Stark-mixing effect on the 6p 2 1 S 0−6p 2 3 P 2 transition in Pb I

The intensity of the 531.5 nm electric-field-quench radiation has been measured on a thermal beam of neutral Pb atoms in the metastable 6p ^{2 1} S ₀ state. The measurement yields a Stark-mixing amplitude for transition between the 6p ^{2 1} S ₀ and 6p ^{2 3} P ₂ states. Combining this result with available experimental data sets an upper limit for the 6p 7s ³ P ₁ → 6p ^{2 1} S ₀ transition probability:A _ki <1.79·10³ s ^?1. Calculations for the 6p 7s → 6p ² and 6p 8s → 6p ², as well as transition rates of forbidden lines inside 6p ² configuration of PbI are presented and compared with existing experimental data.     

Fine structure excitation transfer between the lithiumD-lines by collisions with cesium atoms

Applying resonant Doppler-free 2-photon laser spectroscopy with thermionic diode detection, the cross sections for the excitation energy transfer of the collisional process⁷Li*(2P _1/2+Cs(6S _1/2)→⁷Li*(2P _3/2)+Cs(6S _1/2) have been measured. The experimental cross sections, σ^Li-Cs (1/2→3/2)=890 Ų and σ^Li-Cs (3/2→1/2)=430 Ų, are compared with theoretical data obtained by a sudden impact approximation approach taking into account the long-range interaction potentials only. The calculated cross sections show an excitation mixing process at large internuclear distances where Li-Cs dipole-dipole and dipole-quadrupole interaction forces are predominant.     

Ground- and excited stateg-factors of Ba+

We observed the Zeeman-splitting of the 6S _1/2-6P _1/2 resonance transition of Ba⁺ ions (493.4 nm) in a 6 T magnetic field. The ions were stored in a Penning quadrupole trap. From the splitting and the simultaneously measured cyclotron frequency of stored electrons we derived theg-factors of the 6S _1/2 and 6P _1/2 states. The results areg(6S _1/2)=2.00267(20) andg(6P _1/2)=0.66634(22).     

Observation of the electric-field-inducedE1 transition between 5s4d 1 D 2 and 5s 2 1 S 0 levels in Sr I

The angular distribution of the electric-field-inducedE1 radiation is asymmetric with respect to the external electric field direction. Measurement of this asymmetry for the 5s4d ¹ D ₂ → 5s ^{2 1} S ₀ transition in strontium permits the experimental determination of the 5s5p ¹ P ₁ → 5s4d¹ D ₂ decay rate. The result is:A(¹ P ₁ →¹ D ₂)=(4.37±0.93±0.66) × 10³ s ^?1.     

Dynamic metal-ligand coupling in the infrared MCD spectra of the cobalt(II) tetrahalides

The axial single-crystal magnetic circular dichroism spectra of Cs₃ZnCl₅/Co²⁺ and Cs₃ZnBr₅/Co²⁺ have been measured over the 4000–7000 cm^?1 region of the ⁴A₂ → ⁴T₁ (F) transition at ambient and liquid-helium temperature. The B/D and C/D terms obtained give transition moment ratios, 〈t₂∥m∥t₂〉/〈e∥m∥t₂〉, in accord with the value required (?3^{$\text{1}\text{2}$}/2^{$\text{1}\text{2}$}) by a dynamic ligand-polarisation model for d-electron transition probabilities in tetrahedral metal complexes.     

Fine structure excitation transfer between the potassium 42P states induced by collisions with caesium atoms

Applying diode-laser resonant fluorescence method, the cross sections for the excitation energy transfer of the collisional process K*(4² P _1/2)+Cs(6² S _1/2)?K*(4² P _3/2)+Cs(6² S _1/2) have been measured. The values we have obtained are σ(1/2→3/2)=77 Ų and σ(3/2→1/2)=48 Ų. These results complete the sequence of data for the fine-structure mixing of the first-resonance states of alkali atoms colliding with the ground-state caesium atoms.     

The collision cross sections for the fine-structure mixing of caesium 6P levels induced by collisions with potassium atoms

The cross section for the fine-structure excitation transfer Cs(6P _1/2) → Cs(6P _3/2), induced by collisions with the ground state potassium atoms, has been measured by resonant Doppler-free two-photon spectroscopy. The population densities of caesium 6P _J (J=1/2, 3/2) levels were probed by thermionic detection of the collisionally ionized caesium atoms from the Cs(6P _J ) → Cs(10S _1/2) excitation channel. The cross section for the transfer process at the temperatureT=503 K has been found to be σ(1/2 → 3/2)=45 Ų ± 20%. The result is compared with previously published experimental cross sections for fine-structure transfer in resonance states of other alkali elements perturbed by potassium and a thoeretical value of the Li(2P _J )-K system calculated in a simple approach.     

Etude des premières bandes d'absorption S0→S1 et S0→S2 du 5-méthoxyindole

The experimental and theoretical results obtained for 5-methoxyindole (5MOI) indicate a large energetic difference between the S₀ → S₁ and S₀ → S₂ transitions indicating a charge distribution very different from that of indole. A weak shoulder on the long wavelength side of the S₀ → S₁ band is attributed to a nπ* transition associated with the methoxy substituent. This feature has previously been attributed to a hot band. The effect of solvent on the molecule and the results obtained by the quantitative quantum CNDO/2 and CNDO/M methods are discussed.     

Two-beam linear magneto-optical spectroscopy of atomic transitions between short lived states

Two-beam, linear magneto-optical spectroscopy is a powerful tool for studying short-lived states. We present both measurements and a quantitative theoretical analysis of magneto-rotation observed in the forward scattering of a linearly polarised laser beam passing through an amplifying atomic medium placed in a longitudinal magnetic field. The probed transition connects two short-lived, excited atomic levels, the upper state (here the 7S _1/2 level of cesium) being prepared initially via another transition from ground state, excited by a linearly polarised pump beam. The probe polarisation undergoes three different magneto-optical processes: optical rotation, with separate contributions from the two transitions, and linear dichroism due to Hanle precession of the upper state alignment. Complete resolution of the hyperfine structures and ninety degree switching of the probe polarisation enable us to isolate all of these processes. To lowest order in optical thickness the relative intensities and lineshapes are well interpreted.     

Unprecedented mid-infrared nonlinear optical materials achieved by crystal structure engineering,a case study of (KX)P2S6 (X = Sb,Bi, Ba)

Three acentric type-I phase-matchable infrared nonlinear optical materials KSbP₂S₆, KBiP₂S₆, and K₂BaP₂S₆, showing excellent balance between the second harmonic generation coefficient, bandgap, and laser damage threshold, were synthesized via a high-temperature solid-state method. KSbP₂S₆ is isostructural to KBiP₂S₆, which both crystallize in the β-KSbP₂Se₆ structure type. K₂BaP₂S₆ was discovered for the first time, which crystallizes in a new structure type. KSbP₂S₆ and KBiP₂S₆ exhibit close structural similarity to the parent compound, centrosymmetric Ba₂P₂S₆. The [P₂S₆] motifs, isotypic to ethane, exist in Ba₂P₂S₆, KSbP₂S₆, KBiP₂S₆, and K₂BaP₂S₆. The mixed cations, K/Sb pair, K/Bi pair, and K/Ba pair, play a dual-role of aligning the [P₂S₆] structure motifs, contributing to a high SHG coefficient, as well as enlarging the bandgap. KSbP₂S₆, KBiP₂S₆, and K₂BaP₂S₆ are direct bandgap semiconductors with a bandgap of 2.9(1) eV, 2.3(1) eV and 4.1(1) eV, respectively. KSbP₂S₆, KBiP₂S₆, and K₂BaP₂S₆ exhibit a high second harmonic response of 2.2× AgGaS₂, 1.8× AgGaS₂, and 2.1× AgGaS₂, respectively, coupled with a high laser damage threshold of 3× AgGaS₂, 3× AgGaS₂, and 8× AgGaS₂, respectively. The DFT calculations also confirm that the large SHG coefficient mainly originates from [P₂S₆] anionic motifs.

Three superior type-I phase-matching middle infrared nonlinear optical materials (E_a > 4.0 eV, d_ij > 2× AGS, LDT > 8× AGS) were achieved via crystal structure engineering.     

Effect of rotational relaxation on the intensity and polarization of fluorescence emission caused by sequential two-photo excitation

General formulas are derived for the intensity and the degree of polarization of the S_m-fluorescence emission (m ? 2) of a sample excited by the sequential two-photon excitation process (S_n ← S₁ ← S₀, n ? 2) with plane-polarized pulsed light. They show how the S_m-fluorescence intensity and anisotropy depend on the relative orientation of the relevant transition dipoles within a molecule and on the degree of rotational relaxation of molecules in the intermediate state (S₁) and in the S_m state (for the case m = n), or in the S_n → S_m process (m ≠ n).     

The dehydration of nickel sulfate

Nickel sulfate was recrystallized to obtain the 7 H₂O, β6 H₂O and various habits of α6 H₂O. Dehydration and phase transitions were studied using X-ray analysis and DSC with effluent gas analysis. NiSO₄ · 7 H₂O dehydrates spontaneously via 7 → 6β → 6α at room temperature, while the dehydration pathway of NiSO₄ α6 H₂O is 6α → 6γ → 4 → 1. The effect of time and storage on the 6α—6β phase transition was investigated.     

Cyclic polysilanes : XIX. A temperature study of redistribution equilibria between permethylcyclopolysilanes

Equilibria among the cyclic compounds (Me₂Si)_n where n = 5, 6 and 7 have been studied between 30–58°C. Thermodynamic values for the redistribution reactions between pairs of compounds are, for n = 5 → 6, ΔH = ?18 kcal/mole, ΔS = ?20 cal/deg. mole; for n = 7 → 6, ΔH ?3, ΔS +33; for n = 7 → 5, ΔH +18, ΔS + 51. The enthalpies indicate that the stabilities of the rings increase in the order (Me₂Si)₅ < (Me₂Si)₇ < (Me₂Si)₆. The differences are smaller than corresponding differences among the cycloalkanes, probably because the silicon compounds are less affected by steric repulsions and angle strain.     

Classical trajectory study of orientation and alignment effects for charge exchange processes in H+- Na*(3p m) collisions

The orientation and alignment effects for charge exchange in H⁺ + Na*(3p) collisions are studied using the classical trajectory Monte-Carlo method in the energy range from 1 to 8 keV. For Na*(3p _-1) → H*(2s, 2p _±1) transitions a large orientation effect is predicted by the probability functions, in very good agreement with semiclassical calculations. Angular differential cross sections are also calculated and interpreted using the impact parameter dependence of the proton deflection angle. They predict left-right asymmetry in agreement with semiclassical calculations or experimental results, but slightly smaller. Another geometry, not experimentally realized, is considered, where the proton velocity is parallel to the quantization axis of the p _±1 oriented states. Charge exchange from different aligned states with respect to the direction of the projectile velocity is also investigated, but the alignment effects are not as well described as the orientation effects. Total cross sections from oriented or aligned states with cylindrical symmetry around the projectile velocity direction are calculated and allow the hypothesis of velocity matching to be tested.     

Experimental ground stateg J-factor of Ba+ in a Penning ion trap

We observed the Zeeman-splitting of the 6S _1/2 – 6P _1/2 resonance transition of Ba⁺-ions (493.4 nm) in a 6T magnetic field. The ions were stored in a Penning quadrupole trap. We polarized the ground state by optical pumping and in a microwave-optical double resonance experiment we measured the ground state Zeeman-splitting. From the resonance frequency and the cyclotron frequency of electrons stored in the same trap we derived theg-factor of the 6S _1/2 state. The result isg _J(6S _1/2)=2.002 490 6(11), in reasonable agreement with recent calculations.     

Broadening and shift of thalliumnP 1/2,3/2 states (n=6–10) perturbed by noble gases

Impact broadening and shift of the transitions Tl 6P _1/2?nP _1/2,3/2 (n=7, 9, 10) and 6P _3/2?9P _{1/2, 3/2} are measured by high resolution Doppler-free two-photon spectroscopy. For excited states with small principal quantum numbers the results are in accord with values obtained fromC ₆-C ₈-C ₁₂ potentials calculated semiempirically. For intermediate principal quantum numbers the experiments show that the elastic scattering of the valence electron at the noble gas atom must be considered additionally. The experimental shift rates ofP _1/2 states are found to be larger than ofP _3/2 states. Furthermore, the line shifts of the one- and two-photon transitions concerning the 6P _1/2, 6P _3/2, 9P _1/2, 9P _3/2, 7S _1/2 states show that the contribution of the lower level of the transition must be considered too.     

Metamagnetic phase transition in a diruthenium compound with interpenetrating sublattices

The diruthenium compound [Ru₂(O₂CMe)₄]₃[Cr(CN)₆] may be the only known material that contains two weakly-coupled, magnetically-ordered sublattices occupying the same three-dimensional volume. Due to the strong easy-plane anisotropy on each Ru₂ complex, the moment of each sublattice is constrained to one of the eight cubic diagonals. At low fields, the two sublattices are antiferromagnetically aligned by weak dipolar and deformation energies. But above a metamagnetic critical field of about 1000 Oe, the sublattice moments become ferromagnetically aligned and the net magnetization increases dramatically. We have successfully modeled this metamagnetic transition by assuming that the individual sublattice spin configurations are only weakly distorted by the magnetic field. This model suggests that the ground state of each sublattice undergoes a phase transition at a pressure of about 7 kbar. The drop in the sublattice moment and the rise in the sublattice susceptibility above 7 kbar can be explained by a high- to low-spin transition (S = 3/2 to 1/2) on the mixed-valent diruthenium complexes.     

Magnetic properties of Mn2V2O7 single crystals

Magnetic properties of Mn₂V₂O₇ single crystals are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. A structural phase transition of the α-β forms is clearly observed at the temperature range of 200-250 K and an antiferromagnetic ordering with magnetic anisotropy is observed below 20 K. A spin-flop transition is observed with magnetic field applied along the [110] axis of β-Mn₂V₂O₇, of which corresponds to the [001] axis of α-Mn₂V₂O₇, suggesting that the spins of Mn²⁺ ions locate within honeycomb layers which point likely in the [110] direction of β-Mn₂V₂O₇ or the [001] axis of α-Mn₂V₂O₇. However, a rather small jump of magnetization at spin-flop transition suggests a possible partition of crystal to some domains through β-to-α transition on cooling or much complex spin structure in honeycomb lattice with some frustration.     

ΔF = 2–6 transitions in the hyperfine spectrum of λ = 601.8 nm line of 151Eu

The magnetic field effect of the a ⁸ S _1/2-z ⁸ P _9/2, F = 7 transition lines of ¹⁵¹Eu in magnetic fields up to 400 Gauss has been studied by using laser-atomic-beam-spectroscopy. δF = 2–6 transitions were observed. Results show that the transitions originate from the mixing of the Zeeman states induced by the magnetic field. The condition favourable to observation of the |δF| ≥ 2 transitions especially the transitions with high |δF| values discussed