Distance distributions recovered from steady-state fluorescence measurements on thirteen donor-acceptor pairs with different Förster distances

The end-to-end distance distribution of a flexible molecule was recovered from steady-state fluorescence energy transfer measurements using the method suggested by Cantor and Pechukas (Proc. Natl. Acad. Sci. USA 68, 2099–2101, 1971). In this method, the Förster distance (R ₀) is varied by attaching different donor-acceptor (D-A) pairs to the flexible linker of interest. Distance distributions are then recovered from energy transfer efficiency measurements on the set of D-A pairs with differentR ₀ values. Thirteen D-A pair compounds were synthesized withR ₀ values ranging from 6 to 32 Å. Each compound contained a tryptamine donor linked by an alkyl chain (10 carbons) to 1 of 13 acceptors. Using these compounds, we have experimentally confirmed the Cantor and Pechukas method for recovering distance distributions. The measured transfer efficiencies, as a function ofR ₀, were fit to the transfer efficiencies predicted for both Gaussian and skewed Gaussian distance distributions. The data support the existence of a skewed Gaussian distribution, and we believe that this is the first experimental observation of an asymmetric distribution for a flexible molecule using fluorescence resonance energy transfer measurements. Finally, the experimentally recovered distance distribution was found to be in good agreement with the distribution predicted from the rotational isomeric state model of Flory (Statistical Mechanics of Chain Molecules, John Wiley & Sons, New York, 1969, Chaps. 1, 3, and 5) but not with the predicted distribution for a freely rotating or freely jointed chain.     

Dynamics of photoinduced electron transfer in model multiporphyrin ensembles

It is shown by the methods of picosecond fluorescent and femtosecond absorptive spectroscopy that in the nanosize self-assembled complexes based on a Zn-octaethylporphyrin dimer and electron acceptors of different nature (quinone, anthraquinone, pyromellitimide) deactivation of the energy of electron excitation is caused by the nonradiative S-S-energy transfer (for times ≤10 psec) and photoinduced charge transfer (within the range of 300 fsec-700 psec). In tetrads an electron is transferred from a porphyrin extra-ligand to quinone for distances r_DA≈18–21 Å by the “superexchange” mechanism. In triads, the charge transfer from a dimer to a fluorinated porphyrin extra-ligand is efficient at 120-77 K.     

Luminescence of ZnO films implanted with Au+ ions and annealed in oxygen radicals

Visible and ultraviolet luminescence spectra of ZnO:Au⁺ samples with various dose of implanted gold were studied at a temperature of 77 K. Luminescence peaks of bound and free excitons, D-A pairs, and electron recombination to the gold acceptor level were detected. The optical depth (0.117 eV) of the Au_Zn impurity acceptor level was determined. Diffusive spreading of the implanted impurity profile upon annealing of the zinc oxide film was observed by SIMS.     

ESR of Yb3+ ions in a single crystal of the fluctuating-valence compound YbB12

The temperature and angular dependences of electron spin resonance (ESR) spectra of Yb³⁺ ions in a single crystal of fluctuating-valence compound YbB₁₂ were studied. The existence of Yb?Yb ion pairs was observed in a cubic-symmetry crystal. The ions forming the pairs are coupled by the isotropic exchange but interact also with the other pairs by the dipole and exchange coupling. The occurrence of a slight anisotropy in a cubic semiconductor may be the result of a spontaneous break of symmetry specific for the ground state of the Kondo dielectric. A strong temperature dependence of the amplitude of the ESR signals is found at 1.6–4.2 K and interpreted as a result of the capture of electrons by Yb³⁺ ions from electron traps with a binding energy of 18 K. ESR spectra of Yb³⁺ single ions in the Γ₆ state were observed also. The decrease of temperature from 4.2 to 1.6 K indicates a tendency to the ferromagnetic ordering of Yb?Yb pairs.     

Hyperfine Structure in the OD ESR Spectra of Recombining Charge Pairs in Doped Polyethylene Matrices

The hyperfine structure has been resolved in the optically detected electron spin resonance (OD ESR) spectra of radical ion pairs generated by ionizing radiation at a low dopant concentration in a solid polyethylene matrix at room temperature. The different organic molecules that can capture electrons and holes were used as the dopants. The spin-lattice relaxation times T₁ for radical ions of dopants were estimated to be tens of microseconds. The temperature dependence of OD ESR signal intensity indicates the diffusion-assisted mechanism of recombination of radical ion pairs in polyethylene.     

A photoluminescence study on lithium ternary compounds

Summary The photoluminescence spectra of LiInS₂ and LiInSe₂ single crystals are investigated in this work. The origin of the coloration and deep emission at 460 nm on LiInS₂ is estimated through ESR measurements to be due to Fe³⁺ ions. The so-called ?near-band-edge emission? was observed at 600 nm and at 77 K on the light yellow LiInSe₂ single crystals. Through a comparison between the results of electrical and photoluminescence measurements it is shown that the red LiInSe₂ has a tailing band structure with high-density residual impurities. Some deep emissions are obtained on yellow and red LiInSe₂. Paper presented at the ?V Intenrational Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Nonlinear oscillatory waveguide propagation in a plasma

The presence of an intense Gaussian laser beam gives rise to a ponderomotive force on electrons in a collisionless plasma, leading to a redistribution of electron density along the wave-front and consequently to an intensity dependent dielectric constant which saturates with increasing intensity. The intensity dependent dielectric constant is responsible for beam propagation in an oscillatory waveguide. It is seen that (i) a beam of radiusr ₀ less thanr _0min (?c/ω _p) cannot be focused in the plasma regardless of its power, (ii) minimum dimension of oscillatory waveguide increases with increasing power of the beam. Similar results are also obtained for collisional plasma where nonlinearity arises due to nonuniform heating and consequent redistribution of carriers.     

Electron spin resonance in carbon-doped boron nitride

It is shown that carbon impurity is responsible for the yellow color and paramagnetism in boron nitride which has been heated to 1800–2300°C in graphite furnaces. The yellow BN powders exhibited 10-line ESR spectra with spin concentrations up to 10¹⁹/g. Annealing the yellow BN at 1800–2300°C in a carbon-free environment removed the yellow color and reduced the spin concentration below the observable limit of 10¹⁶/g. Such pure white BN was unaffected by prolonged irradiation with u.v., 50 kV X-rays or 1 MeV γ-rays, whereas other less pure types of BN showed a large enhancement of ESR signal after relatively short exposures to such radiation. The ESR spectra from pure white BN doped with C¹³-enriched carbon were identical to those from BN doped with normal carbon. These results suggest that the yellow color in BN is due to an F-center produced by a carbon impurity ionization mechanism. The anisotropy of the ESR was studied in carbon-doped specimens of highly oriented BN. The room temperature hyperfine splitting at 9·151 GHz was found to vary as (a + b cos²θ), where a = 7·85 ± 0·07 G and b = −1·27 ± 0·07 G. The g-value obeyed a similar relation: g = g_⊥ + Δg cos²θ, where g_⊥ = = 2·00321 ± 0·00002 and Δg = g_∥ − g_⊥ = (−9·5 ± 0·2) × 10⁻⁴.     

Triplet-triplet transfer of electronic excitation energy in vapors of organic molecules

The rate constants of the triplet-triplet (T-T) transfer of energy of the electronic excitation obtained in the gas phase for two sets of donor-acceptor pairs with various spectral characteristics are compared with those predicted by the theories of nonradiative transitions. The rate constants K _TT and efficiencies β_DA of the T-T transfer are estimated from quenching of the phosphorescence of biacetyl by aromatic hydrocarbons (anthracene, 9-methylanthracene, 9,10-dimethylanthracene, 9-phenylanthracene, pyrene, and chrysene) and from quenching of the delayed fluorescence of benzophenone, 4-phenylbenzophenone, anthraquinone, carbazole, and naphthalene by biacetyl, as well as from the sensitization of phosphorescence of biacetyl by these compounds. The causes of the broad variations of β_DA in the gas phase from 0.35 (biacetyl-anthracene) to 10^?4 (biacetyl-chrysene) are discussed for the studied donor-acceptor pairs, which differ in the exothermicity of the process and in the configuration of the lowest triplet levels of the molecules participating in the transfer. The dependences of the transfer rate constants K _TT on the free energy change ΔG (the range from 0.4 to ?1.7 eV) are analyzed. It is shown that, in gas-phase systems, K _TT increase with the exothermicity of the transfer, attain a highest value, and then decrease in the range of negative ?ΔG. The observed differences in the experimental values of β_DA are discussed in terms of changes in the free energy ΔG of the process, in the energy of rearrangement of the nuclear configuration of the molecules during the transfer, and in the matrix elements of the interaction.     

ESR study on the reactivity of two isomers of LaC82 with disilirane

We observed photochemical and thermal addition of disilirane to the LaC₈₂-A isomer, which give new ESR active species. Both ESR spectra of the adducts in photo- and thermal reactions are well simulated by at least six octet components, and the isotropic La coupling constants and g-factors of the six octet components are determined. On the other hand, the reaction of the LaC₈₂-B isomer with disilirane gives no ESR active species. This fact is also verified by mass spectroscopic measurements. The different reactivities with disilirane of the two isomers are not interpreted by a popular view that the two isomers have a different C₈₂ cage structure, but we rather suggest that the origin of the isomers of metallofullerenes should be re-examined.     

KGW 晶体外腔式高功率579 nm喇曼黄光激光器

报道了579 nm高功率KGd(WO₄)₂喇曼晶体外腔式喇曼黄光激光器的输出特性.基于808 nm脉冲激光二极管侧面泵浦Nd:YAG陶瓷、腔内BBO电光晶体同步延迟调Q和Ⅰ类临界相位匹配的LBO晶体腔外倍频方案,并通过外腔式KGW晶体Ng轴二阶斯托克斯喇曼频移,获得了579.54 nm黄光激光输出.当脉冲信号重复频率为1 kHz、532 nm泵浦光最高平均功率为5.02 W、脉冲宽度为10.1 ns时,获得了最高平均功率2.58 W、脉冲宽度7.4 ns、峰值功率348.6 kW的579.54 nm二阶斯托克斯喇曼黄光激光输出;532 nm至579.54 nm的光-光转化效率为51.4%、斜率效率为54.8%,光束质量因子M_x-579.54²=5.829、M_y-579.54²=6.336,输出功率不稳定性小于±2.35%.实验表明:外腔式喇曼结构能够高效地获得喇曼黄光,具有很高的光-光转化效率及良好的功率稳定性,并通过脉冲LD结合同步延迟电光调Q可获得高重复频率、高平均功率、窄脉冲宽度和高峰值功率的黄光激光输出.     

高次正幂与逆幂势函数的叠加的径向薛定谔方程的解析解

当薛定谔方程中出现高次非谐振子势，电偶极矩势，分子晶体势，极化等效势等高次正幂与逆幂势函数以及它们的叠加时，薛定谔方程的求解变得非常复杂，采用奇点邻域附近的级数解法与求解渐近解相结合，在多种相互作用幂函数紧密耦合的条件下，得到势函数为V(r)=a₁r⁶+a₂r²+a₃r^{-4关键词： 级数解法 幂势函数 径向波函数 渐近解}     

Gd3+ electron spin resonance spectroscopy on LaO1 ? x F x FeAs superconductors

We report an electron spin resonance (ESR) spectroscopy study on polycrystalline samples of the LaO_{1 ? x} F _x FeAs (x = 0 and 0.1) compound with small levels of Gd doping (2% and 5%). The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent LaO_{1 ? x} F _x FeAs compounds at T _SDW ?? 130 K. Interestingly, the analysis of the low-temperature ESR spectra of the c-axis oriented Gd_{1 ? y} La _y OFeAs samples gives evidence for the magnetically nonequivalent Gd sites and also for sites having a different local charge environment. The analysis of the temperature dependence of the ESR linewidths gives evidence for a coupling of the localized 4f electrons of Gd to the conduction electrons in the FeAs layers. The ESR data reveal that the fluorine substitution, which provides electron doping, suppresses the SDW order and enhances the density of states in the electronic bands stemming from the xz and yz orbital states of Fe to which the 4f electrons are most strongly coupled.     

Investigation of Paramagnetic Centers in Fullerides A2MC60 and AM2C60 (A = K, Rb, M = Mg, Be)

Paramagnetic centers in heterofullerides with the composition A₂MC₆₀ and AM₂C₆₀ (A = K, Rb, M = Mg, Be) were investigated by the electron spin resonance (ESR) method. It was found that the ESR signal can be interpreted as a composition of two lines with different temperature dependence of ESR absorption magnitude. This gives an evidence of the presence of at least two different types of paramagnetic centers. Centers of the first type behave as localized spins, while the rest can be interpreted as conduction electrons. Authors' address: Vladimir A. Kulbachinskii, Physics Faculty, M. V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russian Federation     

The Circular Loop Equation of a Cosmic String with Time-Varying Tension in de Sitter Spacetimes

In this work the equation of circular loops of cosmic string possessing time-dependent tension is studied in the de Sitter spacetime. We find that the cosmic string loops with initial radius r(t ₀)>0.707L, L de Sitter radius, should not collapse to form a black hole. It is also found that in the case of r(t ₀)<0.707L a loop of cosmic string whose tension depends on some power of cosmic time can not become a black hole if the power is lower than a critical value which is associated with the initial size of the loop. Our research gives rise to some important corrections to the conclusion in the case of loops of cosmic string with constant tension in the same background.     

Laser diode and pumped Cr:Yag passively Q-switched yellow-green laser at 543 nm

Efficient and compact yellow green pulsed laser output at 543 nm is generated by frequency doubling of a passively Q-switched end diode-pumped Nd:YVO₄ laser at 1086 nm under the condition of sup-pressing the higher gain transition near 1064 nm. With 15 W of diode pump power and the frequency doubling crystal LBO, as high as 1.58 W output power at 543 nm is achieved. The optical to optical conversion efficiency from the corresponding Q-switched fundamental output to the yellow green output is 49%. The peak power of the Q-switched yellow green pulse laser is up to 30 kW with 5 ns pulse duration. The output power stability over 8 hours is better than 2.56% at the maximum output power. To the best of our knowledge, this is the highest watt-level laser at 543 nm generated by frequency doubling of a passively Q-switched end diode pumped Nd:YVO₄ laser at 1086 nm.     

Continuous-wave dual-wavelength operation of a diode-end-pumped Nd:LuVO<Subscript>4</Subscript> laser

A dual-wavelength continuous-wave (CW) diode-pumped Nd:LuVO₄ laser that generates simultaneous laser action at the wavelengths 1066 and 1343 nm is demonstrated. A total dual-wavelength output power of 2.58 W was achieved at the incident pump power of 18.2 W. Furthermore, intracavity sum-frequency mixing at 1066 and 1343 nm was then realized in a LBO crystal to reach the yellow range. We obtained a total CW yellow output power of 830 mW at 594 nm.     

End-to-end diffusion coefficients and distance distributions from fluorescence energy transfer measurements: Enhanced resolution by using multiple donors with different lifetimes

We describe a method to improve the resolution of donor-to-acceptor distance distributions in molecules which are flexing on the timescale of the fluorescence lifetime. We measured the timedependent donor decays of two donor (D)-acceptor (A) pairs, where the donor lifetimes were substantially different. The donors were an indole residue (5.7 ns) and a naphthalene residue (24.4 ns). The same dansyl acceptor was used for both D-A pairs. The donor decays are complex due to both a distribution of D-A distances and D-A diffusion. Using the donor decay data for each D-A pair alone, it is difficult to resolve both the distance distribution and the D-to-A diffusion coefficient. However, these values are unambiguously recovered from global analysis of the data from both D-A pairs. The increased resolution from the global analysis is apparently the result of the complementary information content of the data for each D-A pair. The shorter-lived indole donor provides more information on the time-zero distance distribution because there is less time for D-A diffusion, and the longer-lived naphthyl donor is quenched to a greater extent than indole due to the longer time for diffusion-enhanced energy transfer. Simulations were also used to demonstrate the increased resolution of global analysis with different lifetime donors to obtain distance distribution parameters in the presence of D-A diffusion.     

A comparative study of the energy migration and energy transfer in highly doped Nd:YAG single crystal

The donor-donor (D-D) energy migration interaction parameter C_DD in high-concentration Nd³⁺-doped YAG laser crystal is estimated, for the first time, by using the Yokota-Tanimoto (Y-T) model and the spectral overlap model (SOM) of Kushida. Firstly, the experimental luminescence decay curves of ⁴F_3/2 state of Nd³⁺ ions in YAG laser crystal at room temperature for 2.0 and 3.0 at% Nd³⁺ concentrations reported by Mao are fitted successfully by using the Y-T model and the parameter C_DD is obtained to be 1.50×10⁻³⁹ cm⁶/s. Secondly, the parameter C_DD is also directly calculated by using the SOM of Kushida: C_DD is calculated to be 2.73×10⁻³⁹ cm⁶/s. By comparing the energy migration interaction parameter C_DD and the donor-acceptor (D-A) energy transfer interaction parameter C_DA (1.794×10⁻⁴⁰ cm⁶/s), it is concluded that energy migration rate between Nd³⁺ ions in YAG laser crystal was about 11 times larger than energy transfer rate, and that energy migration would play a very important role in high-concentration Nd³⁺ -doped YAG laser crystal.     

The F+ center in reactor-irradiated aluminum oxide

A point-ion calculation has been performed for the F⁺ center in α-Al₂O₃ (one electron in an O^2? vacancy). Optical transitions are predicted at 2·26, 3·39 and 5·15 eV. Contact hyperfine interactions with the two nearest pairs of Al³⁺ ions are calculated to be 151 and 39 G. Single crystals of α-Al₂O₃ were reactor-irradiated up to doses of 10²⁰ fast neutrons per cm², and studied by electron spin resonance (ESR). A broad ESR spectrum with 13 resolved components at g=2·0029±0·0005 was interpreted as the interaction of an unpaired electron with two pairs of Al³⁺ nuclei with hyperfine constants of 49·2 and 13·5 G. These values are in the same ratio as the values calculated for the F⁺ center, to which this ESR spectrum is attributed. The discrepancy of a factor of three is typical of point-ion calculations. The optical absorption spectrum for heavily-irradiated samples is not available for comparison with calculated transition energies.