Benzo[a]azulenediones and 10,10′‐Bibenzo[a]azulene

The oxidation of benzo[a]azulene ( 4 ) with commercial MnO₂ in dioxane/H₂O leads to a number of products in low yield (Table 1). Treatment of 4 with ‘mild’ MnO₂ (MnO₂/C) in dioxane/5% H₂O results in the formation of 10,10′‐bibenzo[a]azulene ( 18 ) in yields of up to 59% of isolated and purified material. Compound 18 exhibits atropisomerism and can be separated by HPLC on a Chiralcel column at room temperature into its stable antipodes (Fig.).     

[ReF6]2−: A Robust Module for the Design of Molecule‐Based Magnetic Materials

A facile synthesis of the [ReF₆]^2? ion and its use as a building block to synthesize magnetic systems are reported. Using dc and ac magnetic susceptibility measurements, INS and EPR spectroscopies, the magnetic properties of the isolated [ReF₆]^2? unit in (PPh₄)₂[ReF₆]?2 H₂O ( 1 ) have been fully studied including the slow relaxation of the magnetization observed below ca. 4 K. This slow dynamic is preserved for the one‐dimensional coordination polymer [Zn(viz)₄(ReF₆)]_∞ ( 2 , viz=1‐vinylimidazole), demonstrating the irrelevance of low symmetry for such magnetization dynamics in systems with easy‐plane‐type anisotropy. The ability of fluoride to mediate significant exchange interactions is exemplified by the isostructural [Ni(viz)₄(ReF₆)]_∞ ( 3 ) analogue in which the ferromagnetic Ni^II–Re^IV interaction (+10.8 cm^?1) dwarfs the coupling present in related cyanide‐bridged systems. These results reveal [ReF₆]^2? to be an unique new module for the design of molecule‐based magnetic materials.     

Si/C/F/H materials from laser-explosive decomposition of fluoromethylsilanes

Solid Si/C/H/F materials laser-deposited from gaseous fluoro-methylsilanes are examined by XPS, SEM, IR and Raman spectroscopy and revealed to be a blend of silicon-carbon-based frameworks, possessing some Si-H and C-H bonds, and carbon whose modification is dependent on the structure of the parent fluoromethylsilane. Their properties are affected by contact with the atmosphere: although incorporation of oxygen into silicon-carbon frameworks is a very fast process, the dramatically improved adhesion to aluminium requires long exposure periods.     

Optical parametric oscillator based difference frequency laser source for photoacoustic trace gas spectroscopy in the 3 μm mid-IR range

We report on the extension of the emission wavelength range of a nanosecond pulsed commercial optical parametric oscillator (OPO) towards the infrared. By difference frequency mixing the idler of the OPO with the fundamental of its Nd:YAG pump laser the wavelength range from 2.5–4.5 μm is covered at considerable pulse energies. This laser source is then applied for the first time to photoacoustic trace gas spectroscopy in the fundamental C---H stretch band located around 3.3 μm. Spectra of single- and multi-component gas mixtures of volatile organic compounds are recorded and analyzed with respect to composition and concentration. The detection limits for the individual substances within the gas mixtures are found in the lower parts per million (ppm) range.     

Temperature-dependent photoacoustic spectroscopy with a Helmholtz resonator

We describe and example the Beam Propagation Method (BPM) used to model and simulate nonlinear refractive and absorptive effects in materials with applications to optical limiting and switching. Various scenarios including laser-beam trapping and laser-beam division are investigated, in order to demonstrate the power of the BPM. A novel technique is also described for efficiently modelling the external far-field propagation from nonlinear media, including the propagation of non-Gaussian-shaped spatial profiles. The methods are finally combined with the phenomenon of nonlinear absorption to demonstrate enhanced power limiting in the presence of self-refraction. Optimal parameters for high-fluence power-limiting are subsequently discussed.     

Theoretical insights into the magnetostructural correlations in Mn3-based single-molecule magnets

Density functional theory (DFT) and the valence bond configuration interaction (VBCI) model have been applied to the oximato-based Mn(III)(3)O single-molecule magnets (SMMs), allowing one to correlate the Mn(III)-Mn(III) exchange coupling energy (J) with the bridging geometry in terms of two structural angles: the Mn-O-N-Mn torsion angle (γ) and the Mn(3) out-of-plane shift of O (angle δθ). Using DFT, a two-dimensional (γ, δθ) energy surface of J is derived and shown to yield essentially good agreement with the reported J values deduced from magnetic susceptibility data on trigonal oximato-bridged Mn(3) SMMs. VBCI is used to understand and analyze the DFT results. It is shown that the exchange coupling in these systems is governed by a spin-polarization mechanism inducing a pronounced and dominating ferromagnetic exchange via the oximato bridge as opposed to kinetic exchange, which favors a weaker and antiferromagnetic exchange via the bridging oxide. In the light of these results, a discussion of the exchange coupling in the Mn(6) family of the SMM with a record demagnetization barrier is given.     

Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for the possible application in cavity-enhanced absorption spectroscopy. In order to estimate the noise level of an absorbance baseline, the triplet of frequency modulated light, i.e. the central laser frequency and the two sidebands, were transmitted through both the V-cavity in open air and a 1.5-cm long optical cell placed behind the cavity output mirror and filled with acetylene (C₂H₂) at low pressure. The performance of the setup was evaluated from the measured relative intensity noise on the cavity output (normalised by the bandwidth) and the frequency modulation absorption signals induced by C₂H₂ absorption in the 1.5-cm cell. From these data, we estimate that the noise-equivalent absorption sensitivity of 2.1 × 10^?11 cm^?1 Hz^?1/2—by a factor of 11.7 above the shot-noise limit—can be achieved for C₂H₂ absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM₀₀ resonances.     

Trace gas monitoring with infrared laser-based detection schemes

The success of laser-based trace gas sensing techniques crucially depends on the availability and performance of tunable laser sources combined with appropriate detection schemes. Besides near-infrared diode lasers, continuously tunable midinfrared quantum cascade lasers and nonlinear optical laser sources are preferentially employed today. Detection schemes are based on sensitive absorption measurements and comprise direct absorption in multi-pass cells as well as photoacoustic and cavity ringdown techniques in various configurations. We illustrate the performance of several systems implemented in our laboratory. These include time-resolved multicomponent traffic emission measurements with a mobile CO₂-laser photoacoustic system, a diode-laser based cavity ringdown device for measurements of impurities in industrial process control, isotope ratio measurements with a difference frequency (DFG) laser source combined with balanced path length detection, detection of methylamines for breath analysis with both a near-IR diode laser and a DFG source, and finally, acetone measurements with a heatable multipass cell intended for vapor phase studies on doping agents in urine samples. PACS 33.20.Ea; 42.62.Fi; 42.72.Ai; 87.64.km; 92.60.Sz     

Eine Anwendung der K-Theorie in der Theorie der H-Räume

Ohne Zusammenfassung