Saturation of resonant frequency conversion processes in cadmium vapour

We have investigated two-photon resonant up conversion of 375 to 159 nm coherent radiation using the four-frequency process ₁₅₉=2₃₇₅+₁₀₄₀ in cadmium vapour, where the ir component (1040 nm) is internally generated by the 375 nm pump tuned to the Cd two-photon transition 5s ^{2 1} S ₀5s6s ¹ S ₀. Scaling laws and tuning behaviour of both the 159 and 1040 nm output power were measured up to 1×10⁸ W cm^–2 pump intensity and 2×10¹⁷ cm^–3 Cd number density. The results are compared to numerical calculations based on semiclassical theory in a stationary perturbation approximation up to third order. Here, in accordance with experimental results, the ir component was assumed to originate from a superposition of stimulated emission and parametric generation. The observed saturation effects turned out to be mainly due to population changes of the atomic levels involved and are qualitatively reproduced by calculations. Discrepancies with respect to absolute output levels are discussed in terms of the approximations in the theoretical formalism.     

A new torsion-rotation fitting program for molecules with a sixfold barrier: Application to the microwave spectrum of toluene

A new program is described for fitting rotation-torsion energy levels in molecules like toluene, in which the frame (C₆H₅) has C_2v symmetry and the methyl top has C_3v symmetry, i.e., for molecules where the internal rotation barrier is expanded in cos6nα, where α is the internal rotation angle and n = 1,2,…. The program is based on the theoretical framework developed by Sørensen and Pedersen in their application of the Longuet-Higgins permutation-inversion group G₁₂ to the microwave spectrum of CH₃NO₂. It is specifically designed for sixfold barrier molecules, and allows the user to select almost any symmetry-allowed torsion-rotation term for inclusion in the fitting Hamiltonian. This program leads to a very successful fit of transitions in the microwave spectrum of toluene characterized by J ? 30, K_a ? 12, and by the free-rotor quantum number ∣m∣ ? 3. In these fits we included both published and rather extensive unpublished new measurements, for which fits using other torsion-rotation programs have not been very successful. The fit presented here uses 28 parameters to give an overall standard deviation of 7.4 kHz for 372 line frequencies, and results in a much improved value for the sixfold barrier for toluene, V₆ = 13.832068(3) cal mol⁻¹.     

Transportation of megawatt millijoule laser pulses via optical fibers?

Laser ignition is considered to be one of the most promising future concepts for internal combustion engines. It combines the legally required reduction of pollutant emissions and higher engine efficiencies. The igniting plasma is generated by a focused pulsed laser beam. Having pulse durations of a few nanoseconds, the pulse energy E _p for reliable ignition amounts to the order of 10 mJ. Different methods of laser ignition with an emphasis on fiber-based systems will be discussed and evaluated.     

Quasi-free contributions in the6Li(d,tp)4He-reaction atE d =0.465 MeV

The⁶Li(d, tp)⁴He reaction was investigated at a bombarding energy ofE _d =0.465 MeV in a kinematically complete experiment. Evidence was found for a quasi-free reaction.     

Perfect double covers with paths of length four

It is shown that for any 4-regular graph G there is a collection F of paths of length 4 such that each edge of G belongs to exactly two of the paths and each vertex of G occurs exactly twice as an endvertex of a path of F. This proves a special case of a conjecture of Bondy. © 1996 John Wiley & Sons, Inc.     

(Diazoalkyl)Phosphoranes from [4 + 1]-cycloadditions of tetrahalo-orthobenzoquinones to (diazoalkyl)phosphanes [1]

The (diazoalkyl)phosphanes 4a–e undergo [4 + 1]-cycloaddition reactions with the tetrahalo-orthobenzoquinones 5a,b to furnish the previously unknown (diazoalkyl)phosphoranes 6a–h . The structure of product 6h has been investigated by X-ray crystallography.     

Swelling and fractal heterogeneities in networks

In this paper we revisit old swelling data on polymer networks that have not been interpreted theoretically on a closed molecular basis. If the osmotic pressure of the swollen network is compared to the osmotic pressure of the corresponding uncrosslinked solution unsolved problems appear, when the relative osmotic pressure is plotted against the degree of swelling, i.e. the deformation due to swelling. A significant maximum appears which cannot be explained by any of the recently derived elastic models, such as junction constraint or other entanglement models. It is suggested in this paper that the maximum is a consequence of structural heterogeneities of fractal nature. If such fractal heterogeneities are assumed a strong maximum in the relative osmotic pressure can be reproduced. The physical reason is the different thermodynamic behavior of uncrosslinked linear chains and crosslinked self-similar (non-linear) polymers. The conclusion is supported by numerical (Monte Carlo) simulations.     

Effect of filler networking on the dynamic mechanical properties of crosslinked polymer solids

The strain amplitude dependence of the viscoelastic properties of eight different carbon blacks dispersed in two different rubbery networks is investigated and evaluated within the Cole-Cole approach for the complex elastic modulus at fixed temperature and frequency. This approach is based on the Kraus model of strain-dependent filler agglomeration-deagglomeration rates. We find a non-integer universal shape exponent of the Cole-Cole process that obviously depends only on the surrounding polymer matrix but is independent of the filler grade. The exponent can be related to the fractal dimension of the carbon black surface.