Optical absorption of lithium metal vapor at high temperatures

Experimental results are reported for a unique spectroscopic device called the Plasma Spectroscopy Cell. Optical absorption of lithium metal vapor was observed at high density and temperature. Absorption spectra are analyzed using theoretical calculations of absorption cross sections for lithium-helium interactions, and singlet and triplet state transitions of diatomic lithium in the visible spectral range. This is believed to be the most complex example yet calculated in which absolute bound-bound, bound-free, free-bound, and free-free contributions for all possible optically allowed transitions are all included, in quite respectable agreement with experiment.     

Vibrational population densities of the A3Σu+, B3Πg, W3Δu, B'3Σu-, and C3Πu states in nitrogen plasmas

Population densities of vibrational levels of triplet excited states have been calculated using a collisional-radiative model applicable to nitrogen gas discharges with a Maxwellian electron energy distribution. Relative contributions of various electron impact and radiative processes, including intrasystem cascade, to the total cross sections of the triplet excited states are investigated.     

Magnetic and orbital dichroism in (e,2e) ionization of sodium

We present the first measurement of (e,2e) ionization cross sections for a laser oriented atomic target by spin polarized electrons. Cross sections are presented as a function of target orientation and polarization direction of the incident electron beam. This study provides insight into mechanisms by which angular momentum is transferred from the valence electron to the two final-state continuum electrons in both singlet and triplet spin channels, by comparing measurement with distorted wave Born approximation and the dynamically screened three Coulomb wave calculations.     

超冷温度下钾和铯原子间弹性散射特性的精确计算

本文分别用量子方法和半经典方法计算了超冷钾和铯原子之间弹性碰撞的s波散射长度,有效力程和p波散射长度等散射参数. 超冷温度下³⁹K-¹³³Cs原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现, 计算发现单重态和三重态截面分别存在显著的g波和d波形状共振.另外,本文应用简并内态近似方法获得了⁴¹K-¹³³Cs 超精细态相互作用时的s波散射长度.     

自旋极化有机电致发光器件中单线态与三线态激子的形成及调控

由于有机半导体（OSC）材料自旋弛豫时间长、自旋扩散长度大,OSC自旋器件逐渐成为研究热点.对于有机电致发光器件（OLED）,通过自旋极化电极调控单线态和三线态激子比率是提高其效率的有效方法.本文从漂移扩散方程和载流子浓度连续性方程出发,结合朗之万定律建立了一个自旋注入、输运、复合的理论模型.计算了OSC中的极化电子、空穴浓度,得出了单线态和三线态激子的比率.分析了电场强度、自旋相关界面电导、电极和OSC电导率匹配和电极极化率等因素的影响.计算结果表明：两电极注入反向极化的载流子并提高载流子自旋极化率,有     

Similarity measures between excited singlet and triplet electron densities in linear acenes: an application to singlet fission

A theoretical framework for modelling the electronic structure of absorbing molecules that may contribute to explain and even predict phenomena where a singlet exciton is converted to triplet is reported. The pathway is suggested to occur when the excited singlet and triplet electron density maps are similarly shaped at Frank Condon states. Calculations were carried for some linear acene excitons from their multielectronic wave functions and corresponding charge maps after a configuration interaction of singly excited determinants. For cases where the singlet fission effect has been reported, charge maps of certain higher energy singlet excitons show a relevant shape similarity with the corresponding lowest triplet. This method could enlighten the way for predicting transit between singlets and triplets of very big systems and also may help to explain experimental phenomena at femto and even attosecond scales.     

Charmonium production in deep inelastic scattering at HERA

The electroproduction of and mesons is studied in elastic, quasi-elastic and inclusive reactions for four momentum transfers and photon-proton centre of mass energies GeV. The data were taken with the H1 detector at the electron proton collider HERA in the years 1995 to 1997. The total virtual photon-proton cross section for elastic production is measured as a function of and W. The dependence of the production rates on the square of the momentum transfer from the proton (t) is extracted. Decay angular distributions are analysed and the ratio of the longitudinal and transverse cross sections is derived. The ratio of the cross sections for quasi-elastic and meson production is measured as a function of . The results are discussed in terms of theoretical models based upon perturbative QCD. Differential cross sections for inclusive and inelastic production of mesons are determined and predictions within two theoretical frameworks are compared with the data, the non-relativistic QCD factorization approach including colour octet and colour singlet contributions, and the model of Soft Colour Interactions. Received: 10 March 1999 / Published online: 8 September 1999     

Elastic scattering of sodium and cesium atoms at ultracold temperatures

The elastic scattering properties in a mixture of sodium and cesium atoms are investigated at cold and ultracold temperatures. Based on the accurate interatomic potential for the NaCs mixture, the interspecies s-wave scattering lengths, the effective ranges and the p-wave scattering lengths are calculated by the quantal method and the semiclassical method, respectively. The s-wave scattering lengths are 512.7a₀ for the singlet state and 33.4a₀ for the triplet state. In addition, the spin-change and elastic cross sections are also calculated, and the g-wave shape resonance is found in the total elastic cross sections.     

Enhanced cooling of hydrogen atoms by lithium atoms

We present calculated scattering lengths for collisions between various isotopic forms of lithium and hydrogen atoms interacting via singlet and triplet molecular states of LiH. We demonstrate that one bound triplet level is supported for each isotopomer 7LiH, 6LiH, 7LiD, and 6LiD. We obtain large calculated triplet scattering lengths that are stable against uncertainties in the potential. We present elastic and momentum transfer cross sections, and the corresponding rate coefficients, for hydrogen atoms colliding with 7Li atoms. We suggest that enhanced cooling of trapped atomic hydrogen by 7Li atoms is feasible.     

An investigation of the structure of free-base porphycene by time-resolved electron nuclear double resonance and density functional theory on the photoexcited triplet state

The photoexcited triplet state of free-base porphycene has been investigated by time-resolved electron nuclear double resonance spectroscopy at 10 K. In order to determine whether porphycene is cis or trans configured, experimentally determined A_zz hyperfine coupling tensor components are compared with those predicted from density functional theory. The structures of cis and trans porphycene, in both the singlet ground state and lowest excited triplet state were calculated using the Becke3-Lee-Yang-Parr composite exchange correlation functional and the 6–31G* basis set. Hyperfine couplings for these structures were then calculated using the same functional but with the extended EPR-II basis set. From the results it can be concluded that porphycene has a trans configuration in both the ground singlet and lowest excited triplet state. Additionally, the significant differences in structure between the singlet and triplet states are reflected in the calculated hyperfine couplings, with those for the trans conformation in the triplet state in reasonable agreement with the experimental values.     

Contributions to the theory of singlet exciton decay into pairs of triplet excitons in organic molecular crystals

Probability of singlet exciton decay into pairs of triplet excitons in organic molecular crystals is calculated. The dependence of the calculated probability on the separation of created pairs of triplet excitons, on temperature as well as on variations of equilibrium positions of molecules caused by changes of the state of electron excitations in the crystal is discussed. In conclusion there is given the estimation of the order of the probability value.Na Slovance 2, Praha 8, Czechoslovakia.     

Non-radiative transformation of a singlet exciton into a triplet one in organic molecular crystals

The probability of non-radiative transformation of a singlet exciton into a triplet one in organic molecular crystals is calculated. Mechanisms caused by intermolecular interactions apparent at grouping molecules into a crystal together with spin-orbital molecular interactions are considered as mechanisms of non-radiative transformation. Dependence of the calculated probability on the temperature and the character of an energetic spectrum of a singlet and a triplet exciton is discussed. On the basis of the calculated probabilities for various specific mechanisms the influence of intermolecular interactions, when arranging molecules into a crystal, on the intensity of non-radiative transitions between singlet and triplet excited states of molecules in the crystal is analyzed.Na Slovance 2, Praha 8, Czechoslovakia.     

超冷钠原子弹性散射特性的精确计算

基于精确的原子之间相互作用势,系统研究了钠原子在超冷温度下的弹性散射特性,精确计算了钠原子间碰撞时的s波散射长度、有效力程、p波散射长度以及束缚态数目等散射参数.超冷温度下单重态和三重态原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现,计算发现单重态和三重态散射截面分别存在显著的f波和i波形状共振.应用简并内态近似方法获得了超精细态相互作用时的s波散射长度,所得结果与精确值比较符合.     

Dissociative excitation of even triplet and quintet levels of the iron atom in collisions of electrons with iron diiodide molecules

The method of extended crossing beams is used to study inelastic collisions of slow electrons with iron dichloride molecules. The dissociative excitation of even triplet and quintet levels of the iron atom at the incident electron energy of 100 eV is examined. The resulting dissociative excitation cross sections are compared to the cross sections of the excitation of FeI in electron-atom collisions.     

Spin exchange during collisions of two sodium atoms

Based on data on the singlet (X ¹Σ⁺) and triplet (a ³Σ⁺) potentials describing the interaction of two Na atoms in the ground state, the complex cross sections of spin exchange for the system under investigation are calculated. The obtained cross sections allow one to determine relaxation times, as well as magnetic-resonance frequency shifts caused by spin-exchange Na-Na collisions.     

Proton impact charge exchange and excitation cross sections for liquid Sn divertor studies

The quantum dynamics of charge exchange (electron capture) and excitation processes in proton collisions with Sn atoms is studied for the first time by using the two-centre atomic orbital close coupling method. The expansion basis includes the lowest nine states of singlet and triplet symmetry of Sn and all the states with n ≤ 8 of the H atom. The spin-resolved state-selective charge exchange (up to n = 5) and excitation cross sections are calculated in the energy range of 0.5 − 100 keV . Strong coupling between the electron capture and excitation channels is observed in the low-energy (E ≤ 10 keV) region associated with the energy quasi-resonance of some projectile and target states. The intensities of some strong Sn spectral lines are also calculated for a number of typical tokamak edge plasma temperatures.     

Excitation of even triplet levels of YII in the e-Y collisions

The excitation of high-lying triplet levels of a singly charged yttrium ion by electron-atom collisions is studied experimentally. The parity of the levels studied coincides with the parity of the initial state YI. Forty one excitation cross sections with simultaneous single ionization are measured for an exciting electron energy of 50 eV. The optical excitation functions (OEF) are recorded for the majority of transitions in the electron energy range 0–200 eV. The total cross sections are determined for 20 energy levels. Possible channels of a simultaneous excitation-ionization process are discussed.     

Lowest Π–Π* electronic transitions in linear and two-dimensional polycyclic aromatic hydrocarbons: enhanced electron density edge effect

Polycyclic aromatic hydrocarbons (PAHs) form an important class of molecules as they are ubiquitous, pollute air and cause severe health problems. Lowest vertical π–π* singlet–singlet or triplet–triplet excitation energies and corresponding oscillator strengths were studied for several linear and two-dimensional PAHs employing time-dependent density functional theory. Excited-state electron density, molecular electrostatic potential (MEP) and spin density distributions in the PAHs, along with ground-state chemical hardness, were also studied. It has been found that, generally, excitation energies and oscillator strengths decrease with increase in PAH size, and excitation energies and chemical hardness are strongly linearly correlated. Enhanced electron density edge effect, which was found to occur in the ground states of the molecules, continues to hold in their excited states also. A strong similarity between the ground and π–π* excited-state MEP maps suggests that σ electrons are the main contributors to the enhanced electron density at the edges. Due to their strong electronic absorption transitions in the visible and infrared regions, the PAHs can be used for harnessing solar energy efficiently.     

Excited-state absorption in a terpyridyl platinum(II) pentynyl complex

The singlet excited-state lifetime of a terpyridyl platinum(II) pentynyl complex was determined to be 268+/-87 ps by fitting femtosecond transient absorption data, the triplet excited-state lifetime was found to be 62 ns by fitting nanosecond transient absorption decay data, and the triplet quantum yield was measured to be 0.16. A ground-state absorption cross section of 2.5 x 10(-19) cm(2) at 532 nm was deduced from UV-vis absorption data. Excited-state absorption cross sections of 3.5 x 10(-17) cm(2) (singlet) and 4.5 x 10(-17) cm(2) (triplet) were obtained by using a five-level dynamic model to fit open-aperture Z scans at picosecond and nanosecond pulse widths and a variety of pulse energies.     

Color and Luminescence Centers in LuAG and Pr:LuAG Crystals Before and After <Superscript>60</Superscript>Co γ-Irradiation

Entire luminescence spectrum of a commercial photosensitizer Radachlorin in aqueous solution has been recorded under laser excitation at 660 nm and analyzed. The peak of singlet oxygen phosphorescence at 1274 nm has been observed. The results obtained were compared with those recorded with laser excitation at 405 nm and reported earlier. The comparison showed the similarity of relaxation and luminescence processes occurring in both cases. Effective absorption cross sections were determined at each excitation wavelength, it was also shown that the singlet oxygen quantum yield is independent of photosensitizer concentration. The lifetime of the first excited triplet state in Radachlorin was determined. The results obtained can be used for optimization of the conditions of singlet oxygen generation and detection in solutions and biological samples.