Mechanism of the electron spin polarization of excited triplet states caused by the mutual annihilation of triplet states

A new mechanism of the triplet spin polarization is considered qualitatively and quantitatively. The spin polarization arises due to the spin-selective triplet-triplet annihilation and the subsequent spin dynamics in the spin-correlated pairs of triplets. With the time-dependent perturbation theory the simple rules are found which specify the polarization pattern of triplets.     

Electron spin polarization of doublet state species due to interaction with excited triplet states in single crystals

Doublet states species trapped in crystalline solids show transient spin polarized EPR spectra if the crystal is illuminated by visible or UV light. The spin polarization is accounted for by the interaction of the doublet species with photoexcited triplet states. The mechanism of the process producing the spin polarization is examined and some experimental examples are discussed. The analysis of the time evolution of the transient variation of the EPR signal allows the measurements of the spin lattice relaxation time and in some cases of the diffusion rate of mobile triplet excitations in the crystal lattice.     

Quenching of excited complexes by Fe-octaethylporphin in triplet-triplet annihilation of Mg-phthalocyanine in liquid solutions

The effect of triplet-state quenchers on the kinetics of triplet-triplet annihilation (TTA) of Mg-phthalocyanine (Mg-Phc) is studied. It is found that the rate constant of triplet-state quenching caused by TTA increases with increasing concentration [Q] of quenchers. The maximum values of the relaxation parameter of triplet states are proportional to [Q]². The experimental data correspond to TTA with the formation of TT complexes from molecules in triplet states. The proportionality of the decay rate of TT complexes into molecules in the ground state to [Q]² suggests that two quenching molecules are required for quenching one TT complex. It seems that the complex contains two locally excited triplet states of individual molecules. The spin correlation time in the triplet state seems to be longer than the average lifetime of complexes (≤10^?4 s). The quenching probability of triplet states in complexes (caused, in particular, by the energy of charge transfer) is lower than the probability of intermolecular triplet energy transfer to the quencher levels.     

Chemically induced dynamic electron polarization of C60 nitroxide bisadducts and spin multiplicity of their excited states

A series of biradicals consisting of a C₆₀ linked to two nitroxide addends have been studied by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The series includes all four trans bisadducts and the equatorial isomer. After a visible laser pulse, the bisadducts in toluene liquid solution show chemically induced dynamic electron polarization (CIDEP) effects with a complex pattern of narrow lines in emission and enhanced absorption. From the knowledge of the sign of exchange interaction between the two nitroxide moieties, it was possible to account for the CIDEP effect in terms of an intramolecular triplet-triplet annihilation. A broad signal attributed to the quintet excited state was observed at short time delay after the laser pulse. The TR-EPR spectra recorded at low temperature in glassy matrix are assigned to an excited triplet state localized on the C₆₀, with a small interaction with the spins of the two nitroxide fragments. The assignment is based on spectral simulation and analysis of the transient nutation frequency. The small exchange interaction between excited triplet C₆₀ and nitroxide is shown to depend on the nature of the nitroxide addend.     

Electron spin polarization in a rotating triplet

The triplet model of electron spin polarization in fluid media is evaluated. The model consists of an initial singlet molecule rotating in a static, externally applied magnetic field. Intersystem crossing into different zero-field states is represented by a rate matrix diagonal in the molecular frame, and this matrix is expressed as an effective spin operator. The triplet rotates, and the motion affects the polarization in the laboratory frame, and also causes spin relaxation in the triplet manifold. The triplet is chemically quenched, and the polarization appears in the doublet fragments. The model is treated in a density matrix formalism and on the basis of anisotropic rotational diffusion of the triplet molecule. Explicit expressions are obtained in terms of the molecular parameters, the various rate constants, and the rotational correlation time.     

Spin polarization in triplet states in solution

A method is described for measuring the absolute value of the spin polarization in a triplet state in solution through CIDEP observations of the radicals formed on its reaction, and through it the anisotropy of the rates of the spin-selective intersystem crossing process. A long-accepted equation concerned with triplet mechanism spin polarization is shown to be inadequate to reproduce observed behaviour, and evidence produced to suggest that the observed polarization is affected by radical relaxation. The method also allows determination of the absolute polarization in the radicals. A novel analysis of the relative contributions of TM and RPM processes to observed spectra provides further evidence for the conclusions.     

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.     

Nuclear spin coupling by electron orbital polarization

A theory is presented of the coupling of nuclear spins in a molecule by the induction of electronic orbital currents. An approximate expression for the contribution of this effect to the total coupling constant is derived in terms of the anisotropy of the magnetic screening of the nuclei involved. The theory is applied to some coupling constants involving fluorine nuclei, for which the electron orbital contribution is found to be an appreciable but not dominant part. It can have either sign, but is usually positive.     

Doubly excited triplet states of highly stripped ions

Transition energies, Coulomb repulsions and effective quantum numbers have been calculated for the doubly excited Nsnp: ³P⁰ (for N = 2, n = N,?, 5n = N,\ldots, 5); Nsnd: ³D^e (for N = 2, n=N+1, ?, 5n=N+1, \ldots, 5) and Npnd: ³F⁰ (for N = 2, n=N+1, ?, 5n=N+1, \ldots, 5) states for the highly stripped ions Na⁹⁺{\rm Na^{9+}}, Mg¹⁰⁺\rm Mg^{10+}, Al¹¹⁺\rm Al^{11+}, Si¹²⁺\rm Si^{12+}, P¹³⁺\rm P^{13+} and S¹⁴⁺\rm S^{14+}. Time-dependent perturbation theory has been applied to calculate such transitions properties. A time-dependent harmonic perturbation causes simultaneous excitation of both the electrons with a change of spin state. The doubly excited energy levels and the analytic representation of their wave functions are obtained by identifying the poles of an appropriately constructed linearized variational functional with respect to driving frequency. Most of the results are new. The transition energies and effective quantum numbers of 2s2p: ³P⁰ states of all the ions agree well with the only available experimental data.     

Studies of hyperfine splittings and spin distributions of the lowest excited triplet states of substituted benzenes and pyridines

Hyperfine splittings and linewidths of the E.P.R. spectra of the lowest excited triplet states of several cyano-substituted benzenes, pyridines and 1,2,4,5-tetrachlorobenzene were studied, and spin distributions were estimated. These spin distributions as well as the zero field parameters were discussed in terms of the modified benzene π electron MO. It is shown that one excited configuration dominates in the studied systems, but a small amount of configuration mixing is necessary in order to explain ZFS. The experimentally determined spin densities were compared with those calculated from UHF methods in the case of benzonitrile and phenylacetylene. The calculated spin densities agree well with those estimated experimentally when proper choice of integral is made. It is also shown that the spin distributions in triplet state and radical anions are similar in the systems studied here. The geometries of the excited triplet states of substituted benzenes are also investigated from UHF calculation.     

The fast and slow rotation approximations for the description of the kinetics of triplet radical quenching and electron spin polarization

The specific features of the mechanisms and kinetics of generation of net chemically induced dynamic electron polarization (CIDEP) in triplet radical quenching (TRQ) in liquids is analysed in detail. The problem reduces to the analysis of fairly strong non-adiabatic transitions between states of the triplet radical spin Hamiltonian which are known to determine CIDEP generation in TRQ. The analysis is performed in two limits of fast and slow rotation of the triplet molecule using the previously developed method of treatment for non-adiabatic transitions. The method made it possible to derive analytical formulas for the CIDEP generation probability P _e and rate K _e, and for the TRQ probability P _q and rate K _q in the case of relatively strong quenching. It is shown that the dependence of K _e on the relative diffusion coefficient D _r is of bell shape and cannot be described correctly by the usually applied relation K _e = K _q P _e.     

Identification of excited states in Na2 by two-step polarization labeling

In two-step polarization labeling, as in polarization labeling, a polarized pump laser is tuned to a molecular transition. This produces an optical anisotropy in the upper and lower levels of the pump transition. A linearly polarized probe laser will experience a change in polarization when it interacts with these oriented levels of the molecule. Probe delays of several nanoseconds distinguish lower level depolarization from upper level depolarization. The technique has been applied to Na₂ to identify three new excited states. Preliminary values of some of the molecular constants are presented.     

Inductive-resonant triplet-triplet annihilation in solid solutions of erythrosine

Experimental investigations of annihilation-induced delayed fluorescence and phosphorescence of frozen ethylene glycol solutions activated by erythrosine were carried out. It is found that the kinetics of the annihilation-induced delayed fluorescence is nonexponential in the initial stage, and approaches an exponential decay at later stages with the lifetime equal to half the triplet-state lifetime. It is shown that experimental data agree well with the theory proposed to describe triplet-triplet annihilation by an inductive-resonant mechanism in solid solutions of complex organic molecules. Belarusian State University, 4, F. Skorina Ave., Minsk, 2200050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 629–632, September–October, 1997.     

Dynamic nuclear polarization in electron spin echo

It is shown that under the action of a proper microwave pulse sequence the equilibrium polarization of the electron spin may be transferred dynamically to the longitudinal nuclear magnetization which will oscillate due to the nuclear spin precession around the effective fields relating to differnt electron quantum number manifolds. These oscillations may be measured directly in the radiofrequency band. Analytical formulae are obtained for the case when all the nuclei coupled to an unpaired electron have spins of 1/2.     

Positron annihilation from excited states of [X-e+] systems

The relative probabilities for the radiative de-excitation 2p₊ → ls₊ versus 2γ-annihilation for the hdot; ns²np⁶2p₊, ²P state of the [X^-e⁺] system with X = F, Cl, Br, and I are presented. It is shown that a positron captured into a 2p₊ orbital undergoes annihilation with electrons of the system instead of radiative transition to the ground state of the [X^-e⁺] system.     

Approximation of the electron excitation cross sections for triplet states excited from the 23 S 1 metastable state in helium

The method of approximated four-parameter representation of the electron-impact cross sections for a helium atom excited from the 2³ S ₁ metastable state into higher triplet states is applied and discussed. The approximation consists in interpolation over the whole set of the cross section values for each helium atomic level measured in our experiments and reported by other researchers. The approximation parameters and the cross sections calculated using these parameters for the maxima of the corresponding excitation functions are presented for 19 triplet levels of the S, P, and D HeI series with n=2–10. The interpolated values are compared to the theoretical cross sections. The serial regularities were investigated for the S, P, and D levels studied and a decrease in the cross sections for excitation from the given metastable state within each series, described by the approximate law Q=Cn ^?5, was revealed. Validity of the similarity relationship and the Bethe approximation for cross sections in the 2³ S?n ³ P series was verified. It is shown that the cross sections for a triplet level excitation from the 2³ S ₁ metastable state exceed the corresponding values for excitation from the ground state of helium by a factor of approximately 10³ for n=2 and 3 and 10 2 for the higher levels. It is concluded that the proposed method of representation of the cross sections for the electron-impact excitation of triplet levels from the metastable state increases accuracy and more importantly, reliability of the final results.     

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.     

Electrically controlled nuclear spin polarization and relaxation by quantum-Hall states

We study interactions between electrons and nuclear spins by using the resistance ( Rxx) peak which develops near the Landau-level filling factor nu = 2/3 as a probe. Temporarily tuning nu to a different value, nu(temp), with a gate demonstrates that the Rxx peak regenerates even after complete depletion ( nu(temp) = 0), while it rapidly relaxes on either side of nu(temp) = 1. This indicates that the nu = 2/3 domain morphology is memorized by the nuclear spins which can be rapidly depolarized by Skyrmions. An additional enhancement in the nuclear spin relaxation around nu = 1/2 and 3/2 suggests a Fermi sea of partially polarized composite fermions.