Optical pumping in an organic crystal: quinoxaline in durene

Experiments have been performed to determine the path of entry into and exit from the phosphorescent triplet state T₀ of quinoxaline in a durene host. First of all the decay of phosphorescence after flash excitation was followed at 4.2 and 1.34 °K. It was found that for both perdeutero- and perhydroquinoxaline the lifetime is shortened by a factor of about three when the temperature is lowered from 4.2 to 1.34 °K. At 1.34 °K relaxation between the spin components (i.e. re-orientation of the triplet spin angular momentum) is slow relative to the decay, and the observed reduction in lifetime indicates that entry into and exit from T₀, are through the same spin component. Similar decay experiments were then carried out at 1.34 °K in a 10 kG magnetic field or in a somewhat weaker field so chosen that the effect of microwave saturation of one of the E.S.R. transitions between the components could be observed. From the results it follows that on intersystem crossing the molecules enter the manifold T₀ through the top zero-field component and thus initially have their spins aligned. Decay departs almost exclusively from the same component, even in the case of perhydroquinoxaline, where at least 45 per cent of it must be radiationless. The decay route agrees with out-of-plane polarization of phosphorescence for the free molecule. Finally, spin alignment on intersystem crossing is discussed from the theoretical point of view. It appears that the phenomenon is clear-cut only in molecules such as those of the aza-aromatics, where strong spin-orbit coupling of the atomic type occurs betwen ππ? and nπ? states.     

Phase separation in mixtures of colloidal platelets and non-adsorbing polymer: a scaled particle treatment

Scaled particle theory is used to calculate the equation of state for and the free volume accessible to a non-adsorbing polymer, represented by spheres, in a system of colloidal platelets, represented by cut spheres. At low densities the predictions agree very well with computer simulations. For infinitely thin discs, the predictions agree with existing simulation data for the isotropic phase. The analytical results are used to predict the isotropic-isotropic phase boundary as a function of platelet aspect ratio and polymer/colloid size ratio.     

Design of sparse matrix representations for the propagator used in the BPM and directional wave field decomposition

Directional wave field decomposition can be accomplished with the aid of pseudo-differential operators. A fast numerical scheme requires sparse matrix representations of these operators. This paper focuses on designing sparse matrices for the propagator while keeping the accuracy high at the cost of ignoring critical-angle phenomena. The matrix representation follows from a rational approximation for the square root operator and the derivatives. The parameterization thus introduced lends itself to an overall optimization procedure that minimizes the errors for a chosen discretization rate. As such, the approach leads to an accurate propagator up to the (local) critical angle on a coarse numerical grid. This revised version was published online in November 2006 with corrections to the Cover Date.     

LIGHT-INDUCED FORMATION OF O-2˙ OXYGEN RADICALS IN SYSTEMS CONTAINING CHLOROPHYLL

It has been shown recently that photosystem 1 particles, photosystem 1 lipid vesicles and chlorophyll-a lipid vesicles show identical photochemical reactions in the presence of oxygen e.g. H⁺-and O₂-uptake (Van Ginkel, 1979). Therefore, spin-trapping experiments were done to identify the oxygen radicals formed. The spintrap phenyltertiarybutylnitrone (PBN) failed to yield information about oxygen radicals. With the spintrap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO), however, we obtained a mixed spectrum of O^-_2˙ and OH·-adducts generated in chloroplasts, photosystem 1 particles or chlorophyll-a lipid vesicles. These data indicate that chlorophyll-a in an artificial membrane can also catalyze O^-_2˙-formation. Chlorophyll-a lipid vesicles catalyze light-induced formation of the Tiron-semiquinone free radical, which has been proposed as a specific O^-_2˙-probe (Greenstock and Miller, 1975). However, OH· scavengers strongly reduce the formation of this radical, whereas superoxide dismutase does not. Pulse-radiolysis measurements showed that the rate constant for the reaction of Tiron with OH· is 8.2 · 10⁹M^-1 s^-1, which is considerably higher than the published Tiron/O^-_2˙ rate constants. Therefore, Tiron is a better spin probe for OH· than for O^-_2˙. We suggest that light-induced H⁺-and O^-_2˙-uptake in membranes containing chlorophyll-a in the presence of ascorbate is caused mainly by the very rapid reaction of OH· with ascorbate.     

Radiation from a Point Charge Moving through a Circular Aperture in a Screen

This paper deals with the problem of diffraction radiation froman electric point charge that moves, at a constant speed, througha circular aperture in a plane screen. The screen is assumedto be electrically perfectly conducting. The problem is reducedto the solution of an integral equation for an unknown fieldfunction in the aperture. Low-frequency solutions as well ashigh-frequency solutions are obtained. Results pertaining tothe far-field behaviour of the radiated field are given. Finally,numerical results pertaining to the radiation loss of the chargeare presented.     

Application of a Direct Stiffness Method to Wave Propagation in Multiphase Poroelastic Media

In this paper, it is demonstrated how a direct stiffness method for wavepropagation in multilayered saturated poroelastic media, based on integraltransform techniques, can be modified to account for a small amount of gasin the pores. Unsaturated media with small gas fractions can be treatedusing Smeulders extension of Biots poroelastic theory. The effect of thepresence of gas bubbles on the fluid bulk modulus and the dispersioncharacteristics of a water-saturated sand of Mol is demonstrated. Thedirect stiffness method is illustrated with a numerical example wheretransient wave propagation in a dry, saturated and unsaturated halfspaceis considered.     

The Interface Between Fresh and Salt Groundwater: A Numerical Study

Present address: Laboratoire National d'Hydraulique, 6 quai Watier 78401 Chatou, France. In this paper the two-dimensional flow of fresh and salt waterthrough a homogeneous aquifer is considered. The two fluidsare assumed to be separated by a sharp interface. They differonly in their specific weight. This difference induces a flowin the aquifer which in turn causes a motion of the interface. We present a mathematical formulation of this problem whichconsists of a Poisson equation for the stream function coupledto a time evolution equation for the moving interface. The equationfor the stream function is solved by means of a finite-elementmethod while a predictor-corrector method (the S^ßscheme) is used for the discretization of the equation for theinterface.     

LASER FLASH SPECTROSCOPY OF METHYLENE BLUE WITH NUCLEIC ACIDS. EFFECTS OF IONIC STRENGTH AND pH

Laser flash spectroscopic measurements were made on methylene blue complexed to the synthetic polynucleotides poly[d(G-C)] and poly[d(A-T)] in solutions of varying ionic strength and pH. Triplet decay rates and rates of triplet quenching by oxygen have been measured for the polymer/dye solutions. The triplet decay and oxygen quenching rates of methylene blue in complex with poly[d(A-T)] are much less sensitive than those with poly[d(G-C)] with respect to variations in ionic strength. It is also shown that protonation of the triplet state of MB+ with poly[d(A-T)] is slower than that of free dye. These results indicate strong binding of the dye to poly[d(A-T)]. Excitation of the MB+/poly[d(A-T)] complex at 665 nm yielded single exponential decay kinetics, in contrast with excitation at 600 nm where double exponential kinetics were measured. This is tentatively assigned to excitation of dye dimer bound to this polymer.