Observation of high transverse momentum electrons at the CERN ISR

A search for high transverse momentum electrons directly produced in proton-proton collisions has been performed at the CERN ISR. The apparatus included a magnetic spectrometer with wire spark chambers, a lead-glass counter array, and a threshold gas ?erenkov counter. At a centre-of-mass energy √s = 52.7 GeV, a signal has been observed in the range $\text{1.6<p}{}_{\mathrm{T}}{}^1\text{<4.7}\text{GeV}\text{/c}$. The signal occurs at a level of approximately 10^?4 of the inclusive pion cross-section.     

Quinoxaline sensitised lanthanide ion luminescence: Syntheses,spectroscopy and X-ray crystal structure of Na{1,4,7-tris[(N-diethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane-10-(2-methylquinoxaline)}I3 C7H8

The synthesis of a quinoxaline-appended aza-macrocyclic ligand together with corresponding Ln^III complexes are described. The luminescence properties of the complexes show that the quinoxaline unit sensitises both visible (Eu^III) and near-IR (Nd^III and Yb^III) emitting lanthanide ions. UV–Vis absorption and time-resolved photophysical studies together with X-ray structural data suggest that as well as contributing chromophorically, the quinoxaline moiety generally participates in the first coordination sphere of Ln^III. The luminescent pH response of the Eu^III complex is also reported. The form of the steady state spectrum changes profoundly in the pH range 2–12, implying a change in the coordination environment, which was confirmed with time-resolved lifetime measurements that suggest an increase in europium inner sphere hydration at acidic pH.     

Cyanide-bridged Os(II)/Ln(III) coordination networks containing [Os(phen)(CN)4]2- as an energy donor: structural and photophysical properties

Slow evaporation of aqueous solutions containing mixtures of Na 2[Os(phen)(CN) 4], Ln(III) salts (Ln = Pr, Nd, Gd, Er, Yb), and (in some cases) an additional ligand such as 1,10-phenanthroline (phen) or 2,2'-bipyrimidine (bpym) afforded crystalline coordination networks in which the [Os(phen)(CN) 4] (2-) anions are coordinated to Ln(III) cations via Os-CN-Ln cyanide bridges. The additional diimine ligands, if present, also coordinate to the Ln(III) centers. Several types of structure have been identified by X-ray crystallographic studies. Photophysical studies showed that the characteristic emission of the [Os(phen)(CN) 4] (2-) chromophore, which occurs at approximately 680 nm in this type of coordination environment with a triplet metal-to-ligand charge transfer ( (3)MLCT) energy content of approximately 16 000 cm (-1), is quenched by energy transfer to those Ln(III) centers (Pr, Nd, Er, Yb) that have low-lying f-f states capable of accepting energy from the Os(II)-based (3)MLCT state. Time-resolved studies on the residual (partially quenched) Os(II)-based luminescence allowed the rates of Os(II) --> Ln(III) energy transfer to be evaluated. The measured rates varied substantially, having values of >5 x 10 (8), approximately 1 x 10 (8), and 2.5 x 10 (7) s (-1) for Ln = Nd, Er or Yb, and Pr, respectively. These differing rates of Os(II) --> Ln(III) energy transfer can be rationalized on the basis of the availability of f-f states of the different Ln(III) centers that are capable of acting as energy acceptors. In general, the rates of Os(II) --> Ln(III) energy transfer are an order of magnitude faster than the rates of Ru(II) --> Ln(III) energy transfer in a previously described series of [Ru(bipy)(CN) 4] (2-)/Ln(III) networks. This is ascribed principally to the lower energy of the Os(II)-based (3)MLCT state, which provides better spectroscopic overlap with the low-lying f-f states of the Ln(III) ions.     

Photophysical and structural properties of cyanoruthenate complexes of hexaazatriphenylene

The tritopic bridging ligand hexaazatriphenylene (HAT) has been used to prepare the mono-, di-, and trinuclear cyanoruthenate complexes [Ru(CN)(4)(HAT)](2-) ([1](2-)), [{Ru(CN)(4)}(2)(mu(2)-HAT)](4-) ([2](4-)), and [{Ru(CN)(4)}(3)(mu(3)-HAT)](6-) ([3](6-)). These complexes are of interest both for their photophysical properties and ability to act as sensitizers, associated with strong MLCT absorptions; and their structural properties, with up to 12 externally directed cyanide ligands at a single "node" for preparation of coordination networks. The complexes are strongly solvatochromic, with broad and intense MLCT absorption manifolds arising from the presence of low-lying pi* orbitals on the HAT ligand, as confirmed by DFT calculations; in aprotic solvents [3](6-) is a panchromatic absorber of visible light. Although nonluminescent in fluid solution, the lowest MLCT excited states have lifetimes in D(2)O of tens of nanoseconds and could be detected by time-resolved IR spectrosocopy. For dinuclear [2](4-) and trinuclear [3](6-) the TRIR spectra are indicative of asymmetric MLCT excited states containing distinct Ru(III) and Ru(II) centers on the IR time scale. The complexes show red (3)MLCT luminescence as solids and in EtOH/MeOH glass at 77 K. Ln(III) salts of [1](2-), [2](4-), and [3](6-) form infinite coordination networks based on Ru-CN-Ln bridges with a range of one-, two-, and three-dimensional polymeric structures. In the Yb(III) and Nd(III) salts of [3](6- )the complex anion forms an 8-connected node. Whereas all of the Gd(III) salts show strong (3)MLCT luminescence in the solid state, the Ru-based emission in the Nd(III) and Yb(III) analogues is substantially quenched by Ru --> Ln photoinduced energy transfer, which results in sensitized near-infrared luminescence from Yb(III) and Nd(III).     

Anthracene as a sensitiser for near-infrared luminescence in complexes of Nd(III), Er(III) and Yb(III): an unexpected sensitisation mechanism based on electron transfer

The ligand L(1), which contains a chelating 2-(2-pyridyl)benzimidazole (PB) unit with a pendant anthacenyl group An connected via a methylene spacer, (L(1) = PB-An), was used to prepare the 8-coordinate lanthanide(III) complexes [Ln(hfac)(3)(L(1))] (Ln = Nd, Gd, Er, Yb) which have been structurally characterised and all have a square antiprismatic N(2)O(6) coordination geometry. Whereas free L(1) displays typical anthracene-based fluorescence, this fluorescence is completely quenched in its complexes. The An group in L(1) acts as an antenna unit: in the complexes [Ln(hfac)(3)(L(1))] (Ln = Nd, Er, Yb) selective excitation of the anthracene results in sensitised near-infrared luminescence from the lanthanide centres with concomitant quenching of An fluorescence. Surprisingly, the anthracene fluorescence is also quenched even in the Gd(III) complex and in its Zn(II) adduct in which quenching via energy transfer to the metal centre is not possible. It is proposed that the quenching of anthracene fluorescence in coordinated L(1) arises due to intra-ligand photoinduced electron-transfer from the excited anthracene chromophore (1)An* to the coordinated PB unit generating a short-lived charge-separated state [An(.+)-PB(.-)] which collapses by back electron-transfer to give (3)An*. This electron-transfer step is only possible upon coordination of L(1) to the metal centre, which strongly increases the electron acceptor capability of the PB unit, such that (1)An* --> PB PET is endoergonic in free L(1) but exergonic in its complexes. Thus, rather than a conventional set of steps ((1)An* -->(3)An* --> Ln), the sensitization mechanism now includes (1)An* --> PB photoinduced electron transfer to generate charge-separated [An(.+)-PB(.-)], then back electron-transfer to generate (3)An* which finally sensitises the Ln(III) centre via energy transfer. The presence of (3)An* in L(1) and its complexes is confirmed by nanosecond transient absorption studies, which have also shown that the (3)An* lifetime in the Nd(III) complex matches the rise time of Nd-centred near-infrared emission, confirming that the final step of the sequence is (3)An* --> Ln(III) energy-transfer.     

Synthesis, structures and properties of a new series of platinum-diimine-dithiolate complexes

The new square-planar platinum-diimine-dithiolate compounds [Pt(mesBIAN)SS] have been synthesised {mesBIAN = bis(mesityl)biazanaphthenequinone; SS = 1,2-dithiooxalate (dto) , maleonitriledithiolate (mnt) , 1,2-benzenedithiolate (bdt) , 3,4-toluenedithiolate (tdt) and 1,3-dithia-2-thione-4,5-dithiolate (dmit) }, and the X-ray crystal structures of and determined. Cyclic voltammetry reveals that all the compounds form stable anions, and ESR spectroscopy of these anions shows that the SOMO is based upon the mesBIAN ligand; compounds also show a reversible oxidation wave in their CV. Computational studies reveal that charge-transfer processes from orbitals that are combinations of metal and dithiolate ligand to a mesBIAN pi-based LUMO are responsible for the low energy absorptions seen in the UV/visible spectra of these compounds, and that the reverse process is responsible for the observed room-temperature solution luminescence of [Pt(mesBIAN)Cl(2)] and , and . Compounds and , containing aromatic thiolates, were not found to luminesce under the same conditions. Resonance Raman experiments have shown the origin of band-broadening of the lowest-energy absorption band in the absorption spectra of to be due to vibronic structure within one electronic transition.     

Comparison of high-resolution MRI, optical microscopy and SEM for quantitation of trabecular architecture in the rat femur

Magnetic resonance imaging (MRI) has been used to analyze trabecular bone architecture in femur heads taken from adult Wistar rats. The aim of this study was to validate the use of MRI in assessing trabecular structure and morphology by comparing standard measures of bone morphology in the rat femur obtained from high resolution MRI with those obtained by conventional optical microscopy and by scanning electron microscopy (SEM). MR images were obtained on a Bruker 4.7 T micro-imaging system using a three-dimensional spin echo sequence with spatial resolution of 23 microm in-plane and a slice thickness of 39 microm. Optical images were obtained by de-calcifying the bone in EDTA and then sectioning 5-microm-thick slices. SEM images were obtained from bone embedded in epoxy resin with surface preparation by diamond polishing. Values of standard bone morphological parameters were compared and correlation coefficients between the MRI and the optical- and SEM-derived measures of morphology were calculated. Partial volume effects in MRI were minimized in this study by the use of very thin slices, yielding better agreement with optical- and SEM-derived measures of trabecular bone morphology than have been obtained in previous studies. Correlations between the MRI and optical data were significantly lower than those between the MRI and SEM data. Effects of de-calcification were also investigated. The results indicate that comparison of MRI with thin (de-calcified) optical images may be inherently flawed due to the destructive de-calcification and sectioning process used to prepare samples for the optical imaging.     

Lagrangian investigation of local extinction,re-ignition and auto-ignition in turbulent flames

Lagrangian PDF investigations are performed of the Sandia piloted flame E and the Cabra H₂/N₂ lifted flame to help develop a deeper understanding of local extinction, re-ignition and auto-ignition in these flames, and of the PDF models' abilities to represent these phenomena. Lagrangian particle time series are extracted from the PDF model calculations and are analyzed. In the analysis of the results for flame E, the particle trajectories are divided into two groups: continuous burning and local extinction. For each group, the trajectories are further sub-divided based on the particles' origin: the fuel stream, the oxidizer stream, the pilot stream, and the intermediate region. The PDF calculations are performed using each of three commonly used models of molecular mixing, namely the EMST, IEM and modified Curl mixing models. The calculations with different mixing models reproduce the local extinction and re-ignition processes observed in flame E reasonably well. The particle behavior produced by the IEM and modified Curl models is different from that produced by the EMST model, i.e., the temperature drops prior to (and sometimes during) re-ignition. Two different re-ignition mechanisms are identified for flame E: auto-ignition and mixing-reaction. In the Cabra H₂/N₂ lifted flame, the particle trajectories are divided into different categories based on the particles' origin: the fuel stream, the oxidizer stream, and the intermediate region. The calculations reproduce the whole auto-ignition process reasonably well for the Cabra flame. Four stages of combustion in the Cabra flame are identified in the calculations by the different mixing models, i.e., pure mixing, auto-ignition, mixing-ignition, and fully burnt, although the individual particle behavior by the IEM and modified Curl models is different from that by the EMST model. The relative importance of mixing and reaction during re-ignition and auto-ignition are quantified for the IEM model.     

A numerical study of auto-ignition in turbulent lifted flames issuing into a vitiated co-flow

This paper presents a numerical study of auto-ignition in simple jets of a hydrogen–nitrogen mixture issuing into a vitiated co-flowing stream. The stabilization region of these flames is complex and, depending on the flow conditions, may undergo a transition from auto-ignition to premixed flame propagation. The objective of this paper is to develop numerical indicators for identifying such behavior, first in well-known simple test cases and then in the lifted turbulent flames. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD code, FLUENT. The in-situ-adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. A simple k–ε turbulence model is used for turbulence along with a low Reynolds number model close to the solid walls of the fuel pipe.

The first indicator is based on an analysis of the species transport with respect to the budget of convection, diffusion and chemical reaction terms. This is a powerful tool for investigating aspects of turbulent combustion that would otherwise be prohibitive or impossible to examine experimentally. Reaction balanced by convection with minimal axial diffusion is taken as an indicator of auto-ignition while a diffusive–reactive balance, preceded by a convective–diffusive balanced pre-heat zone, is representative of a premixed flame. The second indicator is the relative location of the onset of creation of certain radical species such as HO₂ ahead of the flame zone. The buildup of HO₂ prior to the creation of H, O and OH is taken as another indicator of autoignition.

The paper first confirms the relevance of these indicators with respect to two simple test cases representing clear auto-ignition and premixed flame propagation. Three turbulent lifted flames are then investigated and the presence of auto-ignition is identified. These numerical tools are essential in providing valuable insights into the stabilization behaviour of these flames, and the demarcation between processes of auto-ignition and premixed flame propagation.     

Structural, spectroscopic and redox properties of uranyl complexes with a maleonitrile containing ligand

The reaction of uranyl nitrate hexahydrate with the maleonitrile containing Schiff base 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (salmnt((Et(2)N)(2))H(2)) in methanol produces [UO(2)(salmnt((Et2N)2))(H(2)O)] (1) where the uranyl equatorial coordination plane is completed by the N(2)O(2) tetradentate cavity of the (salmnt((Et(2)N)(2)))(2-) ligand and a water molecule. The coordinated water molecule readily undergoes exchange with pyridine (py), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF) and triphenylphosphine oxide (TPPO) to give a series of [UO(2)(salmnt((Et(2)N)(2)))(L)] complexes (L = py, DMSO, DMF, TPPO; 2-5, respectively). X-Ray crystallography of 1-5 show that the (salmnt((Et(2)N)(2)))(2-) ligand is distorted when coordinated to the uranyl moiety, in contrast to the planar structure observed for the free protonated ligand (salmnt((Et(2)N)(2))H(2)). The Raman spectra of 1-5 only display extremely weak bands (819-828 cm(-1)) that can be assigned to the typically symmetric O=U=O stretch. This stretching mode is also observed in the infrared spectra for all complexes 1-5 (818-826 cm(-1)) predominantly caused by the distortion of the tetradentate (salmnt((Et(2)N)(2)))(2-) ligand about the uranyl equatorial plane resulting in a change in dipole for this bond stretch. The solution behaviour of 2-5 was studied using NMR, electronic absorption and emission spectroscopy, and cyclic voltammetry. Complexes 2-5 exhibit intense absorptions in the visible region of the spectrum due to intramolecular charge transfer (ICT) transitions and the luminescence lifetimes (< 5 ns) indicate the emission arises from ligand-centred excited states. Reversible redox processes assigned to the {UO(2)}(2+)/{UO(2)}(+) couple are observed for complexes 2-5 (2: E(1/2) = -1.80 V; 3,5: E(1/2) = -1.78 V; 4: E(1/2) = -1.81 V : vs. ferrocenium/ferrocene {Fc(+)/Fc}, 0.1 M Bu(4)NPF(6)) in dichloromethane (DCM). These are some of the most negative half potentials for the {UO(2)}(2+)/{UO(2)}(+) couple observed to date and indicate the strong electron donating nature of the (salmnt((Et(2)N)(2)))(2-) ligand. Multiple uranyl redox processes are clearly seen for [UO(2)(salmnt((Et(2)N)(2)))(L)] in L (L = py, DMSO, DMF; 2-4: 0.1 M Bu(4)NPF(6)) indicating the relative instability of these complexes when competing ligands are present, but the reversible {UO(2)}(2+)/{UO(2)}(+) couple for the intact complexes can still be assigned and shows the position of this couple can be modulated by the solvation environment. Several redox processes were also observed between +0.2 and +1.2 V (vs. Fc(+)/Fc) that prove the redox active nature of the maleonitrile-containing ligand.