Configuration interaction in the 6s n d 1 D 2 and 6s n s 1 S 0 states of Yb probed by lifetime measurements

Radiative lifetimes in the even parity 6s n d ¹ D ₂ (n=6?13) and 6s n s ¹ S ₀ (n=8?14) level series of neutral Yb have been measured. In contrast to a monotonicn*³ behaviour which is expected in the absence of configuration interaction a drastic decrease of the lifetime data was observed in the vicinity of multiply excited states. In addition, for certain levels a strong increase of the lifetime values has been found which may be ascribed to destructive interference due to configuration mixing which can also affect the branching ratios of the radiative decay of these levels.     

Lifetime and collisional depopulation of the metastable 5D 3/2-state of Yb+

The lifetime and collisional depopulation rates of the metastable 5D _3/2 state of Yb⁺ have been determined in a radiofrequency ion trap by observation of the fluorescence count rate after ion excitation by a short laser pulse. From measurements using He, N₂ and H₂ as buffer gases between 10^?8 and 10^?6 mbar pressure and linear extrapolation to zero pressure we obtain a lifetime of τ=52.15±1.00 ms and rate constants ofR(H₂)=(1.02±0.10)×10^?9 cm³/s andR(N₂)=(1.78±0.19)×10^?10 cm³/s. The lifetime is in fair agreement with a calculated value of 74 ms.     

Proton-impact excitation of HeI triplet levels

We investigated the excitation of the λ(1s3d ³ D?1s2p ³ P)=588 nm line of atomic helium by proton and deuteron impact for projectile energies 10 keV≦E _p≦25 keV. In apparent contradiction to Wigner's spin conservation rule, the emission cross section does not vanish. By measuring the intensity of the impact radiation as a function of homogeneous magnetic and electric fields applied to the collision volume, it has been shown thatp- andd-impact excitation of the 1s3d ³ D level of HeI proceeds via 1snl states withl≧3, which populate the 3³ D states by cascade decays. The well-known strong singlet-triplet mixing of these 1snl states enables a population of triplet states in accord with Wigner's rule. Accordingly, we determine the excitation cross section of the 1s4f multiplet from the measured emission cross section of the 588 nm line. The field-dependent signals give evidence that predominantly substates with |m _L|≦1 are excited.     

Laserspectroscopic investigations of the lithium-D-lines in magnetic fields

Laser-atomic-beam investigations of the lithiumD ₁- andD ₂-line in magnetic fields were performed using cw-laser excitation and fluorescence detection. For both isotopes⁶Li and⁷Li, the hyperfine splittings of the ground level 2² S _1/2 and the upper level of theD ₁-line, 2² P _1/2, as well as the isotopic shifts of theD ₁- and theD ₂-line were determined from the registrations without field. In magnetic fields, Zeeman- and Paschen-Back-effects of the lines were studied. Using the Zeeman pattern of theD ₁-line for a calibration of the field strength, values for the hyperfine constantsA andB of the 2² P _3/2-level of⁷Li could be derived from the Zeeman pattern of theD ₂-line.     

Carrier-free separation of167Tm from Yb,Er and Ho targets by reversed-phase extraction chromatography and preparation for its medical use

Reversed-phase extraction chromatography employing a Daiflon-supported HDEHP column and HCl as eluent was examined for carrier-free separation of¹⁶⁷Tm from a photo-irradiated Yb target and also from particle-bombarded Er and Ho targets. The examination was performed with radioassays of radioactive Tm and Yb tracers and⁵⁷Co--induced X-ray analysis of Er and Ho. An example of the separation of about 1 mCi of¹⁶⁷Tm from a 100 mg irradiated Yb target enriched in¹⁶⁸Yb and a procedure for the final preparation for medical use are described. The YbSO₄ precipitation could be combined with this chromatographic separation of¹⁶⁷Tm from a more massive Yb target of natural abundance, weighing several tens of grams. A procedure for YbSO₄ precipitation is also presented in the Appendix.     

Investigations of radiative lifetimes in the 3p5p configuration of neutral silicon

Lifetimes for the³ S ₁,³ P _{0, 1, 2} and³ D _{1, 2, 3} states in the 3s ²3p5p configuration of silicon have been determined using stepwise dye laser excitation and time resolved detection. A comparison is made with theoretical values, calculated using multi-configuration Hartree-Fock wavefunctions. Laser-evaporation was used to produce free silicon atoms by focusing a Nd: YAG laser on a rotating silicon target.     

Collision cross sections for excitation energy transfer in Na* (3P 1/2)+K(4S 1/2)⇔Na*(3P 3/2)+K(4S 1/2) processes

The cross sections for the excitation energy transfer between the 3² P _J states of sodium atoms by collisions with ground-state potassium atoms have been measured by resonant Doppler-free two-photon spectroscopy, where the population densities of directly pumped and collisionally excited Na(3P _J )(J=1/2, 3/2) levels were probed by counter-propagating Na(3P _J ) → Na(4D _{3/2, 5/2}) excitation and detected with the thermionic diode. Cross sections of σ(3P _1/2 → 3P _3/2)=190 Ų±20% and σ(3P _3/2 → 3P _1/2)=100 Ų±20% were found. The theoretical calculations taking into account the long-range interaction termsR ^?6,R ^?8 andR ^?10 yield a value σ(3P _1/2 → 3P _3/2)=165 Ų. On the basis of these long-range interaction potentials the differential cross section has been calculated and compared with recently published experimental data. Very good agreement between the theoretical and experimental data was found.     

Reinvestigation of Reactions of HgPh2 with Eu and Yb Metal and the Synthesis of a Europium(II) Bis(tetraphenylborate)

Europium bis(tetraphenylborate) [Eu(thf)₇][BPh₄]₂⋅thf containing a fully solvated [Eu(thf)₇]²⁺ cation, was synthesized by protolysis of “EuPh₂” (from Eu and HgPh₂) with Et₃NHBPh₄, and the structure was determined by single-crystal X-ray diffraction. Efforts to characterize the putative “Ph₂Ln” (Ln = Eu, Yb) reagents led to the synthesis of a mixed-valence complex, [(thf)₃Yb^II(μ-Ph)₃Yb^III(Ph)₂(thf)]⋅2thf, resulting from the reaction of Yb metal with HgPh₂ at a low temperature. This mixed-valence Yb^II/Yb^III compound was studied by ¹⁷¹Yb-NMR spectroscopy and single-crystal X-ray diffraction, and the oxidation states of the Yb atoms were assigned.     

Potential energy surfaces for OsH2

Summary We compute the potential energy surfaces of 12 electronic states of OsH₂ (four quintet, four triplet, and four singlet) arising from⁵ D ground state of the Os atom as well as triplet and singlet excited states using the complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by multireference configuration interaction (MRCI) and relativistic CI (RCI) calculation which include up to 430,000 configurations. We find that the⁵ D ground state of Os atom does not insert into H₂ while the excited³ F state of Os does. The³ B ₁ ground state of OsH₂ (there are two other nearly degenerate states) in the absence of spin-orbit coupling was found to be 22 kcal/mol more stable than Os(⁵ D)+H₂. The spin-orbit mixing of³ B ₁,³ B ₂,³ A ₂, and¹ A ₁ states was so strong that it induces significant change in bond angles (up to 10°) for OsH₂.Dedicated to Prof. Klaus RuedenbergCamille and Henry Dreyfus Teacher-Scholar     

Enhancing Dye-Triplet-Sensitized Upconversion Emission Through the Heavy-Atom Effect in CsLu2F7:Yb/Er Nanoprobes

Lanthanide (Ln³⁺)-doped upconversion (UC) nanoprobes, which have drawn extensive attention for various bioapplications, usually suffer from small absorption cross-sections and weak luminescence intensity of Ln³⁺ ions. Herein, we report the controlled synthesis of a new class of Ln³⁺-doped UC nanoprobes based on CsLu₂F₇:Yb/Er nanocrystals (NCs), which can effectively increase the intersystem crossing (ISC) efficiency from singlet excited state to triplet excited state of IR808 up to 99.3 % through the heavy atom effect. By virtue of the efficient triplet sensitization of IR808, the optimal UC luminescence (UCL) intensity of IR808-modified CsLu₂F₇:Yb/Er NCs is enhanced by 1309 times upon excitation at 808 nm. Benefiting from the intense dye-triplet-sensitized UCL, the nanoprobes are demonstrated for sensitive assay of extracellular and intracellular hypochlorite with an 808-nm/980-nm dual excited ratiometric strategy.     

Chemiluminescence and catalysis of decomposition of dispiro(diadamantane-1,2-dioxetane) in the presence of EuIII and TbIII tris(benzoyltrifluoroacetonate) complexes

Catalysis of decomposition of dispiro(diadamantane-1,2-dioxetane) (1) in the presence of Eu^III and Tb^III tris(benzoyltrifluoroacetonate) complexes (Ln(btfa)₃) accompanied by the formation of adamantanone (2) and chemiluminescence (CL) was studied. The rate constants (k ₂) of decomposition of compound1 in the1·Ln(btfa)₃ complexes and their stability constants (K ₁) have been determined. The Arrhenius parameters of decomposition of1 (E _a= 22.4±0.7 kcal mol^?1, logA=10.2±0.8 for1·Tb(btfa)₃ andE _a=23.4±0.6 kcal mol^?1, logA=10.6±0.8 for1·Eu(btfa)₃) and thermodynamic parameters of complex formation (ΔH=?5.5±0.5 kcal mol^?1, ΔS=?10.4±0.7 e.u. for1·Tb(btfa)₃ and ΔH=?5.8±0.5 kcal mol^?1, ΔS=?10.9±0.7 e.u. for1·Eu(btfa)₃) have been calculated from the temperature dependences ofk ₂ andK ₁. The yields of excitation of the Ln(btfa)₃ chelates φ _Eu ^* =0.021±0.006 and φ _Tb ^* =0.12±0.04 have been determined. A higher efficiency of the occupation of the⁵D₄-level of Tb³⁺ compared to those of the⁵D₁- and⁵D₀-levels of Eu³⁺ is caused by different efficiencies of the non-radiative energy dissipation in the Ln³⁺ ion after the intracomplex energy transfer from the³n,π^*-state of2 to the resonance excited levels of lanthanides.     

Radiative lifetimes and depopulation cross-sections of excited rubidium2 S and2 D states

The radiative lifetimes of excited rubidiumn ² S (n=8?13) andn ² D (n=6?11) states have been measured using the time resolved laser induced fluorescence method. The cross-sections for depopulation of the excited states in collisions with the ground-state Rb atoms have been determined. The influence of the blackbody radiation on the measured values is discussed.     

The Rydberg Nature And Assignments Of Excited States Of The Water Molecule

We report ab initio theoretical calculation on 32 excited states of H₂ O found to lie below 11.7 eV. Of the eight states observed experimentally, the average discrepancy between theoretical and experimental excitation energies is 0.1 eV. We find that the excited states can each be characterized as arising from an excitation to a Rydberg orbital. Our results indicate that the ? and F? states are both 3d-like excited states rather than one 3d state and one 4s state as previously assumed and similarly for the two Rydberg series joining onto ? and F?. The nsa₁ Rydberg series is found to have a quantum defect of 1.38. joining onto the Ã(¹B₁ state. We have assigned the 9.81 eV transition observed by electron impact as the 1b₁ – 3pb₁ excitation to a ³A₁ state.     

Depolarization and fine structure effects in half collisions of sodium-noble gas systems

Population ratios and polarization of the Na3P _J fine structure states following far wing photon excitation of Na-noble gas collision pairs are studied with respect to the underlying interaction potentials and molecular coupling schemes. For this purpose spectral profiles of these quantities, i.e. its dependencies on excitation frequency, have been measured up to ±200 cm^?1 detuning from the NaD lines for NaAr and NaHe under nearly single collision conditions. Comparing the measured population ratios with quantummechanical coupled channels calculations the Σ potential well depths differ considerably from results of model potential calculations. Large residualJ=3/2 alignment observed in the far wings disagrees with simple models assuming incoherent excitation and/or full adiabatic reorientation of the radiating dipole along straight trajectories. By comparison with Lewis model calculations, using realistic trajectories and decoupling radius, it is found instead, that realistic trajectories are constitutive for alignment after Σ-excitation (blue wing), whereas coherence between the²Π_1/2 and²Π_3/2 states determine primarily alignment after Π-excitation (red wing).     

Landé g J factor measurements in the 6snd 1,3 D 2 sequences of Yb I

The Landé g _J factors in the 6snd ^1,3 D ₂ sequences as well as states from the perturbing configurations of neutral Ytterbium have been measured by timeresolved fluorescence spectroscopy. Strong singlet-triplet mixing effects were observed. The g _J factors were measured using the Zeeman quantum-beat technique following step-wise laser excitation. The experiment provided new data for checking the Multi-channel Quantum Defect Theory (MQDT) used in energy-level and lifetime analysis.     

‘Non-Koopmans’ States in Stilbene Cations and a Remarkable Example to the «SDT-Equation»: Indeno [2,1-a]indene

The radical cations of indeno [2, 1-a]indene ( 1 ), stilbene ( 2 ) and 3, 5, 3', 5'-tetramethylstilbene ( 3 ) were prepared by γ-irradiation of the neutral precursors in an electron-scavenging matrix at 77 K . Their electronic spectra were recorded and compared to the photoelectron spectra ( PE .) of the neutral precursors. The results show that either the fourth or the fifth excited doublet state of the cations is of «Non-Koopmans» type, with specific doublet energy (D) D (²B_g)=2.74 eV ( 1 ⁺), =2.59 eV ( 2 ⁺), =2.49 eV ( 3 ⁺). Remarkably, 1 ⁺ possesses two electronic states in the 2.7-2.8 eV energy range: ²A_u («Koopmans»-type) and ²B_g («Non -Koopmans»-type). The «SDT»-equation \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm D} = \sqrt {{\rm S} \cdot {\rm T}} $\end{document} approximately connecting excited singlet (S) and triplet (T) states of a neutral alternant system with the excited doublet (D) states of its radical cation - provided e-promotion occurs For all three excited states between the same (paired) orbitals-is satisfyingly exemplified by 1 : S¹ = 3.92 eV and T¹= 2.06 eV for 1 , D^{4 or 5}=2.74 eV for 1 ⁺.     

Chemiluminescence and catalysis of decomposition of dispiro(diadamantane-1,2-dioxetane) in solutions of lanthanide perchlorates

Chemiluminescence (CL) accompanying the decomposition of dispiro(diadamantane-1,2-dioxetane) (1) in acetonitrile solutions of Eu^III, Gd^III, Tb^III, Pr^III, and Ce^III perchlorates was studied. In the presence of Eu^III, Tb^III, and Pr^III ions, the chemiluminescence spectra contain the luminescence bands of these ions. In the cases of Gd^III and Ce^III, the chemiluminescence is caused by deactivation of singlet-excited adamantanone (2). The excitation of the lanthanide ion depends on the existence of suitable energy levels at which intracomplex excitation transfer from the³n,π^* of ketone is possible. Chemiluminescence of1 increases in solutions of Eu^III and Tb^III. The yields of CL and excitation of the lanthanide ions in the decomposition of1 in the1·Eu^III and1\Tb^III complexes were determined: φ_Eu · =0.013 ± 0.003 and φ_Tb · =0.08±0.02. The fact that the efficiency for the population of the⁵D₄-level of Tb^III is higher than that for the⁵D₁ and⁵D₀-levels of Eu^III is related to the difference in the energy gap between the triplet level of2 and the excited levels of the lanthanides. For Part I, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 730–735, April, 1997.     

Ab initio MRD-CI study of neutral and charged Ga2, Ga3 and Ga4 clusters and comparison with corresponding boron and aluminum clusters

MRD-CI calculations were performed on Ga, Ga₂, Ga₃, Ga₄ and on the corresponding positive and negative ions. In general, pseudopotentials were used, and 4s4p/2s2p basis sets withs andp diffuse functions and one or twod functions. For Ga₂, all-electron calculations were also performed. For Ga ₂ ^(±) , potential functions for ground and low-lying excited states are given. For Ga ₃ ^(±) , geometries were optimized both inC _2v andD _∞h symmetry. The lowest state of Ga ₃ ⁺ is found to be¹Σ _g ⁺ , of Ga₃ ⁴ A ₂, and of Ga ₃ ^{? 1} A ₁ (D _3h ). Ionization potentials and electron affinities of Ga₃ were evaluated. Many low-lying excited states of Ga ₃ ^(±) were found. Rhombic (D _2h , including squareD _4h ), tetrahedral (T _d ), T-shaped (C _2v ) and linear structures (D _∞h) were investigated in the search for the lowest state of Ga₄. A square-planar arrangement of the nuclei, withR _e = 5.30 a₀, was found to have the lowest energy. The other geometries lie about 0.5 eV higher. InD _2h symmetry, low-lying excited states of Ga₄, as well as ground and excited states of Ga ₄ ⁺ and Ga ₄ ^? were studied. Geometries, ionization potentials, electron affinities, atomization and fragmentation energies of Ga _n are compared with corresponding data for B _n and Al _n . Typical changes in going from first-row to third-row atoms are observed.     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.     

Excitation and luminescence spectra of UO2F2−4

The luminescence and excitation spectra of [(CH₃)₄]₂UO₂F₄, M₃UO₂F₅ (M = K and Rb) and Cs₂SnCl₆:UO₂Cl²⁻₄ have been recorded at temperatures down to 10 K. The excitation spectrum of [(CH₃)₄]₂UO₂F₄ is unique because the electronic origin is located at the lowest energy reported for any uranyl compound. The analysis of the excitation spectrum is consistent with a D_4h, but not a D_5h coordinated uranyl chromophore. A detailed interpretation of the vibronic structure of the spectrum enables the lower excited states of the uranyl ion to be located, and the symmetries of these are consistent with the model of Denning. A comparison with the excitation spectra of salts of the UO₂F³⁻₅ anion is made.