Evaluation of a Continuous Flow Furnance Atomizer for Laser Excited Atomic Fluorescence Spectrometry

A system for continuous sample introduction into a ceramic atomization furnace for laser excited atomic fluorescence spectrometry (LEAFS) was developed. The system was evaluated using two different excitation sources, a nitrogen laser-pumped dye laser and a copper vapor laser-pumped dye laser. Pure aqueous Pb solutions were used for system optimization and analytical calibration curves. These curves were linear over at least three     

Charge-transfer pipi* excited state in the 7-azaindole dimer. A hybrid configuration interactions singles/time-dependent density functional theory description

The hybrid configuration interaction singles/time dependent density functional theory approach of Dreuw and Head-Gordon [Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007] has been applied to study the potential energy landscape and accessibility of the charge-transfer pipi* excited state in the dimer of 7-azaindole, which has been traditionally considered a model for DNA base pairing. It is found that the charge-transfer pipi* excited state preferentially stabilizes the product of a single proton transfer. In this situation, the crossing between this state and the photoactive electronic state of the dimer is accessible. It is found that the charge-transfer pipi* excited state has a very steep potential energy profile with respect to any single proton-transfer coordinate and, in contrast, an extremely flat potential energy profile with respect to the stretch of the single proton-transfer complex. This is predicted to bring about a pair of rare fragments of the 7-azaindole dimer, physically separated and hence having very long lifetimes. This could have implications in the DNA base pairs of which the system is an analogue, in the form of replication errors.     

4,6-bis(supermesitylphosphanylidenemethyl)dibenzofuran. Synthesis, X-ray structure and reactivity towards group 11 metals

New kinetically stabilized mono- and bis-phosphaalkene ligands (2 and 3, respectively) were synthesized via the phospha-Wittig approach. Ligand 3 was characterized by X-ray diffraction. The coordinating behaviour of the bidentate ligand was investigated towards group 11 metal centers in order to test its capacity to bind two coordination sites located in a trans-fashion. The [Au(3)][BF4], [Ag(3)(H2O)][BF4] and [Cu(3)(CH3CN)][BF4] complexes 4 , 5 and 6, respectively, were characterized by X-ray diffraction. The peculiar geometries of 4 and 6 were rationalized by means of DFT calculations.     

Optical phase conjugation and related experiments with surface plasma waves

We report on the experimental observation of optical phase conjugation using surface plasma waves in two different configurations: one involving four surface plasma waves and the other one involving two surface pump waves and bulk probe and conjugate waves. The major nonlinear contribution comes from heating of the metal film. In a related experiment, we have also observed the coupling of a bulk probe wave to the corresponding surface wave through the grating created by two counter-propagating surface plasma waves. In this case, we could observe the creation of surface acoustical waves.     

Emission spectroscopy detection of N2 active species in plasma nitriding process

The present paper is a review of recent works related to the optical emission spectroscopy applied to the determination of ground state densities of N atoms in Ar-N₂ flowing post-discharges. The effect of small quantities of H₂ in the Ar-N₂ discharge on N atom densities within the post-discharge has been analysed. The authors demonstrate that optical emission of Na impurity may be applied to the determination of the vibrational temperature of N₂(X,v) states at the temperatures usual for nitriding of metals in Ar-N₂ post-discharge. From the diagnostic of Ar-N₂ post-discharges, it is clearly specified that nitriding of iron base alloys in a flowing post-discharge reactor is originating in N atoms, especially when a few H₂ molecules are admixed into the Ar-N₂ discharge. Finally, correlation between the N-atom density and the thickness of the iron nitrided layers when H₂ is introduced into the Ar-N₂ discharge are given. Dedicated to Prof. Jan Janča on the occasion of his 60^th birthday.     

The optical kerr effect in small metal particles and metal colloids: The case of gold

We first consider theoretically the various mechanisms contributing to the Kerr nonlinearity in small gold particles. The major ones are the conduction electron intraband contribution, the saturation of direct interband transitions and the change in dielectric constant due to hot photoexcited electrons. We present experimental results obtained using optical phase conjugation in gold-doped glasses. By varying several parameters, we were able to ascertain the origin of the nonlinear response: the main contributions are the hotelectron and the interband contributions. All experimental results, including saturation behaviour, are fully understood.     

Evidence for dynamic behavior of O2 in oxyheme model compounds

We have performed Mössbauer studies on several oxy-heme model compounds, and for two of them we also have derived the threedimensional structure from X-ray studies; I: [Fe(O₂) (SC₆HF₄) TP_pivP] [Na?18C6] and II: [Fe(O₂) (SC₆HF₄)TP_pivP] [K?222]. Compound I shows a pronounced temperature-dependence of ΔE_Q, i. e. 2.17 mms^?1 at 4.2 K and 1.55 mms^?1 at 295 K, while ΔE_Q of compound II is practically temperature-independent, i.e. ca. 2.05 mms^?1. The X-ray structure analysis of these model compounds provides us with the fact that O₂ occupies three different sites in I and only two sites in II. From the experimentally known occupancies of these sites at specific temperatures we were able to apply relaxation calculations for the electric field gradient tensors in I and II, and we found for both compounds that our calculated ΔE_Q(T)-data are in agreement with our experimental results. These findings imply that the dynamic jump behavior of O₂ among the different sites in oxy-heme model compounds is responsible for the temperature-dependence (like in HbO₂) or temperature-independence of ΔE_Q (like in cytochrome P450).     

Distribution Radiale de la Densité des Atomes Métastables dans les Plasmas Produits par un Champ Electrique Continu ou de Haute-Fréquence

The radial density distribution of metastable Ar(³P₂) and resonant Ar(¹P₁) atoms is determined experimentally in two types of electrical discharges of cylindrical shape: the positive column plasma of a DC discharge and the plasma produced by an electromagnetic surface wave. The exitation and deexcitation rates for Ar(³P₂) by electron collisions are determined as a function of radius from the measured radial population density profile using a population density balance equation. These rate coefficients are obtained for various electron density values on the axis. The published values for these coefficients in the positive column plasma assume that they are independent of the discharge current. In this work, it is shown that these coefficients actually decrease as the electron density increases. In a more general way, the results obtained indicate that the examination of the radial density distributions of exited atoms is a powerful method for determining the kinetics of their creation and destruction.     

A theoretical study of the ground and first excited singlet state proton transfer reaction in isolated 7-azaindole–water complexes

A systematic study of the proton transfer in the 7-azaindole–water clusters (7-AI(H₂O)_n; n=1–4) in both the ground and first excited singlet electronic states is undertaken. DFT(B3LYP) calculations for the ground electronic state shows that the more stable geometry of the initial normal tautomer presents a cyclic set of hydrogen bonds that links the two nitrogen atoms of the base across the waters. For the n=4 cluster the water molecules adopt a double ring structure so that two cycles of hydrogen bonds are found there. From this structure full tautomerization implies only one transition state so that a concerted but non-synchronous process is predicted by our theoretical calculations. This behavior is found both in the ground and the excited states where CIS geometry optimizations and TD(B3LYP) energy calculations are performed. The difference between both states is the height of the energy barrier that is much lower in the excited state. Another clear difference between both electronic states is that full tautomerization is an endergonic process in the ground state whereas it is clearly exergonic (then favorable) in the excited state. This is so because electronic excitation implies a charge transfer from the five-member cycle to the six-member one of 7-azaindole so that the proton transfer from the pyrrolic side to the pyridinic one is favored. These results clearly indicate that full tautomerization will not likely occur in the ground state but it will be quite easy (and fast) in the excited state. Reaction is already feasible in the S₁ 1:1 complex but it is faster in the 1:2 complex. However the reaction slows again for the 1:3 complex and, finally, reaches a new maximum for the largest cluster studied here, the n=4 case. These results, which are in agreement with experimental data, are explained in terms of the number of hydrogen bonds that are involved in the transfer. The proton transfer through a ring formed by the substrate and two water molecules is found to be the more efficient one, at least in this system.