ZnO:V3+晶体光谱和电子顺磁共振谱的理论研究

采用强场耦合图像,建立了3d2电子组态在三角对称中的45×45能量矩阵,通过对角化完全能量矩阵的方法,计算出了ZnO:V3 晶体的45条光谱能级和5个电子顺磁共振谱参量(零场分裂D,g因子g//,g⊥和超精细结构常数A//,A⊥),计算结果与实验数值能很好地符合.计算中还发现V3 杂质中心的局部结构与基质晶体的结构不一致,即V3 在ZnO中并不占据准确的Zn2 位置,而是沿c3轴方向位移一段距离△Z≈0.003 nm.作者对上述这些结果的合理性进行了讨论.     

Improved measurement of (3)J(H(alpha)(i),N(i+1)) coupling constants in H(2)O dissolved proteins.

A modification to the recently proposed alpha/beta-HN(CO)CA-J TROSY pulse sequence (P. Permi et al., J. Magn. Reson. 146, 255-259 (2000)) makes it possible to determine (3)J(H(alpha)(i), N(i+1)) coupling constants from a single E.COSY-type cross-peak pattern rather than from two (1)H(alpha) spin-state-edited subspectra. Advantages are increased (15)N resolution, critical to extracting accurate (1)H(alpha)-(15)N coupling constants, and minimized differential relaxation due to nested (13)C(alpha) and (15)N evolution periods. Application of the improved pulse sequence to Desulfovibrio vulgaris flavodoxin results in (3)J(H(alpha)(i), N(i+1)) values being systematically larger than those obtained with the original scheme. Parametrization of the coupling dependence on the protein backbone torsion angle psi yields the Karplus relation (3)J(H(alpha)(i), N(i+1))=-1.00 cos(2)(psi-120 degrees )+0.65 cos(psi-120 degrees )-0.15 Hz, with a residual root-mean-square difference of 0.13 Hz between measured and back-calculated coupling constants. The curve compares with data derived from ubiquitin (A. C. Wang and A. Bax, J. Am. Chem. Soc. 117, 1810-1813 (1995)), although spanning a slightly larger range of J values in flavodoxin. The orientation of the Ala39/Ser40 peptide link, forming a type-II beta-turn in flavodoxin, is twisted against X-ray-derived torsions by approximately 10 degrees in the NMR structure as evident from the analysis of straight phi- and psi-related (3)J coupling constants. The remaining deviation of some experimental values from the prediction is likely to be due to strong hydrogen bonding, substituent effects, or the additional dependence on the adjacent torsions straight phi.     

O~++H_2→OH~++H反应的动力学研究

采用含时量子波包方法结合二阶分裂算符传播子对初始态为(v=0, j=0)的O~++H_2→OH~++H反应体系在0.01—1.00 eV的碰撞能范围内进行了态分辨理论水平上的动力学计算.对反应概率、积分截面、微分截面以及固定初始态的热速率常数等动力学信息进行了计算并与文献报道的实验和理论结果进行了比较.结果表明本文的理论结果与实验结果十分符合.从微分截面的散射信息可知,在低碰撞能范围内,插入反应机制在反应中占据主导地位.随着碰撞能的增加,反应机制逐渐由插入机制变为抽取反应机制.     

Investigations on the local structure and EPR parameters for the trigonal Nd3+ centre in CdS

The local structure and the electron paramagnetic resonance (EPR) parameters (g factors and hyperfine structure constants) for the trigonal Nd³⁺ centre in CdS are theoretically investigated by considering the local lattice relaxation. The impurity Nd³⁺ is found not to occupy exactly the host Cd²⁺ site, but to suffer a shift of about 0.31 Å away from the sulphide triangle along the C₃ axis due to size and charge mismatch. The theoretical EPR parameters based on the above impurity axial displacement are in good agreement with the observed values. The local structure and the EPR parameters are discussed for this centre.     

Structural and electronic properties of Fe 3+ -O 2− Pairs in KMF 3 lattices (M=Mg,Zn): Is the superposition model valid?

The geometrical and electronic properties of the tetragonal FeF 5 O 4 m complex which is likely formed in KMgF 3 and KZnF 3 fluoroperovskites have been investigated by means of Density Functional calculations. The properties of this centre are dependent on the Fe-O distance ( R ox ) and also on the distances R eq and R ax between Fe 3+ and the four equatorial F m ions and the apical F m ion respectively. The calculated equilibrium distance for the isolated Fe 3+ -O 2 m pair is equal to R ox =1.63 v + while when it is introduced in KMF 3 lattices ( M =Mg, Zn) R ox ～1.82 v + . Compared to FeF 6 3 m in KMgF 3 ( R eq = R ax =1.94 v + ) the obtained equilibrium distances R ax =2.05 v + and R eq =2.02 v + mean an outwards relaxation of 6% and 4% respectively induced by the F m M O 2 m substitution. The ratio of the associated superhyperfine ( shf ) constants of equatorial ( A eq ) and axial ( A ax ) F m ligands is A eq / A ax , 2 and thus account for an experimental fingerprint of this centre. The huge difference between A s (eq) and A s (ax) cannot be explained assuming that the law relating A s to the metal-ligand distance, R , for the octahedral FeF 6 3 m unit can be transferred to the present case involving a lower symmetry . The present analysis casts doubts on the conclusions previously reached using the empirical superposition model.     

Correlation between TL and OSL signals in KMgF3:Ce3+: Bleaching study of individual glow-peaks

KMgF₃:Ce³⁺ is an ultra sensitive Thermoluminescence (TL) material with a complex TL and OSL glow-curve structure. The aim of the present work is to attempt a one-to-one correspondence between specific TL glow-peaks and OSL components in KMgF₃:Ce³⁺. The correlation study involves the deconvolution of the TL curves and the estimation of the bleaching decay constants for individual glow-peaks followed by the deconvolution of the LM-OSL curve using the estimated decay constants. It was found that the bleaching of each individual glow-peak takes place in three different rates; namely in a fast, medium and slow rate.     

o-Semiquinonato and o-iminosemiquinonato rhodium complexes. EPR study of the reactions in coordination sphere of rhodium

Chemical exchange saturation transfer (CEST) processes in aqueous systems are quantified by evaluation of z-spectra, which are obtained by acquisition of the water proton signal after selective RF presaturation at different frequencies. When saturation experiments are performed in vivo, three effects are contributing: CEST, direct water saturation (spillover), and magnetization transfer (MT) mediated by protons bound to macromolecules and bulk water molecules. To analyze the combined saturation a new analytical model is introduced which is based on the weak-saturation-pulse (WSP) approximation. The model combines three single WSP approaches to a general model function. Simulations demonstrated the benefits and constraints of the model, in particular the capability of the model to reproduce the ideal proton transfer rate (PTR) and the conventional MT rate for moderate spillover effects (up to 50% direct saturation at CEST-resonant irradiation). The method offers access to PTR from z-spectra data without further knowledge of the system, but requires precise measurements with dense saturation frequency sampling of z-spectra. PTR is related to physical parameters such as concentration, transfer rates and thereby pH or temperature of tissue, using either exogenous contrast agents (PARACEST, DIACEST) or endogenous agents such as amide protons and -OH protons of small metabolites.     

Zeeman relaxation of CaF in low-temperature collisions with helium

The collision-induced Zeeman relaxation rate for collisions of CaF X2Sigma(v(')=0) with 3He is measured to be Gamma(Z)=(7.7+5.4/-2.5)x10(-15) cm(3)/s at 2 K. This rate is a direct measurement of the influence of spin-rotation coupling on Zeeman relaxation in the first rotational level of CaF. The relationship of this rate to known molecular constants is consistent with recent theory of cold molecular collisions and outlines the (2)Sigma molecules conducive to magnetic trapping.     

(1+2+1)双共振多光子电离概率的理论研究

共振增强多光子电离光谱技术已成为研究原子、分子高激发态能级结构的重要方法。运用光和物质相互作用的速率方程理论,推导出四能级物质系统1+2+1双共振增强多光子电离概率的解析表达式,以此为基础,理论模拟了电离概率随激发光强、激光脉冲宽度和碰撞弛豫速率的变化,发现在1+2+1多光子电离机制中,电离概率随光强的增加而增大,继而出现单步、双步激发饱和的现象,直至饱和值1;继续增大光强,电离概率将围绕饱和值1窄幅振荡,振荡幅度随光强增加而增大。随激光脉冲宽度的增大,电离概率从零开始逐渐增大直至饱和值1。而随碰撞弛豫速率的增大电离概率以线性规律减小。     

Multiple time scales in diffraction measurements of diffusive surface relaxation

We grew SrTiO3 on SrTiO3(001) by pulsed laser deposition, using x-ray scattering to monitor the growth in real time. The time-resolved small-angle scattering exhibits a well-defined length scale associated with the spacing between unit-cell high surface features. This length scale imposes a discrete spectrum of Fourier components and rate constants upon the diffusion equation solution, evident in multiple exponential relaxation of the "anti-Bragg" diffracted intensity. An Arrhenius analysis of measured rate constants confirms that they originate from a single activation energy.     

锰原子的二步多光子与三步三光子共振电离研究

激光共振电离技术是痕量分析中的重要手段之一。文章以速率方程理论为基础,对锰原子的激光共振电离过程进行了分析,讨论了电离过程中各级激发光功率密度及激光作用时间对电离效率的影响;提出了根据所要求的电离效率和激光作用时间计算所需要的各激发光或电离激光的功率密度的方法;得到了饱和激发或饱和电离的规律及阈值条件。研究发现,在激光作用时间为10 ns时,锰原子饱和电离的激光强度阈值基本都在108 W·cm-2的量级,只有“1+1”双色双共振低三个量级;而“1+1”和“1+1+1”饱和激发的激光强度阈值则在102～103 W·cm-2量级;并且随着激光作用时间的增加,各过程的饱和激发和饱和电离的激光强度阈值将单调减少。     

HBr(X~1Σ~+ν~″=5)分子与H_2,N_2,CO_2和HBr间的近共振振动-振动能量转移

利用简并受激超拉曼泵浦激发HBr(Χ~1Σ~+ν~″=5)振动态,由高分辨瞬时激光感应荧光(LIF)探测碰撞弛豫后HBr(ν~″≤5)各振动态时间分辨布居数的演化过程,得到了HBr(ν~″=5)分别与分子M(H_2,N_2,CO_2和HBr)的碰撞弛豫速率系数。对于M=CO_2,近共振的1-1振动-振动(V-V)能量转移是有效的,这一结果表明CO_2强的红外振动模对近共振V-V能量转移是有利的。而红外禁戒跃迁的N_2(0-1)的近共振V-V转移虽然也能观察到,但相应速率系数比CO_2小2个量级。碰撞分子的振动跃迁红外活性越强,能量转移速率系数越大。在HBr(ν~″=5)+HBr的自弛豫过程中,单量子弛豫率占总弛豫率的70%,而双量子弛豫约占25%。在HBr(ν~″=5)+H_2中,只有2-1的V-V近共振过程是重要的。同时还研究了V-V近共振能量转移速率系数与温度变化的关系,对于CO_2的1-1近共振,V-V能量转移速率系数随温度的增加而减小;对于H_2和HBr,其弛豫速率系数随温度的增加而增加;对于N_2,其弛豫速率系数随温度的增加而缓慢增加。     

Excitation and relaxation of metastable state NaK(1 3pi) at high vibrational levels

The authors have investigated collision vibrational energy transfer rate constants in NaK[1 3pi(v)] and He system. Pump laser excitation of the spin-forbidden band was used to produce very highly vibrationally excited metastable state NaK[1 3pi (v = 22, 21, 20)]. The probe laser was used to excite the 1 3pi (v = 22, 21, 20) to 5 3pi(v'). Laser induced fluorescence (LIF) from 5 3pi --> 1 3sigma+ transition was used to follow the collision dynamics. The semilog plots of time-resolved LIF was obtained. The slopes yielded the effective lifetimes. From such data several Stern-Volmer plots could be constructed and the relaxation rate constants could be extracted for the sum of all processes that give rise to the decay of the prepared vibrational state. The rate constants (in units of 10(-11) cm3 x s(-1)) for v being 22, 21 and 20 are 1.4 +/- 0.1, 1.2 +/- 0.1 and 1.0 +/- 0.1, respectively. The vibrational relaxation rate is increasing with vibrational quantum number. In order to determine the importance of multiquantum relaxation, it is necessary to measure the relative population of both the prepared state and collisionally populated states. By the kinetic equations governing up to delta(v) = 2 transitions, the time dependence of populations of the vibrational states were obtained. With the help of the integrating the population equations over all time, the importance of the two-quantum relaxation could be studied experimentally. By varying the delay between the pump and the probe laser, the He pressure dependent vibrational state specific decay could be measured. The time evolutions and relative intensities of the three states v = 22, 21 and 20 by preparing v = 22 were obtained. Using experimental data the rate constants (in units of 10(-11) cm3 x s(-1)) for v = 22 --> 21 and v = 22 --> 20 are 0.67 +/- 0.15 and 0.49 +/- 0.12, respectively. The single quantum relaxation accounts for only about 48% of the total relaxation out of v = 22. Multi-quantum relaxation (delta(v) > 1) was found to be important at high vibrational states.     

Dielectric torque and orientation dynamics of liquid crystals with dielectric dispersion

We demonstrate that the finite rate of dielectric relaxation in liquid crystals which has been ignored previously causes profound effects in the fast dielectric reorientation of the director. We propose a theory of dielectric response in which the electric displacement depends not only on the present (as in the standard theory) but also on the past values of electric field and director. We design an experiment with a dual-frequency nematic in which the standard "instantaneous" model and our model predict effects of opposite signs; the experimental data support the latter model.     

Er3+/Yb3+共掺NaYF4/LiYF4微米晶体的上转换荧光特性

采用水热法成功制备了Er³⁺/Yb³⁺共掺杂的NaYF₄和LiYF₄微米晶体. 通过X射线衍射仪和环境扫描电子显微镜对样品的晶体结构及形貌进行表征. 实验结果表明: 六方相NaYF₄微米晶体为棒状结构, 而四方相LiYF₄微米晶体则为八面体结构. 在近红外光980 nm激发下, NaYF₄:Yb³⁺/Er³⁺和LiYF₄:Yb³⁺/Er³⁺ 微米晶体均展现出很强上转换荧光发射. 且NaYF₄:Yb³⁺/Er³⁺微米晶体的荧光发射强度大约是LiYF₄:Yb³⁺/Er³⁺微米晶体的2倍, 但红绿比明显较低. 根据荧光光谱, 并借助激光光谱学及发光动力学深入探讨基质变化及表面修饰剂乙二胺四乙二酸(EDTA)对荧光特性的影响. 实验结果发现: 影响荧光强度的主要因素是基质环境的局域对称性, 而导致不同红绿比则是由于样品表面较多的EDTA分子所引起. Er³⁺掺杂的NaYF₄和LiYF₄ 微米晶体呈现出很强的绿光发射可被应用于全色显示, 荧光粉和微光电子器件中.     

共掺杂Ce~(3+)调控β-NaLuF_4:Yb~(3+)/Ho~(3+)纳米晶体的上转换荧光发射

在980 nm近红外光激发下,通过共掺杂Ce~(3+)离子调控六方相NaLuF_4:Yb~(3+)/Ho~(3+)纳米晶体的上转换荧光发射.实验结果表明,当掺杂Ce~(3+)离子浓度从0增加到12.0%时,Ho~(3+)离子的上转换荧光发射实现了由绿光向红光的转变,其红绿比提高了近24倍.根据Ho~(3+)离子的能级结构发现,Ho~(3+)离子的红光发射源自~5F_5能级到5I8能级的辐射跃迁,因此要增强红光发射,必须提高该能级粒子数布居.Ho~(3+)与Ce~(3+)离子之间相近的能级差促使它们之间产生了共振交叉弛豫,从而有效地提高了Ho~(3+)离子~5F_5能级的粒子数布居,增强了红光发射.同时对Ho~(3+)离子的上转换调控机理进行讨论,并借助不同的激发策略,进一步证实了Ho~(3+)与Ce~(3+)离子之间相互作用的发生.     

Exchange-mediated spin-lattice relaxation of Fe3+ ions in borate glasses

Spin-lattice relaxation times (T1) of two borate glasses doped with different concentrations of Fe2O3 were measured using the Electron Spin-Echo (ESE) technique at X-band (9.630 GHz) in the temperature range 2-6K. In comparison with a previous investigation of Fe3+-doped silicate glasses, the relaxation rates were comparable and differed by no more than a factor of two. The data presented here extend those previously reported for borate glasses in the 10-250K range but measured using the amplitude-modulation technique. The T1 values were found to depend on temperature (T) as T(n) with n approximately 1 for the 1% and 0.1% Fe2O3-doped glass samples. These results are consistent with spin-lattice relaxation as effected by exchange interaction of a Fe3+ spin exchange-coupled to another Fe3+ spin in an amorphous material.     

Redox reaction kinetics of UO2+x

Oxidation and reduction kinetics of UO_2+x was investigated in CO₂/CO gas mixtures at elevated temperatures of 1000°C to 1300°C by monitoring the electrical conductivity relaxation induced by an abrupt change of ambient oxygen partial pressure. By fitting the experimental relaxation data to theoretical equations, the oxidation and reduction kinetics was found to be controlled mostly by the surface reaction step in the ranges of oxygen partial pressure and temperature examined and the surface reaction rate constants were derived therefrom.     

Relaxation of the remanent resistance of granular HTSC Y-Ba-Cu-O + CuO composites after magnetic field treatment

The hysteresis of magnetoresistance R(H) and relaxation of the remanent resistance R _rem with time after magnetic field treatment of HTSC (Y-Ba-Cu-O) + CuO composites are studied. Such a composite constitutes a network of Josephson junctions wherein the nonsuperconducting component (CuO) forms Josephson barriers between HTSC grains. By comparing the experimental R _rem(t) and R(H) dependences, it is shown that the relaxation of the remanent resistance is caused by the decreased magnetic field in the intergrain medium due to relaxation of magnetization. The reason is uncovered for the differences between the published values of pinning potentials determined by measuring the relaxation of magnetization or resistance and fitting them by the Anderson law.     

Estimation of free copper ion concentrations in blood serum using T(1) relaxation rates

The water proton relaxation rate constant R(1)=1/T(1) (at 60 MHz) of blood serum is substantially increased by the presence of free Cu2+ ions at concentrations above normal physiological levels. Addition of chelating agents to serum containing paramagnetic Cu2+ nulls this effect. This was demonstrated by looking at the effect of adding a chelating agent-D-penicillamine (D-PEN) to CuSO4 and CuCl2 aqueous solutions as well as to rabbit blood serum. We propose that the measurement of water proton spin-lattice relaxation rate constants before and after chelation may be used as an alternative approach for monitoring the presence of free copper ions in blood serum. This method may be used in the diagnosis of some diseases (leukaemia, liver diseases and particularly Wilson's disease) because, in contrast to conventional methods like spectrophotometry which records the total number of both bound and free ions, the proton relaxation technique is sensitive solely to free paramagnetic ions dissolved in blood serum. The change in R(1) upon chelation was found to be less than 0.06 s(-1) for serum from healthy subjects but greater than 0.06 s(-1) for serum from untreated Wilson's patients.