Dead-zone-free atomic magnetometry with simultaneous excitation of orientation and alignment resonances

Atomic magnetometers have very high absolute precision and sensitivity to magnetic fields but suffer from a fundamental problem: the vectorial or tensorial interaction of light with atoms leads to "dead zones," certain orientations of the magnetic field where the magnetometer loses its sensitivity. We demonstrate a simple polarization modulation scheme that simultaneously creates coherent population trapping (CPT) in orientation and alignment, thereby eliminating dead zones. Using 87Rb in a 10?Torr buffer gas cell we measure narrow, high-contrast CPT transparency peaks for all orientations and also show the absence of systematic effects associated with nonlinear Zeeman splitting.     

Molecular alignment during gel formation from methyl methacrylate: An excimer fluorescence study

Glass transition during bulk polymerization was studied in free-radical cross-linking copolymerization (FCC) of methyl methacrylate (MMA) using the steady-state fluorescence (SSF) technique. Naphthalene (N) was used as a monomer and excimer forming probe. Changes in the viscosity of the pregel solutions due to gel formation dramatically enhance both monomer and excimer fluorescent yield of N molecules. The reaction time at which the monomer and excimer intensities exhibit a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effect. This effect was used to study the gelation of MMA, as a function of time, in various N concentrations. The results were interpreted in the view of percolation theory. The gel fraction, β, and weight average degree of polymerization, γ, exponents β?=?0.40?±?0.02 and γ?=?1.70?±?0.07 were found in agreement with percolation results for both monomer and excimer measurements, respectively.     

Collisional alignment and orientation of atomic outer shells. III. Spin-resolved excitation

The review generalizes the formalism of the first part of this series [N. Andersen, J.W. Gallagher and I.V. Hertel, Phys. Rep. 165 (1988) 1–188] on electron-atom collisions to cases where the spin polarization of the electron is investigated at least once, i.e., before and/or after the collision. In addition, the target atom may be spin polarized. The preparation of initially polarized beams and/or the analysis of photon and spin polarization in the final state significantly increase the number of cases where a “perfect scattering experiment” can be performed. The connection between the scattering amplitudes and the generalized Stokes and STU parameters is analyzed. Favorable scattering geometries for perfect experiments and possibilities for consistency checks are pointed out. Recommendations are made about directions of future work.     

Theoretical study of the effect of orientation and solvent on energy transfer in bichromophore systems

Results of a quantum-chemical study of the effect of relative orientation of chromophores and solvent on energy transfer in bichromophore systems are presented. The results of theoretical calculations and the experimental data were found to be inconsistent with the prediction of the Förster theory relative to the effect of the orientation factor on the efficiency of energy transfer. It is shown that the effect of solvent on energy transfer is similar to the effect of a connecting bridge in bichromophore molecules. An interpretation of theoretical and experimental results is presented.     

Selective alignment of carbon nanotubes on sapphire surfaces: bond formation between nanotubes and substrates

We present our first-principles total-energy calculations performed for carbon nanotubes (CNTs) on sapphire substrates. We find that the formation of covalent and partly ionic bonds between Al and C atoms on the Al-rich surfaces causes the selective alignment of CNTs, this being the principal reason for the CNT growth along particular crystallographic directions. We also find that the van der Waals interaction which is important on the stoichiometric surfaces produces no directional preference. The characteristic features in the electron states of the CNT on the substrate are clarified.     

N2HF体系的结构与势能函数

用QCISD/6-311++G方法对N2HF体系进行优化得到其基态的平衡几何结构,属于C∞v构型,(1∑)态.计算表明N2HF分子是一特殊的van der Waals分子.应用多体项展式方法,导出了N2HF分子的解析势能函数,该函数正确的复现了N2HF体系的平衡结构及能量变化.势能面的静态特征表明:N2+HF→N2-HF反应是一个有阈能的反应,即是需活化能的反应,反应过程需克服144.21 kJ/mol的势垒.然后,就N2HF→N2+HF反应机理进行了讨论.     

Luminescence and energy transfer in Eu2+,Mn2+ codoped Na2BaMgP2O8 phosphor

Eu²⁺ single-doped and Eu²⁺/Mn²⁺-codoped Na₂BaMgP₂O₈ phosphors were prepared by a combustion-assisted synthesis method. The phase formation was confirmed by X-ray powder diffraction measurement. Na₂BaMgP₂O₈:Eu²⁺,Mn²⁺ shows a broad blue emission band and a red emission band, which originate from Eu²⁺ occupying the Ba²⁺ sites and Mn²⁺ occupying the Mg²⁺ sites, respectively. The efficient energy transfer from Eu²⁺ to Mn²⁺ is verified by the excitation and emission spectra together with the luminescence decay curves. Based on the principle of energy transfer, the relative intensities of blue and red emissions could be tuned by adjusting the contents of Eu²⁺ and Mn²⁺.     

高激发振动态RbH（Х1Σ+,ν″=15～22）与H2,N2的振动-振动碰撞能量转移

脉冲激光激发Rb原子至6 D态,Rb(6 D)与H2反应生成RbH(Х1Σ+,ν″=0～2)振动态。研究了RbH(Х1Σ+)高位振动态与H2,N2间的碰撞弛豫过程,利用泛频泵浦分别激发Х1Σ+(ν″=0)至Х1Σ+(ν″=15～22)各振动态,检测激光激发Х1Σ+(ν″)至A1Σ+(ν′),测量A1Σ+(ν′)的时间分辨激光感应荧光光谱,利用Stern-Volmer方程,得到振动能级ν″的总的弛豫速率系数kν(H2)。在H2和N2的混合气体中,总弛豫速率系数kν(H2+N2)与α(H2的摩尔配比)成直线的关系,其斜率为kν(H2)-kν(N2),而截距为kν(N2)。对于ν″<18主要发生单量子弛豫(Δν=1)过程,kν(H2)和kν(N2)与振动量子数ν″均成线性增加关系。对于ν″≥18,多量子弛豫(Δν≥2)过程及共振振动-振动转移起重要作用。对于RbH(ν″=21)+N2(0),测量ν″=16的布居数时间演化轮廓,在20μs内有一个锐锋,在100～200μs内有一个较低的宽峰,锐锋相应于RbH(ν″=21)+N2(0)→RbH(ν″=16)+N2(1)的共振转移过程,而宽峰是由相继的单量子过程产生的。     

CN bond orientation in metal carbonitride endofullerenes:A density functional theory study

The geometric and electronic structures of scandium carbonitride endofullerene Sc_3CN@C_(2n)(2 n = 68, 78, 80, 82,and 84) and Sc(Y)NC@C_(76) have been systematically investigated to identify the preferred position of internal C and N atoms by density functional theory(DFT) calculations combined with statistical mechanics treatments. The CN bond orientation can generally be inferred from the molecule stability and electronic configuration. It is found that Sc_3CN@C_(2n) molecules have the most stable structure with C atom locating at the center of Sc3 CN cluster. The CN bond has trivalent form of [CN]~(3-)and connects with adjacent three Sc atoms tightly. However, in Sc(Y)NC@C_(76) with [NC]~-, the N atom always resides in the center of the whole molecule. In addition, the stability of Sc_3CN@C_(2n)has been further compared in terms of the organization of the corresponding molecular energy level. The structural differences between Sc_3CN@C_(2n) and Sc_3NC@C_(2n)are highlighted by their respected infrared spectra.     

Numerical study of energy transfer between a beam and its internal reflection component in a nonlinear medium

A theoretical study is presented of a special case of stationary energy transfer in degenerate two-wave mixing in a reflection geometry. The two interacting beams consist of a beam and its first-order internal reflection component created at the boundary of a nonlinear medium. Numerical results obtained from the computer calculations are presented as graphs. It is found that, under suitable conditions, this phenomenon can be used to eliminate the multiple internal reflections.     

Pulse shaping control of alignment dynamics in N2

Control on the alignment transients of impulsively aligned ensembles of N₂ molecules has been demonstrated by the use of laser pulses shaped by a spatial light modulator. An alignment experiment has been inserted in the feedback loop of an evolutionary algorithm that found optimum pulse shapes for a set of criteria. Optimum pulse shapes for the maximization of total alignment and for the control of certain aspects of the revival structures are given. The physical mechanisms responsible for the control are analysed with the help of single‐parameter control schemes and numerical simulations, which allowed us to explore the low‐temperature region. This approach sheds light on the role played by different control mechanisms for the alignment dynamics of a molecular ensemble. Copyright © 2006 John Wiley & Sons, Ltd.     

Photoionization detection of stimulated Raman scattering on electron transitions of Na in an atomic beam

Stimulated Raman scattering has been experimentally studied by using the process of two-step ionization of a Na atomic beam. A dip that is due to a resonance at a combination frequency corresponding to the forbidden 3S–4S transition of a Na atom was observed in the signal of nonresonant photoionization when two laser frequencies λ₁=284.5 nm and λ₂=1.06 μm were overlapped.     

Molecular beam surface ionization detection: II. Field reversal and surface ionization study of Na on Re with adsorbed oxygen

The desorption and surface ionization of Na on a polycrystalline Re surface with various amounts of adsorbed oxygen have been studied by field reversal, surface ionization and thermoelectronic emission methods. In this work the unique properties of the field reversal method are taken advantage of, i.e. that both neutral and ionic desorption rate constants can be determined simultaneously. Absolute ionization coefficients have been measured by field reversal and have been compared with values found by the “oxygen coverage” method and by static surface ionization. The application to beam flux density determinations is discussed. The simultaneous variation of the neutral and ionic desorption rate constants during oxygen adsorption and the temperature dependence of them have been studied. The Re surface in 2 × 10^?8 Torr of oxygen and at 1300–1500 K is shown to be very stable and to behave differently than in studies at higher temperatures. The very rapid change in both desorption rate constants at an effective work function Φ^e = 5.35 V is here correlated with the results of LEED experiments (Gorodetskii and Knysh) and is proposed to indicate a change from a stable Re oxide surface at low Φ^e (and oxygen coverage) to a different surface structure at higher Φ^e. Desorption energies have been determined at various values of Φ^e. The neutral desorption energy at low oxygen pressure is 2.70 ± 0.06 eV, which agress well with earlier, here corrected modulated beam results. The energy (Schottky) cycle for surface ionization is shown to be closed at low Φ^e, which has been difficult to show with other methods in any other case.     

三硝基甲烷键离解能和生成焓的理论计算

采用密度泛函(DFT)四种交换/相关函数(B3LYP、B3P86、B3PW91和PBE0)结合不同的基函数,求得了三硝基甲烷C-NO2键的离解能(BDE),并且通过合理选择参考物硝基甲烷,设计等键等电子对反应,计算了气相三硝基甲烷分子的生成焓(HOF).与实验数据进行比较,PBE0/6-31g*计算出的BDE值最好,误差为-2.1 kcal mol-1;PBE0密度泛函结合带极化函数的6-31g基组得到的HOF值与实验值吻合的最好(误差在0.1 kcal mol-1以内).     

Raman regime energy dependence of alignment and orientation of KrII states populated by resonant Auger effect

The energy dependencies of alignment parameters A20 for KrII 4p(4)5p states after the Auger decay of the KrI 3d(9)np resonances were investigated theoretically and experimentally for the first time in the Raman regime with the bandwidth of the exciting radiation (deltaE(FWHM)=20 meV) smaller than the natural width of the resonances (Gamma approximately 80 meV). The observed energy dependence is due to the in-terference between the different resonance channels and the direct photoionization channel. A strong energy dependence for both the orientation parameter O10 and the photoelectron angular distribution parameter beta(el) is also predicted.     

Resonant optical orientation and alignment of excitons in superlattices

Exciton states in short-period GaAs/AlGaAs superlattices have been studied by optical orientation and optical dipole-moment alignment methods. The effect of magnetic field in the Faraday and Voigt geometries on the degree of linear and circular polarization of photoluminescence have been studied under resonant and nonresonant excitation. The constants of electron-hole exchange interaction in the exciton have been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 2229–2235 (December 1998)     

Optical orientation and alignment of excitons in quantum dots

Optical orientation and alignment of excitons in InAlAs quantum dots in the AlGaAs matrix have been studied both theoretically and experimentally. Experiments performed in a longitudinal magnetic field (Faraday geometry) reveal transformation of optical orientation to alignment and alignment to orientation, which is caused by exchange splitting of the dipole-active exciton doublet and allowed by the quantum-dot low symmetry. A comparison of theory with experiment made with inclusion of the anisotropy of exciton generation and recombination along the and [110] axes permits one to determine the character of dipole distribution in direction for resonant optical transitions in the self-organized quantum-dot ensemble studied. Fiz. Tverd. Tela (St. Petersburg) 40, 858–861 (May 1998)