Observation of oriented 23 S 1 helium atoms in a sodium-helium plasma irradiated by polarization-modulated laser radiation

An experiment on the observation of spin polarization of metastable helium atoms interacting with optically oriented sodium atoms under continuous rf discharge conditions is described. Laser radiation with alternating-sign circular polarization, tuned to the resonance excitation of the 3² S _1/2–3² P _1/2 transition in Na atoms, is used as the source for optical pumping of ground-state sodium atoms. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 385–387 (10 March 1997)     

Molecular structure of trimethyl[(3-indole)ethoxy]silane cooled in a supersonic jet

Conformations of He-jet-cooled trimethyl[(3-indole)ethoxy]silane (TIES) have been studied using a laser spectroscopy technique in combination with quantum-chemical computations. Six probable conformers of the molecule were computed, of which only two conformations were observed. Based on an analysis of fluorescence excitation spectra, fluorescence spectra, shapes of rotational band contours at the electronic S₀–S₁ transition of TIES, and theoretical computations, the above conformers were assigned to steric structures. Twisted structures have the lowest energy due to intramolecular hydrogen bonds C - H ?O < _C^Si C - H \cdots O <_C^{Si} between hydrogen atoms of methyl groups and an oxygen atom and C–H···π between H and the π-electron cloud of the indole ring.     

Effect of excitation level on the photoluminescence of barium thiogallate activated with europium and cerium ions

The photoluminescence of barium thiogallates doped with Eu²⁺, Ce³⁺, and Eu²⁺ + Ce³⁺ ions is studied over a wide range of excitation levels (10^–3–10⁶ W/cm²). Introduction of 3 at.% Eu and 3 at.% Ce instead of 5 at.% Eu into a BaGa₂S₄ matrix doubles the luminescence quantum yield of the phosphor. Doped BaGa₂S₄ exhibits a high linearity in its luminescence intensity as a function of excitation level (a constant efficiency) up to 2·10⁴ W/cm² for excitation pulse durations of 8 ns, which corresponds to cw pumping at a power density of about 5·10² W/cm² in terms of the concentration of excited ions. It is shown that using BaGa₂S₄:Eu,Ce along with the “yellow” phosphor of a Nichia NS6L083 LED may increase its color rendering index from 0.64 to 0.80 with no reduction in its luminous efficiency.     

Limitations of an optically pumped rubidium laser imposed by atom recycle rate

A rubidium laser pumped on the 5²S_1/2–5²P_3/2 D₂ transition by a pulsed dye laser at pump intensities exceeding 3.5 MW/cm² (>1000 times threshold) has been demonstrated. Output energies as high as 12 μJ/pulse are limited by the rate for collision relaxation of the pumped ²P_3/2 state to the upper laser ²P_1/2 state. More than 250 photons are available for every rubidium atom in the pumped volume during each pulse. For modest alkali atom and ethane spin–orbit relaxer concentrations, the gain medium can only process about 50 photons/atom during the 2–8 ns pump pulse. At 110°C and 550 Torr of ethane, the system is bottlenecked in the ²P_3/2 state and all of the incident photons cannot be absorbed. The output energy is linearly dependent on pump pulse duration for a given pump energy. The highly saturated pump limit of the recently developed three-level model for diode pumped alkali lasers (DPALs) is developed. The system efficiency based on absorbed photons approaches 36% even for these extreme pump conditions.     

Observation of signature inversion in the πh9/2?vi13/2 oblate band

High-spin states in ¹⁹⁰Tl have been studied via the ¹⁶⁰Gd(³⁵Cl, 5nγ) reaction. The level scheme, consisting of the πh _9/2⊗vi _13/2 oblate band and a cascade with character of single particle excitations, has been established. Spin values have been firmly assigned to the oblate band in ¹⁹⁰Tl, resulting in low-spin signature inversion in the πh _9/2⊗vi _13/2 oblate band for the first time. Based on the similarity of the level structure in doubly odd Tl nuclei, spin values for the oblate bands in ^192–200Tl should be re-assigned, and a consistent low-spin signature inversion has occurred in these oblate deformed nuclei. The low-spin signature inversion phenomena can be interpreted qualitatively by using the 2-quasiparticle plus rotor model including p-n residual interactions.     

Rate constants of nonradiative intersystem crossing transition S1(ππ*) ? T1(ππ*) in molecules of aromatic compounds with heteroatoms

In terms of the adiabatic theory of interactions, we consider the results of the theoretical estimation of the rate constants K _ST of the nonradiative intersystem crossing conversion S ₁(ππ*) ⇝ T ₁ ^S (ππ*) for nine aromatic molecules containing 10–14 carbon atoms and one or two heavy (many-electron) oxygen or chlorine atoms.     

The Muonic Hydrogen Lamb Shift Experiment at PSI

A measurement of the 2S Lamb shift in muonic hydrogen (μ⁻p) is being prepared at the Paul Scherrer Institute (PSI). The goal of the experiment is to measure the energy difference ΔE(2⁵ P _3/2−2³ S _1/2) by laser spectroscopy (λ≈6μm) to a precision of 30 ppm and to deduce the root mean square (rms) proton charge radius with 10⁻³ relative accuracy, 20 times more precise than presently known. An important prerequisite to this experiment is the availability of long-lived μp_2S -atoms. A 2S-lifetime of ∼1 μs – sufficiently long to perform the laser experiment – at H₂ gas pressures of 1–2 hPa was deduced from recent measurements of the collisional 2S-quenching rate. A new low-energy negative muon beam yields an order of magnitude more muon stops in a small low-density gas volume than a conventional cloud muon beam. A stack of ultra-thin carbon foils is the key element of a fast detector for keV-muons. The development of a 2 keV X-ray detector and a 3-stage laser system providing 0.5 mJ laser pulses at 6 μm is on the way. This revised version was published online in August 2006 with corrections to the Cover Date.     

Determination of the absorption constant in the interband region by photocurrent measurements

We determined high absorption constants of crystals from photocurrent measurements within the interband absorption region (10–10⁴ cm^-1). The method has been demonstrated in the interband absorption regime near 530 nm in Sn₂P₂S₆, a novel infrared sensitive photorefractive material, and in the interband absorption regime near 257 nm of near stoichiometric LiTaO₃. Besides the verification of older measurements with our new technique, precise absorption data for Sn₂P₂S₆ in the wavelength range 488–514 nm are presented. PACS 42.70.Nq; 72.41.+w; 78.20.Ci     

Precision Lamb shift measurement in hydrogenic nitrogen by fast beam laser spectroscopy

A measurement of the 2S_1/2–2P_3/2 (fine structure–Lamb shift) transition in N⁶⁺ is under way. The technique involves fast beam spectroscopy with a transverse interaction geometry and uses an isotopic CO₂ laser and a resonant build up cavity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Short-Range Order of the Rotational Degree of Freedom of the Glycine Molecule in Triglycine Sulfate Glass State

The unit cell of triglycine sulfate (TGS) contains the following glycine ions: dimers (GIISHBGIII)⁺ with a short hydrogen bond (SHB) and a monomer (SO₄LHBGI)⁺ with a long hydrogen bond (LHB) to the SO₄ ²⁻ anion. The spontaneous polarization results from a statistical charge distribution and the Coulomb interaction between SO₄ ²⁻ and fast rotating –NH₃ ⁺ groups. In the lamellar model, chains of –SO₄–(GIISHBGIII)–SO₄–(GIISHBGIII)–SO₄ lying along the polar b-axis are linked perpendicularly to this axis by SO₄LHBGI⁺ units. Tilting the LHBGI⁺ ion around the axis in the mirror plane decides on the direction of the charge displacement from this symmetry plane. The reversal of the spontaneous polarization, i.e., −P _S↔ +P _S, is related to the rotation of the NH₃ group. If this rotation becomes slowed down at low temperature and/or under high pressure, the coercive field increases dramatically. Application of an external electric field E _⊥ perpendicular to the b-axis leads to a hysteresis loop of the polarity P(E), and finally to its disappearance. This phenomenon comes from an ordering of protons in hydrogen bonds perpendicular to the polar axis. This is the first study of the E _⊥ effect by nuclear magnetic resonance. Authors' address: Jan Stankowski, Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, Poznań 60-179, Poland     

Inert gases’ collisional broadening and shift of thallium rydberg states

Summary The broadening and shift of thallium 6P _1/2–10P _1/2,3/2 transitions perturbed by argon and helium gas have been measured by two-photon Doppler-free absorption and ionization detection. Broadening and shift coefficients for He+Tl and Ar+Tl are obtained and van der Waal constants are determined.

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Riassunto L’allargemento e lo spostamento delle transizioni 6P _1/2–10P _1/2,3/2 del tallio disturbate dai gas argon ed elio sono statimisurrrati da un’osservazione di assorbimento e ionizzazione in assenza di effetto Doppler a due fotoni. Si ottengono i coefficienti di allargamento e spostamento per He+Tl e Ar+Tl e si determinano le costanti di van der Waals.

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Резюме Измеряются уширение и сдвиг 6P _1/2–10P _1/2,3/2 переходов в таллии, возмущенном аргоном и гелием. Получаются козффициенть уширения и сдвита для He+Tl и Ar+Tl. Определяются постояннйе ван дер Ваальса.

     

Mechanisms of broadening of emission lines of a laser-induced plasma generated on the surface of rigid targets

This article reports the results of an experimental investigation of the basic mechanisms that cause the broadening of emission lines of a laser-induced plasma generated on the surface of rigid targets in a gaseous atmosphere. The contribution of Stark and resonance mechanisms of emission-line broadening for the resonance doublet Al I 3s²3p²P_3/2,1/2-3s²4s²S_1/2 and triplet Mg I 3s3p³P _0,1,2 ⁰ -3s4s³S₁ is discussed. The concentrations of neutral Al atoms in the laser-induced plasma are evaluated by the magnitude of resonance line broadening. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 234–237, March–April, 2000.     

Depolarization of radiation upon coherent excitation

The possibility of cancelling the signal of linear polarization upon coherent excitation of 4s ³ S ₁ atoms via a Λ scheme from 3p ³ P ₁ and 3p ³ P ₂ levels is demonstrated using an ensemble of sulphur atoms as an example.     

Electrical and optical emission measurements of a capillary dielectric barrier discharge

We report a capillary dielectric barrier discharge (Cap-DBD) plasma operated in atmospheric pressure air. The plasma reactor consists of metal wire electrodes inside quartz capillary tubes powered with a low kilohertz frequency AC high voltage power supply. Various reactor geometries (planar, 3-D multilayer, and circular) with wall-to-wall separation ranging from zero up to 500 micron were investigated. For the electrical and spectral measurements, three reactors, each with six tubes, six inches in length, were assembled with gap widths of 500 micron, 225 micron, and 0 micron (i.e. tubes touching). The discharges appear homogenous across the whole device at separations below 225 micron and turned into filamentary discharges at larger gap spaces. The operating voltage was generally around 3–4 kV (rms). The power consumption by the Cap-DBD was calculated using voltage/charge Lissajous figures with observed powers of a few watts to a maximum of about 14 W for the reactor with no gap spacing. Further studies of optical emission spectroscopy (OES) were employed to evaluate the reactive species generated in the microplasma source. The observed emission spectrum was predominantly within the second positive system of N₂\mbox{N}_2(C³\mbox{C}^3 P_u\Pi_u–B³\mbox{B}^3 P_g\Pi_g) and the first negative system of N⁺₂\mbox{N}^+_2(B²\mbox{B}^2 S⁺_u\Sigma^+_u–X²\mbox{X}^2 S⁺_g\Sigma^+_g).     

Spectral characteristics of heterocyclic compounds with a chain structure, cooled in an ultrasonic jet

We have recorded the fluorescence excitation spectra of three heterocyclic compounds with a chain structure [BPO (2-phenyl-5-(4-diphenylyl)oxazole), POPOP (1,4-di[2-(5-phenyloxazolyl)]benzene, and TOPOT (1,4-di[2-(5-n-tolyloxazolyl)]benzene] and the fluorescence spectra of POPOP, under conditions where the molecules were cooled in an ultrasonic helium jet. A line structure is observed in the spectra of POPOP and TOPOT; for the BPO molecules, whose configuration changes considerably during electronic excitation, vibrational structure is apparent only in the low-frequency region of the excitation spectrum, and a diffuse spectrum is recorded starting from ν ₀ ⁰ + 200 cm⁻¹. For all the compounds, in the spectra we recorded vibrations with frequencies up to 100 cm⁻¹, arising due to the flexibility of the molecular structure. The rotational contours of the lines for the electronic and vibronic transitions of the POPOP molecules (T_rot = 10.5 K) and TOPOT molecules (T_rot = 15 K) are structureless and bell-shaped. The degree of polarization of the fluorescence P_fl for the jet-cooled POPOP molecules for excitation of vibrations along the absorption band up to 2000 cm⁻¹ above ν ₀ ⁰ is practically constant (∼8.4%) and matches P_fl for high-temperature vapors. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 728–734, November–December, 2006.     

Spectroscopic investigation of the plasma of a high-current diffuse discharge in He/Ar/CF2Cl2(CCl4) mixtures

Survey emission spectra in the region of 190–600 nm and time and service-life characteristics of a transverse nanosecond discharge in He/Ar/CF₂Cl₂(CCl₄) mixtures at a pressure of 10–100 kPa are investigated. In the emission spectra, excited products of the decomposition of freons—C₂(A−X), CN(B−X), Cl ₂ ^* , C^*, Cl^*, and Cl^+*— and the emission of ArF at λ=193 nm are revealed. The emissions of Cl ₂ ^* at λ=258 nm and ArF at λ=193 nm were the most intense. The discharge in the He/Ar/CF₂Cl₂ mixture is a multiwave emission source with λ=258 nm Cl ₂ ^* 193 nm ArF, and probably, 175 nm Arcl. It is of interest for applications in UV-VUV-range pulse photometry. The duration of the emission on Cl ₂ ^* , ArF, ArI, ClI, and ClII transitions in the discharge in the Ar/CF₂Cl₂ mixture (P=10–20 kPa) was 200–300 nsec. With adding He and increasing pressure to 100 kPa the duration of the emission decreased by a factor of 1.5–2. The basic mechanisms of the formation of Cl₂, ArF, and CN(B) molecules in the transverse-discharge plasma are considered. Uzhgorod State University, 46, Pidgirna Str., Uzhgorod, 294000, Ukraine. Translated from Zhurnal. Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 241–246, March–April, 1999.     

CARS study of the vibrational kinetics of nitrogen molecules in the burning and afterglow stages of a pulsed discharge

The vibrational kinetics of the nitrogen molecule in the ground state X ¹Σ _g ⁺ in the burning and afterglow stages of a pulsed discharge are investigated by coherent anti-Stokes Raman spectroscopy (CARS). The total cross section for vibrational excitation of the nitrogen molecule by electron impact to the first eight vibrational levels is determined. The rate constant for the associative ionization reaction involving nitrogen atoms in the metastable states ² P and ² D is estimated. It is found that the best agreement between the calculated and measured populations of the nitrogen molecules in the ground state X ¹Σ _g ⁺ in the afterglow stage of a pulsed discharge is obtained when the rate constant for VV exchange K ₀₁ ¹⁰ has the value predicted by the quantum-classical Billing-Fisher model. Zh. Tekh. Fiz. 67, 34–42 (May 1997)     

Polarized fluorescence of β-carotene and related polyenes

The spectral and polarization characteristics of trans-isomers ofβ-carotene, lycopene, retinal, and 1,6-diphenylhexatriene (DPH) in solutions have been investigated by the methods of polarized fluorescence and semiempirical calculations of AM1 and RM3. The high and constant values of the degree of polarization P ∼0.46, closely coinciding with the limiting value P₀=0.5, according to the S₂→S₀-fluorescence and S₂←S₀-fluorescence excitation spectra ofβ-carotene and lycopene in n-hexane at room temperature and also high and coinciding according to the S₁→S₀-fluorescence and S₂←S₀-fluorescence excitation spectra for retinal in EPA at 77 K and DPH in a polyvinylbutyral film, point to the coincidence of the directions of absorption and emission oscillators. These oscillators are aligned along the major axis of the polyene molecule, and the longwave S₂←S₀-absorption band is formed only by one electronic¹B _u ⁺ ←¹A _g ⁻ -transition. The oscillators of shortwave S_a←S₀-absorption bands are aligned at an angle to the oscillators of the S₂←S₀-transitions, which is related to the distortions in the structure of molecules (β-carotene, lycopene) or, as in the case of DPH, to the influence of transitions localized on the orbitals of phenyl rings and polarized perpendicularly to the major axis of the molecule. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 69–74, January–February, 2000.     

Lamb shift, fine structure and hyperfine structure in helium like ions by fast beam laser spectroscopy

Using Doppler tuned fast beam laser spectroscopy with a continuous wave CO₂ laser the following energy intervals in helium like ions have been measured: 1s2s ¹S₀–1s2p ³P^1,F, ³P₀ in ^14,15N⁵⁺; 1s2p ³P_2,F–1s2p ³P_1,F' in ¹⁹F⁷⁺; and 1s2p ³P₀–1s2p ³P₁ in ²⁴Mg¹⁰⁺. The results test modern relativistic and QED atomic theory. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of peripheral radial electric field on L-H transition in T-10 tokamak

In this paper the peripheral plasma behaviour during and after L-H transition on the tokamak T-10 is considered. Abrupt changes of Scrape-Off Layer (SOL) plasma parameters (ion saturation current, floating potential, electron temperature) are observed in regimes with transition to the improved plasma confinement. Such changes begin (5–150) ms prior to the L-H transition. The time delay of a transition depends on auxiliary plasma heating powerP _aux and plasma currentI _p (at constant toroidal magnetic field inductionB _t). Presented at the Workshop on the Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June, 2000. This work is supported by Ministry of Atomic Energy of Russia (contract 69F) and by Ministry of Science and Technology of Russia (Federal Program “Controlled Thermonuclear Fusion and Plasma Processes”).