The lowest triplet state of pyrimidine in a crystalline host at 1.2 K. The spin-sublevel emission spectra and their vibrational assignment

Spin-sublevel phosphorescence spectra of pyrimidine in benzene are reported. The spectra related to the in-plane spin axes consist of progressions of totally symmetric bands, whereas that of the third sublevel in addition shows strong non-totally symmetric bands. Assignments are given, and some out-of-plane modes previously in dispute are identified (ν_10b = 808, ν_17b = 960, ν_s = 992 cm^?1, A₂). As a check similar experiments were carried out of pyrimidine incorporated in p-xylene and cyclohexane.     

Electron-impact excitation of the transition 1s 22s 22p 5(2 P 3 2/0 −2 P 1 2/0 ) of Mg IV

Effective collision strengths for electron-impact excitation of the ground state fine structure transition² P ₃ ^2/0 ?² P ₁ ^2/0 have been calculated by using theR-matrix method. Twelve lowest target states, represented by configuration interaction wavefunctions are included in the scattering calculations.M1 andE2 transition probabilities are also calculated by employing the Breit-Pauli Hamiltonian.     

Intra- and inter-atomic auger processes in collisions of low energy He++, He+, He* (23,S, 21 S) and Li+ with Li covered W(110) surfaces

Electron spectra from He⁺⁺, He⁺ and Li⁺ (10 to 1500 eV) ions colliding under grazing incidence with Li covered W (110) surfaces are reported. The results are compared with those obtained from thermal collisions of (2³ S; 2¹ S) metastable He atoms. In these collisions 1s vacancies are either produced during the collision event (energetic He⁺ (Li ⁺) collisions) or are brought into the collision (slow He⁺⁺ (He⁺, He*) collisions). Population of the 2s orbitals by two electrons produces states which decay by intraatomic Auger processes: we observe autoionization of He** (2s ²) and Li** (1s 2s ²) as well as autodetachment of He^?* (1s 2s ²). Alternatively the 1s-holes in the projectile or target (Li) can be filled by Auger processes involving one or two surface electrons. The processes leading to electron emission are studied as a function of the Li coverage in the submonolayer region (0≦Θ_Li≦1Ml) and as a function of the projectile energy. It is concluded that with one or two 1s vacancies present in the projectile the double capture of two surface electrons constitutes an important process responsible for electron emission of low work function surfaces.     

The atomic form factor for vanadium: a treatment including correlation andLS-dependent relativistic effects

The atomic form factor for the ground state of vanadium is evaluated using non-relativistic and “relativistic-corrected” configuration interaction wavefunctions in the |LSM _L M _s 〉 representation. Relaxing the constraints imposed by the Hartree-Fock model results in a very small reduction of the atomic factor at small momentum transfer with a negligible change at higher momentum transfer. Better agreement with the relativistic Hartree-Fock atomic form factor at small momentum transfer is obtained when theLS-dependent relativistic effects are included in the Breit-Pauli approximation. The sensitivity of the atomic form factor to small changes in the magnitude of the expansion coefficients of the configurational functions is also discussed.     

Angular distributions of photoelectrons in resonantly enhanced multiphoto ionization via the 7s[1 1/2]0 1 and 8s[1 1/2]0 1 states of Xe

Photoelectron angular distributions have been measured for the three-photon resonant one-photon ionization, (3+1), of Xe via the 7s[1 1/2]⁰ ₁ and 8s[1 1/2]⁰ ₁ states. The results are in good agreement with the theoretical predictions of Tang and Lambropoulos [13] for the 8s[1 1/2]⁰ ₁, but not for the 7s[1 1/2]⁰ ₁ state. Furthermore, the results are compared to those which have been obtained by Blazewicz et al. [1] for the three-photon resonant two-photon ionization, (3+2), of Xe via the 6s[1 1/2]⁰ ₁ state.     

Valence bond studies of AH2 molecules

Minimal basis set (STO) molecular orbital and valence-bond calculations are reported for the³ B ₁ and¹ A ₁ states of CH₂. The open-shell molecular orbital calculations used the Roothaan formulation. The valence-bond calculations used the Prosser-Hagstrom biorthogonalisation technique to evaluate the cofactors required in using Löwdin's formulae. Optimisation of geometry and orbital exponents in the molecular orbital calculation on the³ B ₁ state gave a geometry of R_C-H=2.11 a.u. and H-C-H=123.2 °. The energy obtained was ?38.8355 a.u. The molecular orbital and valencebond calculations are compared. In the valence-bond calculations the variation with bond-length and bond-angle of the configuration energies was studied. Valence bond “build-up” studies are also reported. Valence-bond calculations using hybrid orbitals instead of natural atomic orbitals showed that the perfect-pairing approximation is not as good for CH₂ as BeH₂. The nature of the lone-pair and bonding orbitals is found to be significantly different between the³ B ₁ and¹ A ₁ states. In the³ B ₁ state the 2s and 2p orbitals are fairly equally mixed between both types of orbital. However in the¹ A ₁ state the bonding orbitals have mainly 2p character and the lone pair orbitals have mainly 2s character. As was found for H₂O, the bonding hybrid orbitals do not follow the hydrogen nuclei as the bond angle varies but continue to point approximately in their equilibrium directions.     

Hartree-fock energies ofL,S multiplets of open-shell Nan-type jellium spheres

Numerical Hartree-Fock calculations on the^2S+1 L states arising from 1s ² 1 p^v and 1s ² 1p ⁶ 1 p^μ open-shell configurations of Na_n-type (n≦20) jellium spheres are reported for the first time. TheL, S multiplet energy spread is substancially larger than that expected from earlier estimates of exchange interaction energies. A new way of considering cluster abundance structures in mass spectra emerges from the results.     

Autoionisation resonances in the photoabsorption spectra of Ge I and Sn I

We report new measurements in the photoabsorption spectra of Ge I and Sn I in the vacuum ultraviolet region. The spectra were recorded in the first order of a 1-metre vacuum spectrograph using synchrotron radiation as the background source of continuum. Autoionisation resonances above the first ionisation were observed which are identified as the 4s 4p ^{3 1} P ₁ and 5s5p ^{3 1} P ₁ levels in Ge I and Sn I respectively. Resonances due to excitation to these levels from thermally populatedms ² mp ^{2 3} P _{0, 1, 2} levels in the ground states have also been observed. Single configuration Hartree-Fock calculations are reported for the 4s4p ³ and 5s5p ³ configurations for Ge I and Sn I. Fano parameterisation was used to determine the Resonance wavelengths, the line widths and the asymmetry or line shape parameters.     

Hyperfine structure and isotope shift of some even parity levels in the YbI spectrum

Using laser induced fluorescence spectroscopy the hyperfine structure of the even parity levels 4f ¹⁴6s6d ³ D ₁, 4f ¹⁴ 6s8s ³ S ₁ and 4f ¹³ 5d6s6p (7/2, 5/2)J=1,2,3 as well as of the odd parity level 4f ¹⁴ 6s6p ³ P ₂ in neutral ytterbium has been investigated. The isotope shift of the transitions 4f ¹⁴6s6p ³ P ₀ → 4f ¹⁴ 6s6p ³ D ₁ and 4f ¹⁴ 6s6p ³ P ₂ → 4f ¹⁴ 6s8s ³ S ₁, 4f ¹³ 5d6s6p (7/2, 5/2)J=1,2,3 could be measured with high accuracy. The results for the 4f ¹⁴ 6s6p ³ D ₁ level show a considerable influence of second order effects of the hyperfine interaction. The isotope shifts of the 4f ¹⁴ 6s8s ³ S ₁ and 4f ¹³ 5d6s6p (7/2, 5/2)J=1 levels indicate a possible configuration mixing for these levels.     

Preparation and crystallographic investigation of the spinel series: Mn1 + 2sCr2 − 3sSbsO4 (0.05 ⩽ , s ⩽ 0.30)

Mn_{1 + 2s}Cr_{2 ? 3s}Sb_sO₄, a new series of spinels, have been prepared and studied using X-ray powder data. For s going from 0.05 to 0.30, a gradually increases from 8.441(1) to 8.472(1) Å, and u slightly decreases, from 0.262 to 0.258. Interatomic distances are given. The Mn_{1 + 2s}Cr_{2 ? 3s}Sb_sO₄ (0.05 < s < 0.30) series may be conceived as the result of partial substitution of Cr^{3 +} by 2/3Mn^{2 +} + 1/3Sb^{5 +} in the normal spinel, MnCr₂O ₄.     

Calculation of the photo-electrons angular distribution asymmetry parameter β2p near the (3s3p)1 P resonance of He*

Near the (3s3p)¹ P resonance of He, we have calculated the photoelectrons angular distribution asymmetry parameter β_2p in the diagonalization approximation. Using the measured value of β _n=2 near the (3s3p)¹ P level obtained by Lindle et al. in the resonance photo-ionization of He to He^?(n=2), we have estimated the ratioR=σ_2p /σ_2s of the partial 2p photo-ionization cross section to the partial 2s photo-ionization cross section. Our calculation supports the result that in the resonance region, the formation of ions in the 2p level dominates over the 2s level. This is in good agreement with the experimental and most of the theoretical results reported to date.     

Collisions of metastable Ne*, He* atoms with ground-state He,Ne atoms studied by atomic beam and laser techniques

A crossed nozzle-beam experiment is used to investigate thermal energy collisions: Ne*(2p ⁵3s,³ P _{0, 2})+He(1s ²,¹ S ₀), almost purely elastic, and He*(1s2s,^{1, 3} S)+Ne(2p ⁶,¹ S ₀), in which inelastic excitation transfers occur. State and velocity selection of the scattered Ne* atoms is performed using a tunablecw dye laser frequency locked on a definite Zeeman component of the transition 1s ₅→2p ₆ (λ=614.3 nm) of²⁰Ne or²²Ne. In the purely elastic case, this technique allows the selection of one of the two final velocities, and then an unambiguous LAB-CM transformation. The differential cross section at 62 meV tallies on accords with a calculation using a single effective potential. In He* on Ne collisions, the main inelastic processes are endothermic excitation transfers from He*(2¹ S). Experimental results obtained at different energies (62, 95, 109, 124 meV) show that the transfers essentially result in levels 3s and 4d of Ne.     

Dielectronic recombination of lithium-like Ar15+

Dielectronic recombination (DR) of Ar¹⁵⁺(1s ²2s) ions was studied in a single-pass merged-beams experiment at the UNILAC (universal linear accelerator) of GSI. Absolute recombination rates and cross sections were measured for electron-ion center-of-mass energies from 0 to 580 eV. A number of Rydberg states formed by DR with 2s → 2p (Δn=0) and 2s → 3? (Δn=1) core excitations and even individual terms in the 1s ²3?3?′ configuration could be resolved. Theoretical calculations of DR cross sections are in good overall agreement with the data. In the calculations for Δn=0 transitions, effects of electric fields have to be included to reproduce the magnitude of the measured DR rates at the limit of the 2 _{p 1/2}? and 2 _{p 3/2}? Rydberg series. Discrepancies between theory and experiment are observed at the series limits of the (1s ²3?n?′) Rydberg series.     

Quantum chemical ab initio calculations for excited states of F IV

Quantum chemical ab initio calculations have been performed for the ground state and for several excited states of the F³⁺ ion (F IV). Three levels of accuracy have been used: Frozen-core SCF calculations (FRC-SCF) to determine orbital energies ε _nl and quantum defects δ _l for excited Rydberg orbitalsnl; frozen-core SCF followed by CI calculations (FRC-CI) which account for multiplet splittings and configuration mixings, and multi-configuration coupled-electron-pair approximation (MC-CEPA) calculations which include dynamic correlation effects. The accuracy of the calculated excitation energies is in the order of 5000 cm^?1 at the FRC-CI level and in the order of 500 cm^?1 at the MC-CEPA level. This latter error amounts to about 0.1% for excitation energies in the range of 400000 to 600000 cm^?1. The MC-CEPA calculations have been performed for 17 experimentally known states and for 14 experimentally unknown states, in particular for the configurations 2s2p ² (² D)3s, 2s 2p ²(² S)3s, 2s ² 2p 4p, and 2s ² 2p 5p.     

Performance evaluation of k 0-instrumental neutron activation analysis and flame atomic absorption spectrophotometry in the characterization of various types of alloys

Certified alloys of Ni–Cu based, Fe based and Cu–Sn based were analysed by semi-absolute, standardless k ₀-instrumental neutron activation analysis (k ₀-INAA) and flame atomic absorption spectrophotometry (FAAS) aiming at evaluating their comparative performances. In k ₀-INAA measurements, the irradiations were performed at miniaturized neutron source reactor having thermal neutron flux of about 1 × 10¹² cm^?2 s^?1. The experimentally optimized parameters for INAA suggested a maximum of three irradiations for the quantification of 21 elements within 5 days. The same experiments also produced quantitative results of 13 elements not reported in the certificates of the reference materials. AAS was, however, unable to determine any of those elements. Accuracy of the two techniques was assessed by comparing their average root mean squared errors. The data analysis concluded that k ₀-INAA had better sensitivity and accuracy than FAAS.     

Elektrometrische Untersuchung der Zusammensetzung und Stabilität von Thiomilchsäurekomplexen mit Zn2+ und UO 2 2+

The complexation of thiolactic acid with Zn²⁺ and UO₂ ²⁺ has been investigated by potentiometric and conductometric titration techniques. Both the metals form 1∶1 and 1∶2 complexes in different pH ranges. Their logK _stab values have been determined byCalvin andMelchior's extension ofBjerrum's method and were further refined by alternative methods at temperatures 20, 30, and 40°C. The values of overall changes in ΔG, ΔH and ΔS for complexation have also been reported.     

Experimental study of triplet exciton diffusivity in crystalline pyrene at 300 K

Preliminary measurements of the triplet diffusivity tensor in crystalline pyrene at 300 K are reported. Maximum diffusivity occurs along the b axis and D_ab = (1.25 ± 0.3) × 10^?4 cm² s^?1. Diffusion in the ac′ plane is nearly isotropic with D ≈ (0.3 ± 0.1) × 10^?4 cm² s^?1. These results are tentatively interpreted as diffusion dominated by nonlocal scattering.     

Calculations of high lying energy resonances in the photoionization spectra of Ne+ using the screening constant by unit nuclear charge method

We report in this paper, energy resonances of the 2s²2p⁴ (¹D₂)ns,nd, 2s²2p⁴(¹S₀) ns,nd and 2s2p⁵ (³P₂)np series originating from both 2s²2p^{5 2}P_1/2 metastable and 2s²2p^{5 2}P_3/2 ground state of Ne⁺. Calculations are performed using the Screening Constant by Unit Nuclear Charge (SCUNC) method and high lying states up to n=75 are investigated. The results obtained in this work agree very well with Advanced Light Source experiments on Ne⁺ (Covington et al., 2002). The analysis of the present results is achieved by calculating the effective charge number in the framework of the SCUNC formalism and by evaluating the quantum defect from the standard quantum defect expansion formula. Both effective nuclear charge and quantum defect are almost constants up to n=75.     

Absolute detection of metastable rare gas atoms by a cw laser photoionization method

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔI _S in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔI _P (γ = ΔI _S/ΔI _P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s ³ P ₂), Ar* (4s ³ P ₂), and Kr* (5s ³ P ₂) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (2³ S, 2¹ S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.