Time-Resolved Studies of Light Propagation in Crassula and Phaseolus Leaves

Abstract— Time-resolved transmittance was used to extract in vivo optical properties of leaves of green plants experimentally. In time-resolved transmittance measurements an ultrashort light pulse is directed onto the surface of the object and the transmitted light is measured with a time resolution in the range of picoseconds. A table-top terawatt laser was used to generate 200 fs light pulses at 790 nm with a repetition rate of 10 Hz. The light pulses were focused through a cuvette filled with water to produce white light pulses and optical filters were placed in the beam path to select the wavelength of the light focused onto the leaf surface. The time profiles of the light transmitted through the leaves was recorded with a streak camera having a time resolution of about 2.5 ps. Results from Crassula falcata and Phaseolus vulgaris studied at 550, 670 and 740 nm are reported. The three selected wavelength regions represent medium, high and a low absorption of light, respectively. A library of curves was generated using Monte Carlo simulation, and the absorption and scattering coefficients were extracted by comparison of experimental curves with this library. Our results suggest that in the case of the thin (200 μm) Phaseolus leaves and certainly in the case of the thick (4 mm) Crassula leaves, light scattering plays an important role in light transport through the leaf and should also affect light flux in these leaves.     

Optical map transformations

An optical technique for performing general types of map transformations is introduced. Using a coherent system, the object distribution is multiplied by a phase function so that its frequency spectrum constitutes the transformation wanted. Also shown is how imaging systems can be made space variant. Computer-generated holograms are used to give examples of optical arrangements and experimental results.     

Correlated flipping of four benzene rings in substituted [2.2.2.2]Paracyclophanetetraenes

Dynamic NMR spectroscopy and molecular mechanics calculations show that the flipping of the four benzene binge in certain substituted [2.2.2.2]paracyclophanetetraenes occurs in a correlated fashion.     

Balancing Neumann‐Neumann methods for incompressible Stokes equations

Balancing Neumann‐Neumann methods are introduced and studied for incompressible Stokes equations discretized with mixed finite or spectral elements with discontinuous pressures. After decomposing the original domain of the problem into nonoverlapping subdomains, the interior unknowns, which are the interior velocity component and all except the constant‐pressure component, of each subdomain problem are implicitly eliminated. The resulting saddle point Schur complement is solved with a Krylov space method with a balancing Neumann‐Neumann preconditioner based on the solution of a coarse Stokes problem with a few degrees of freedom per subdomain and on the solution of local Stokes problems with natural and essential boundary conditions on the subdomains. This preconditioner is of hybrid form in which the coarse problem is treated multiplicatively while the local problems are treated additively. The condition number of the preconditioned operator is independent of the number of subdomains and is bounded from above by the product of the square of the logarithm of the local number of unknowns in each subdomain and a factor that depends on the inverse of the inf‐sup constants of the discrete problem and of the coarse subproblem. Numerical results show that the method is quite fast; they are also fully consistent with the theory. © 2002 John Wiley & Sons, Inc.     

Rational carrier-period for binarization of sampled images and holograms

Introduction of a rational period in dither methods to binarize graytone images offers an additional degree of freedom. This can be used to overcome the inevitable tradeoff between graytone reproduction and detail resolution in these techniques. The concept is demonstrated with a one-dimensional triangular-shaped carrier function. Experimental results are shown.     

Photoexcited C60: fragmentation and delayed ionization

We investigate two reaction channels of C₆₀, excited by a pulsed laser at 355 nm: i) The relative and absolute yields of even-sized fragment ions are measured over a wide range of laser fluences. At low fluence we find a dramatic dependence of the abundance ratio of C _n?2 ⁺ versus C _n ⁺ . This result argues against the notion that unimolecular emission of C₄ or larger fragments contributes significantly to the production of even-sized fragment ions. ii) We count the number of delayed electrons emitted from C₆₀, or one of its products, after photo-excitation. For high laser fluences, this number reaches a value of (2.6 ± 1.1)% per photoexcited C₆₀. This sets a lower bound to the ratio of effective rate constants, k_e(E^* )/∑k_j(E^*), where k_e refers to electron emission, and the sum in the denominator extends over all reaction channels.     

Macrocyclic π-systems with C2-symmetry as the sum of π-systems with hückel and möbius topology

The π-system of a macrocycle with twofold rotation symmetry and 2N conjugated π-electron can conveniently be regarded as the sum of two cyclic N π-electron systems having Hückel and Möbius topology, respectively.     

A simple synthesis of a doubly-bridged [16]annulene

Reductive coupling of cychoheptatriene-1,6-dialdehyde using a low-valent titanium reagent provides a one-step route to a paratropic bismethano [16] annulene.