Nonresonant ionization of oxygen molecules by femtosecond pulses: plasma dynamics studied by time-resolved terahertz spectroscopy

We show that optical pump-terahertz probe spectroscopy is a direct experimental tool for exploring laser-induced ionization and plasma formation in gases. Plasma was produced in gaseous oxygen by focused amplified femtosecond pulses. The ionization mechanisms at 400- and 800-nm excitation wavelengths differ significantly being primarily of a multiphoton character in the former case and a strong-field process in the latter case. The generation of the plasma in the focal volume of the laser and its expansion on subnanosecond time scale is directly monitored through its density-dependent susceptibility. A Drude model used to evaluate the plasma densities and electron-scattering rates successfully captures the observations for a wide range of pump intensities. In addition, rotational fingerprints of molecular and ionic species were also observed in the spectra.     

Conformations of the 10-membered Ring in 5, 10-Secosteroids. II. (E)-3α-Acetoxy-5, 10-seco-1 (10)-cholesten-5-one and (E)-5, 10-seco-1 (10)-cholestene-3, 5-dione

(E)-3α-Acetoxy-5, 10-seco-1(10)-cholesten-5-one ( 3 ) was synthesized by fragmentation of 3α-acetoxy-5α-cholestan-5-ol ( 1 ) using the photochemical version [3] of the lead tetraacetate reaction [4], and transformed into the corresponding 3-oxo-compound ( 5 ). Two conformations ( A and B ) were deduced for the 10-membered ring of 3 by analysis of the ¹H- and ¹³C-NMR. spectra in toluene. The major conformation ( A ) corresponds to that found in the solid state by X-ray analysis. According to its NMR. spectra in toluene, the medium-sized ring of the diketone 5 exists also predominantly in two conformations, the major one being analogous to A (the solid-state conformation of the 3β-acetoxy isomer ( 9 ) [1]) and the minor one to A (see above). The stereochemistry of the acidcatalyzed and thermal cyclisations of 3 as well as of the corresponding 5-oxime is discussed in terms of conformational factors.     

Determination of phenols with ICN and BrCN in nonaqueous media

Phenol, cresols, ethylphenols, thymol, catechol, resorcinol, hydroquinone, phloroglucinol and β-naphthol have been titrimetrically determined with ICN and BrCN in 10–100% acetic acid and 1:1 glacial acetic acid-acetic anhydride mixture. The phenols undergo substitution; the extent of which depends upon the entire system involved. As little as 0.5 mg of the phenol can be determined with ±0.25% error in these nonaqueous media.     

Reaktionen von Steroiden mit Dialkylaminoschwefeltrifluoriden. I. 11β-Hydroxysteroide. Über STeroide, 233. Mitteilung

Reactions of Steroids with Dialkylaminosulfur Trifluorides. I. 11β-Hydroxysteroids. Reactions of three types of 11β-hydroxy steroids with dialkylaminosulfur trifluorides (DAST) have been studied. 9Δ-unsubstituted 11-alcohols are dehydrated with DAST to Δ⁹⁽¹¹⁾-unsaturated compounds under very mild conditions. 9α-Chloro-11β-hydroxy steroids are quantitatively fluorinated in position 11 yielding the known 9α-chloro-11β-fluoro derivatives. Finally, 9α, 11Δ-fluorohydrins at 0–25° are selectively transformed with DAST into Δ¹¹-9α-fluorides. When the latter reaction is run at lower temperature, the corresponding 11-piperidinesulfinates, products of partial hydrolysis of the alkoxy-dialkylaminosulfur difluoride intermediates postulated by Middleton, can be isolated in high yields. A rational explanation of the results is presented.     

Poly(phenylenediamine) Dispersions

The formation and properties of poly(phenylenediamine) colloidal dispersions were investigated. Oxidative dispersion polymerization of 1,3-phenylenediamine dihydrochloride stabilized with poly(N-vinylpyrrolidone) was taken as a reference experiment. Conductivity, temperature, and acidity of the reaction mixture were recorded during the polymerization. Oxidations of all three phenylenediamine bases, of corresponding dihydrochlorides, and of dihydrochlorides in excess acid are compared. The effect of the nature of steric stabilizer on the course of polymerization was found to be marginal. Dispersion polymerization was observed to proceed faster than the precipitation one. Dynamic light scattering was used to assess particle sizes in poly(phenylenediamine) dispersions. The results are discussed in relation to an analogous polymerization of aniline leading to polyaniline dispersions. Copyright 2001 Academic Press.     

(1)H NMR and small-angle neutron scattering investigation of the structure and solubilization behavior of three-layer nanoparticles

Three-layer nanoparticles were prepared by radiation-induced polymerization of 1-10 g/L of methyl methacrylate dissolved in a 0.1 wt % D(2)O solution of polystyrene-poly(methacrylic acid) (PS-PMA) micelles. According to NMR and small-angle neutron scattering (SANS), most of the poly(methyl methacrylate) (PMMA) is adsorbed at the core-shell interface of the particles. A small fraction of shorter PMMA probably sticks to outer parts of the PMA chains. The absorption kinetics and equilibria of benzene and chloroform were studied by NMR and SANS time-resolved experiments. The diffusion front in the PS core is very narrow but quite broad in the PMMA sheet suggesting, thus, a less compact state of PMMA. According to SANS, the diffusion kinetics is almost independent of the PMMA sheet thickness. In contrast to it, the absorption capacity, reflected by both SANS and NMR, increases markedly with the PMMA content in the particle. The maximum amount of solubilized compound depends on its positive interaction with PMMA (expressed by the chi parameter) but is restricted by the growing interface tension between swollen PMMA and D(2)O. In accordance with this conclusion, a particle saturated with benzene can absorb chloroform only at the expense of a part of benzene expelled into the surrounding medium and vice versa. Starting with 10 g PMMA/L (10 times the weight of the original micelles), the particles become unstable when being swollen with a good solvent.     

Chemie der Seltenerdmetalle, 7. Mitt.: Tartratkomplexe des dreiwertigen Cers

Zusammenfassung Es wurde die systematische Untersuchung des Systems im sauren Bereich durchgeführt. Es wurden Stoffe der Zusammensetzung Ce₂(H₂ T)₃·6H₂O, Ce₂(H₂ T)₃·2H₂O, H[Ce(H₂ T)₂]·2H₂O isoliert^***. Diese wurden röntgenographisch untersucht und es wurde ihre thermische Analyse durchgeführt. Auf Grund von pH-Messungen wurden die Dissoziationskonstanten bestimmt.

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The systems were investigated in the acidic range. The compounds Ce₂(H₂ T)₃·6 H₂O, Ce₂(H₂ T)₃·2 H₂O and H[Ce(H₂ T)₂]·2 H₂O^*** were isolated. X-ray powder diagrams were taken, and a thermal analysis performed. The dissociation constants were determined, based upon pH-measurements.

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Mit 2 Abbildungen

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6. Mitt.:F. Bezina, J. Rosický undR. Pastorek, Acta Univ. Palack., im Druck.