Slow transient response of Ge:Ga far-infrared photoconductors for space applications

Two slow transient times in the response of Ge:Ga photoconductors to photon influx are found to fit well to τ_a=τ_o(N_d/p₁) and τ_m=τ_o(S₁/p₁), derived by analyzing a two-region model, where τ_o is hole lifetime, N_d is the donor concentration, and p₁ and S₁ are concentrations of hole and space charge, respectively in the region near the metal–p⁺ ohmic contact. Both τ_a and τ_m are consistent with the response times observed in Ge:Ga photoconductors with Ga concentrations of 2×10¹⁴ cm⁻³ and 1×10¹⁴ cm⁻³ as well as in a stressed Ge:Ga photoconductor under low background photon influx at low temperatures: Those characteristic times are mainly determined by the compensation and carrier density in the crystal. We discuss the relation between the characteristic times proposed in this study and those theoretically derived by Westervelt and Teitsworth [R.M. Westervelt, S.W. Teitsworth, J. Appl. Phys. 57 (1985) 5457–5469], by Fouks [B.I. Fouks, Proc. ESA Symp. Photon Detectors for Space Instrumentation (ESA SP-356), 1992, pp. 167–174], and by Haegel et al. [N.M. Haegel, C.A. Latasa, A.M. White, Appl. Phys. A 56 (1993) 15–21; N.M. Haegel, C.R. Brennan, A.M. White, J. Appl. Phys. 80 (1996) 1510–1514]. As the compensation of Ge:Ga photoconductors decreases, responsivity to step change in photon influx and the ratio of slow response in the total response increases. These results also indicate that the slow response characteristics depend mainly on crystal qualities such as compensation if the metal–p⁺ contact is good enough.     

Coherence time measurement of picosecond pulses by a light-induced grating method

The temporal coherence function |Γ(τ)| of picosecond pulses from a Nd: YAG laser has been measured by diffraction at a transient grating. The coherence timet _c =7 ps is small compared to the pulse widtht _p =22 ps but is in correspondence with the spectral bandwidth Δv=12·10¹⁰ Hz. The new method for measuring the coherence function |Γ(τ)| is discussed and compared with a previous experiment.     

Complexation of bis‐crown stilbene with alkali and alkaline‐earth metal cations. Ultrafast excited state dynamics of the stilbene‐viologen analogue charge transfer complex

The complex formation of bis(18‐crown‐6)stilbene ( 1 ) and its supramolecular donor‐acceptor complex with N,N′‐bis(ammonioethyl) 1,2‐di(4‐pyridyl)ethylene derivative ( 2 ) with alkali and alkaline‐earth metal perchlorates has been studied using absorption, steady‐state fluorescence, and femtosecond transient absorption spectroscopy. The formation of 1 ?Mⁿ⁺ and 1 ?(Mⁿ⁺)₂ complexes in acetonitrile was demonstrated. The weak long‐wavelength charge‐transfer absorption band of 1 · 2 completely vanishes upon complexation with metal cations because of disruption of the pseudocyclic structure. The spectroscopic and luminescence parameters, stability constants, and 2‐stage dissociation constants were calculated. The initial stage of a recoordination process was found in the excited complexes 1 ?M⁺ and 1 ?(M⁺)₂ (M = Li, Na). The pronounced fluorescence quenching of 1 · 2 is explained by very fast back electron transfer (τ_et = 0.397 ps). The structure of complex 1 · 2 was studied by X‐ray diffraction; stacked ( 1 · 2 )_m polymer in which the components were connected by hydrogen bonding and stacking was found in the crystal. These compounds can be considered as novel optical molecular sensors for alkali and alkaline‐earth metal cations.     

Time evolution of the Infrared Laser Induced Breakdown Spectroscopy of DNA bases Guanine and Adenine

Laser-Induced Breakdown Spectroscopy (LIBS) of DNA bases Guanine and Adenine was studied using a high-power CO₂ pulsed laser (λ=10.591 μm, τ _FWHM=64 ns and fluences ranging from 25 to 70 J/cm²). The strong emission of the adenine and guanine plasma, collected using a high-resolution spectrometer, at medium-vacuum conditions (4 Pa) and at 1 mm from the target, exhibits excited molecular bands of CN (B² Σ ⁺–X² Σ ⁺) and excited neutral H and ionized N⁺ and C⁺. The medium-weak emission is due to excited species C²⁺, C³⁺, N, O, O⁺, O²⁺ and molecular band systems of $\mathrm{C}_{2}(\mathrm{d}^{3}\varPi_{\mathrm{g}}\mbox{--}\mathrm{a}^{3}\varPi_{\mathrm{u}};\ \mathrm{D}^{1}\varSigma_{\mathrm{u}}^{+}\mbox{--}\mathrm{X}^{1}\varSigma_{\mathrm{g}}^{+})$ , OH(A² Σ ⁺–X² Π), NH(A³ Π–X³ Σ ^?), CH(A² Π–X² Π), $\mathrm{N}_{2}^{+}(\mathrm{B}^{2}\varSigma_{\mathrm{u}}^{+}\mbox{--} \mathrm{X}^{2}\varSigma_{\mathrm{g}}^{+})$ and N₂(C³ Π _u–B³ Π _g). We focus our attention on the temporal evolution of different atomic/ionic and molecular species. The velocity distributions for various (different) species were obtained from time-of-flight (TOF) measurements. Intensities of some lines from C⁺ were used for determining electron temperature and their Stark-broadened profiles were employed to estimate the temporal evolution of electron density.     

Limits on a lepton-number violating process from e+e− collisions

If a new hypothetical particle induces a lepton-number violating process, it could also affect the reactions e⁺e⁻→μ⁺μ⁻ and e⁺e⁻→τ⁺τ⁻ through the t-channel exchange of the particle. A fit of the cross sections and forward-backward asymmetries measured by experiments at PEP and PETRA yields the 90% confidence level limits on the coupling of the partile: G_eμ < 0.119G_F and G_eτ < 0.085G_F, where G_F is the Fermi coupling constant. These limits exclude certain regions of couplings allowed by other lepton-number violating searches.     

飞秒时间分辨离子成像研究NO2分子通道分辨超快解离动力学

本文利用飞秒激光泵浦-探测质谱和离子成像研究了NO_2分子的超快解离动力学.结果表明NO~+离子的动能释放包含两个部分,分别对应的能量是0.05和0.25 eV,并且指认了它们叫能的解离通道.NO~+离子通道分辨的瞬态测量提供了区分超快解离路径贡献的方法,不同动能释放的离子信号变化曲线可以通过双e指数函数进行拟合.其中衰减时间为0.25 ps的快速变化部分产生于里德堡态的演化.变化较慢的信号部分是山两个竞争的通道产生的,其中一个通道是吸收一个400 nm光子到A~2B_2激发态,它的衰减寿命是30 ps;另一个慢的通道是吸收三个400 nm光子到一个价电子类型的里德堡态,它的衰减寿命是短于7.2 ps.通道和时间分辨的实验测量对于区分分子复杂的超快解离动力学具有非常大的潜力.     

ΛN and ΣN Interactions from Lattice QCD

We present our recent study on ΛN and ΣN (isospin I = 3/2) interactions by measuring Nambu–Bethe–Salpeter wave functions on the Lattice QCD. The lattice QCD calculation is performed by using the N _f  = 2 + 1 gauge configurations generated by PACS-CS collaboration together with employing an improved method to obtain potentials in lattice QCD simulations. For the ¹ S ₀ channel, the central ΣN (I = 3/2) potential and the central ΛN (¹ S ₀) potential are found to be very similar. For the spin triplet (³ S ₁?³ D ₁) channels, the central ΛN(³ S ₁?³ D ₁) potential is attractive while the central ΣN(I = 3/2, ³ S ₁?³ D ₁) potentials is repulsive. Tensor potentials, on the other hand, are rather weak in both ΛN and ΣN(I = 3/2) systems.     

AB initio calculations of the electronic properties of N4− in KN3

The energies of the ground state and low-lying excited states of the N₄^? defect in KN₃ have been calculated using ab initio techniques. A rectangular equilibrium geometry with dimensions X = 2.76 and Z = 2.47 a.u. and ground state symmetry of Γ₄⁺ was determined by calculating N₄^? as a free radical. For this ground state the unpaired electron is in a π orbital which is consistent with the experimental hyperfine tensor only if one edge of the N₄^? radical is parallel to the c axis in KN₃. These results were used to calculate the X²Γ₄⁺ state of N₄^? in the crystal field of KN₃, yielding an energy of ?217.899 Hartrees. The isotropic hyperfine constant was calculated to be a = 2.1 G and the components of the anisotropic hyperfine tensor as B_xx = ?3.4 G, B_yy = 7.0 G and B_zz = ?3.6 G, in good agreement with experimental and INDO results. Several excited states were calculated for the N₄^? defect in KN₃. When an estimate was made of the correlation energy, the transition energy of the X²Γ₄⁺→A²Γ₃^? transition agreed well with the peak energy of the 780 nm absorption band which has been attributed to N₄^?.     

Lifetimes of high-spin levels in39Ar

The high-spin states in³⁹Ar at 2651, 3992, 4543, and 5536 keV were excited in oxygen-induced fusion-evaporation reactions. Mean lifetimes ofτ _m =1.0±0.2, 1.2±0.3, 1.6±0.3, and <1 ps were determined for these levels, respectively, by means of the recoil distance Doppler shift technique. The transition rates are discussed in terms of the Bansal-French weak-coupling model.     

Properties of excited levels in102Rh by the102Ru(p,nγ) reaction

The level scheme of¹⁰²Rh has been investigated by the¹⁰²Ru(p, nγ) reaction in the proton energy range 3.1 MeV≦E _p≦6 MeV. Excitation functions,γ-γ coincidences and pulsed beam measurements have been performed and a level scheme including 33 excited states up toE _x≈650 keV has been constructed. Theγ-branchings of several levels have also been measured. The lifetimes of 5 excited states have been determined with the centroid-shift method:τ _m(105.2 keV)=(1.3±0.3) ns,τ _m(123.7 keV)=(0.6±0.3) ns,τ _m(178.6 keV)=(1.5±0.3) ns,τ _m(208.7 keV)=(0.5±0.3) ns,τ _m(359.6 keV)=(0.8±0.4) ns. Upper limits have been placed for the lifetimes of 20 other levels.     

Dependence of nuclear deorientation on nuclear spin for recoils in vacuum

Time-integral measurements have been made of the vacuum deorientation of the 2⁺ and 4⁺ states in ¹⁵⁰Sm and the 6⁺ and 8⁺ states in ¹⁵⁶Gd Coulomb-excited by 133 MeV ³⁵Cl ions. The ¹⁵⁰Sm results were deduced from γ-ray angular distributions measured in coincidence with backscattered ³⁵Cl ions. In the ¹⁵⁶Gd measurements, a fixed counter array and a sandwich target were used to measure directly the differences between the γ-ray angular distributions from nuclei recoiling in vacuum and the unperturbed γ-ray angular distributions from nuclei stopped in a thick ¹⁵⁶Gd target. The measured deorientation time constants in ps are τ₂(2⁺) = 27±4; τ₂(4⁺) = 14±4; τ₂(6⁺) = 24±3; τ₂(8⁺) = 23⁺¹⁰_?6; τ₄(2⁺) = 12±2; τ₄(4⁺) = 5±3; τ₄(6⁺) = 7.6±2.5. The 8⁺ data were analyzed assuming τ₂/τ₄ = $\text{10}\text{3}$ as predicted by the Abragam-Pound theory for $\text{I ≧ 4}$ essentially independently of the degree of quadrupole admixture. The other results are consistent with this except for the 2⁺ level which shows some quadrupole effect. The present results, which show strong deorientation of high spin levels, are in contrast to earlier work on neutron deficient Er isotopes. In light of our findings we suggest that the failure to observe deorientation in the high spin states in Er is primarily due to anomalously low g-factors associated with the backbending observed in these nuclei.     

Excitation and Emission Spectra of Cs2NaLnCl6 Crystals Using Synchrotron Radiation

ABSTRACT

The visible emission and vacuum ultraviolet excitation spectra of the series Cs₂NaLnCl₆ (Ln = Y, Nd, Sm, Eu, Tb, Er, Yb) and Cs₂NaYCl₆:Ln³⁺ (Ln = Sm, Er) have been recorded using synchrotron radiation at room temperature, and in some cases at 10 K. The excitation spectra comprise features associated with charge transfer, excitation from the valence to conduction band, and impurity bands. No d–f emissions were observed for these Ln³⁺ ions, so that the emission bands comprise intraconfigurational 4f ^N –4f ^N transitions and impurity bands, whose natures are discussed. Theoretical simulations of the f–d absorption spectra have been included. The comparison with data from the synchrotron at Desy enables a comprehensive account of the ground (or vibrationally excited ground for Ln²⁺) states of the Ln³⁺ 4f ^N , Ln³⁺ 4f ^N?15d, and Ln²⁺ 4f ^N+1 configurations relative to the valence and conduction bands of Cs₂NaLnCl₆, for which the band gaps are between 6.6 and 8.1 eV.     

Radiative lifetimes and electronic quenching rate constants for single-photon-excited rotational levels of no (A2Σ+, ν′ = 0)

A narrow-band, frequency-doubled, tunable dye laser has been used to excite fluorescence from the A²Σ⁺, ν′ = 0 state of NO. Collision-free lifetimes were measured for 21 different K′ levels giving a mean radiative lifetime τ = 217 ± 4 ns. Electronic quenching rate constants of NO (A²Σ⁺, ν′ = 0) were measured for O₂, N₂, H₂O, CO₂ and Ar. No dependence of the quenching-rate constant on the initially excited rotational level was observed.     

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: Effect on the Generation of P 700 ·+ A 1 ·− Radical Pairs as Studied by W-band Transient EPR

Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A_0A (M688N_PsaA) and A_0B (M668N_PsaB) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668N_PsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688N_PsaA mutant. In particular, the P ₇₀₀ ^·+ decay kinetics in the M688N_PsaA mutant is significantly slower than in the WT or the M668N_PsaB mutant. The analysis of the out-of-phase electron–electron dipolar electron spin echo envelope modulation shows that in the M668N_PsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P ₇₀₀ ^·+ A _1A ^·? radical pair measured in the X-ray crystal structure. In the M688N_PsaA mutant, two populations are found with estimated distances of 26.0 ± 0.3 and 25.0 ± 0.3 Å in a ratio of 0.7–0.3, which agree well with the 25.8 Å distance for the P ₇₀₀ ^·+ A _1A ^·? radical pair and the 24.6 Å distance for the P ₇₀₀ ^·+ A _1B ^·? radical pair measured in the X-ray crystal structure. The data confirm that under the experimental conditions employed in this work, which involve dark-adapted samples without the pre-reduction of the iron–sulfur clusters, electron transport in cyanobacterial PS I is asymmetrical at 100 K, with the majority of electron transfer taking place through the A-branch of cofactors.     

Electroweak tests with τ leptons at LEP

We report on two sensitive tests of lepton universality carried out by the 4 LEP experiments at the Z⁰ pole. From measurements of the τ polarization in e⁺ e^?→τ ⁺ τ ^?, the ratios of the vector and axial vector coupling constants of the electron and the tau lepton to the weak neutral current are obtained to beg _{v e} /g _{a e} =0.066±0.015 andg _{V τ} /g _{A τ} =0.070±0.009 respectively. From measurement of the τ lifetime and the τ leptonic branching ratios, the ratio of the coupling constants describing weak leptonic decays of the τ and the μ is measured to beG _τ /G _μ =0.996±0.008.     

Ultrafast dynamics and temperature effects on the quantum efficiency of the ring-opening reaction of a photochromic indolylfulgide

The ultrafast ring-opening reaction of the molecular switch 1,2-Dimethyl-3-indolylfulgide dissolved in acetonitrile is investigated by temperature dependent quantum efficiency measurements and time-resolved transient absorption spectroscopy in the ultraviolet and visible spectral range. The photoreaction is found to be thermally activated with an activation energy of about 1640 cm^− 1. The transient absorption signal is bi-exponential with the time constants τ₁ = 0.7 ps and τ₂ = 12 ps. The fast time constant is due to solvation dynamics, while the main component τ₂ is attributed to the excited state lifetime and product formation. A long-lived intermediate state in the photoreaction can be excluded.     

Preparation of Sm3+–Eu3+ coactivating red-emitting phosphors and improvement of their luminescent properties by charge compensation

By charge compensating, a series of red-emitting phosphors Ca_0.54Sr_0.16Ca_0.54Sr_0.31Eu_0.08Sm_0.02(MoO₄)_0.6(WO₄)_0.4 were synthesized. Two approaches to charge compensation were used: (a) 2Ca²⁺/Sr²⁺→Eu ³⁺/Sm³⁺+M ⁺, where M⁺ is a monovalent cation like Li⁺, Na⁺ or K⁺; (b) Ca²⁺/Sr²⁺→Eu ³⁺/Sm³⁺+N ^?, where N⁺ is a monovalent anion like F^?, Cl^?, Br^?, or I^?. One red LED was made by combining the phosphor and 390–405 nm emitting LED chip under 20 mA forward-bias current, the color purity, chromaticity coordinates and the luminous intensity of which were 99.5%, x=0.66, y=0.33, 5600 mcd, respectively.     

DasL/K- und dasM/L-Einfangverhältnis von65Zink

We have investigated theL/K- and theL/M-capture ratio of⁶⁵Zn. For the transitions to the first excited state of⁶⁵Cu we obtained (N _L/N _K) _A =0.120±3% and (N _M/N _L) _A =0.153±13% and for both transitions to the groundstate and the first excited state simultaneously we found (N _L/N _K) _G =0.118±3%. The values for (N _L/N _K) agree well with the results of Santos Ocampo and Conway but are higher than the results of Totzek and Hoffmann. All values are discussed in view of the different theoretical values obtainable.     

High-velocity DSA measurements in the 1–20 ps lifetime region: Comparison between DSA and RD

Mean lives in the range 1–20 ps of low-lying states of ¹³C, ¹⁶N, ²⁰O and ³⁶Cl have been measured with the Doppler-shift attenuation method by heavy-ion bombardment of ²H and ³H targets. The recoils are slowed down in Mg, Al, Cu, Ag and Au. The γ-ray patterns are observed with a large Ge(Li) detector at 0° in coincidence with protons; for ¹³C the patterns are measured in singles with a Compton-suppression spectrometer. Analysis of the γ-ray patterns with ⁴He-scaled stopping power data of Northcliffe and Schilling yields the following results: ¹³C, τ_m(3.85 MeV) = 12.6 ± 0.3ps; ¹⁶N, τ_m(0.40 MeV) = 5.1 ± 0.3 ps; ²⁰O, τ_m(1.67 MeV) = 9.8 ± 0.7 ps; ³⁶Cl, τ_m(0.79 MeV) = 19.9 ± 1.7 ps, τ_m(1.16 MeV) = 9.2 ± 0.6 ps and τ_m(1.60 MeV) = 0.94 ± 0.06 ps. A comparison with results obtained with the recoil-distance method shows agreement to about 10 %, with a slight tendency to somewhat longer lifetimes for the recoil-distance technique. The above stopping power is also used to reanalyze our previously published measurements. The new mean lives differ less than 4.5 % from the previous results.     

Electron Paramagnetic Resonance and Electron Spin Echo Studies of Co2+ Coordination by Nicotinamide Adenine Dinucleotide (NAD+) in Water Solution

Co²⁺ binding to the nicotinamide adenine dinucleotide (NAD⁺) molecule in water solution was studied by electron paramagnetic resonance (EPR) and electron spin echo at low temperatures. Cobalt is coordinated by NAD⁺ when the metal is in excess only, but even in such conditions, the Co/NAD⁺ complexes coexist with Co(H₂O)₆ complexes. EPR spin-Hamiltonian parameters of the Co/NAD⁺ complex at 6 K are g _z  = 2.01, g _x  = 2.38, g _y  = 3.06, A _z  = 94 × 10^?4 cm^?1, A _x  = 33 × 10^?4 cm^?1 and A _y  = 71 × 10^?4 cm^?1. They indicate the low-spin Co²⁺ configuration with S = 1/2. Electron spin echo envelope modulation spectroscopy with Fourier transform of the modulated spin echo decay shows a strong coordination by nitrogen atoms and excludes the coordination by phosphate and/or amide groups. Thus, Co²⁺ ion is coordinated in pseudo-tetrahedral geometry by four nitrogen atoms of adenine rings of two NAD⁺ molecules.