₁-T _n absorption (250–700 ns) is due to the recombination processes in ion-radical pairs whose lifetime decreases with an increase in the polarity of the medium and is almost independent of the presence of molecular oxygen in the solution. The PET is completely absent at 77 K in the compounds under investigation when they are dissolved in rigid solutions.     

Magnetic fluctuations of filled skutterudites emerging in the transition region between singlet and triplet states

In order to clarify magnetic properties of filled skutterudites, we analyze the Anderson model including seven f orbitals hybridized with an au conduction band using a numerical technique. For n=2 corresponding to Pr-based filled skutterudites, where n is the local f-electron number, even if the ground state is a singlet, there remain significant magnetic fluctuations from a triplet state with a small excitation energy. This result can be understood by the fact that f-electron states are clearly distinguished as itinerant and localized ones in the filled skutterudite structure. This picture also explains the complex results for f-electron magnetic susceptibility and entropy for n=1-13.     

Energy transfer by singlet and triplet excitons in carbazole-containing polymers

Quantum yields of pyrene fluorescence and bis[2-(2′-benzothienyl)-pyridinato-N,C^3′](acetylacetonate)iridium [Btp₂Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole–dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the “tail” energy states, dissociation of singlet excitons into geminal electron–hole pairs (EHP), and capture of triplet excitons by polymer oxidation products.     

Intramolecular energy transfer by singlet and triplet excitons in macromolecules

Absorption, luminescence and photo-oxidation of polyvinylcarbazole (PVCa) and polyvinylbenzocarbazole (PVBCa) in weak solutions have been studied. It is found that fluorescence and phosphorescence spectra, observed at 293, 77 and 4.2 K, depend on the molecular mass M of the polymer and on the experimental conditions. Due to efficient intramolecular energy transfer the fluorescence spectrum includes the emission of excimers, emission of end groups and that of internal structural defects of a macrochain. The relative role of each emission markedly depends on M. Phosphorescence is largely determined by the annihilation effects between triplet excitations, these effects being pronounced in long macrochains. The data show the excitonic nature of the singlet and triplet excitation energy transfer in the polymers studied. The migration of one-dimensional singlet and triplet excitons strongly influences the luminescent properties of PVCa and PVBCa macromolecules, as well as their photochemical reactions. The range of singlet excitons in PVCa and PVBCa macromolecules is determined both at room temperature and at low temperatures.     

Nonradiative heterofission of a singlet host exciton into a pair of triplet electronic excitations in doped aromatic hydrocarbon molecular crystals

The nonradiative heterofission process of a singlet host exciton into one triplet host exciton and one triplet guest molecule electronic excitation in some aromatic hydrocarbon molecular crystals is investigated theoretically. The formulae for the heterofission rate constants are derived and discussed and used for the numerical estimates. The conditions for the occurence of the heterofission of a singlet host exciton in the investigated crystals are discussed.     

Zero-field splitting of methyl-substituted phosphorescent aromatic molecules in their lowest triplet states

The effect of methyl substitution on the ZFS of phosphorescent aromatic compounds in their lowest triplet states has been investigated for both homocyclic and heterocyclic molecules. The experimental results of ESR measurements on fifteen molecules are given. The ZFS parameters have been calculated theoretically applying the semiempirical WM method. The experimental and theoretical D and E values are briefly discussed.     

Excited states of a bound polaron

The results of the calculations of energies of states 2S and 2P of a bound electron interacting weakly with optical phonon field are presented.     

Excited states of a bound polaron

Expressions for the energies of the 3s, 3p and 3d states of a polaron, bound to a positively charged impurity ion in a polar crystal through a Coulomb potential, are obtained in analytic forms using perturbation theory. The values of these energies are calculated in several polar crystals.     

Stability of singlet and triplet trions in carbon nanotubes

We investigate singlet and triplet trion states in semiconducting carbon nanotubes using a one-dimensional model. It is concluded that singlet trion states in bind up to 13.6% stronger than exciton states, and that they lower the optical transition energy with up to 50% of the tight binding band gap energy.     

Semi-empirical calculations of the zero-field-splitting parameters of the lowest triplet states of aromatic ketones

Semi-empirical calculations of zero-field-splitting parameters have been performed for the lowest triplet states of aromatic ketones, the mixing between the nearly degenerate n, π* and π, π* triplet states through the vibronic and spin-orbit interactions being considered. The results show that the mixing through the spin-orbit interaction is important to explain the absolute values and signs of the zero-field-splitting parameters observed for acetophenone, p-chlorobenzaldehyde, and benzophenone. The observed and calculated D values are -0·1943 cm^-1 and -0·180 cm^-1 for the lowest triplet state of p-chlorobenzalhyde in p-dibromobenzene.