Electromagnetic and nuclear fission of238U in the reaction of 100, 500, and 1000 A·MeV208Pb with238U

The folding- and azimuthal-angle and velocity distributions for the²³⁸U fission fragments have been measured in reactions with 100, 500, and 1000 A·MeV²⁰⁸Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16±0.07, 0.48±0.08, and 0.60±0.04, respectively. The electromagnetic fission cross section as a function of the²⁰⁸Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (≈1.5 b) is approximately constant between 100 and 1000 A·MeV.     

A pulse-train laser driven XUV source for picosecond pump–probe experiments in the water window

We describe the development of a table top soft X-ray source based on a laser produced plasma. The plasma is produced by a Nd:YLF pulse-train laser which delivers pulse trains of up to 400 pulses each of about 25 ps duration. Number and energy of the micropulses are fully computer controlled adjustable for an optimum interaction with a rotating solid target. Spectral, spatial and temporal characteristics of the X-ray emission from Mylar and copper targets have been studied. A calibrated back-illuminated CCD camera together with a slit grating spectrograph were utilized for the measurement of the absolute soft X-ray photon fluxes. The temporal measurements were performed using a streak camera while the plasma size was measured with a zone plate. An application of the system in a X-ray absorption experiment is shown. The advantages of the system in view of possible optical pump and soft X-ray probe experiments will be discussed.     

EXCITED STATE BEHAVIOR OF PHYTOCHROME Pr

The primary excited state processes of phytochrome, the plant chromoprotein responsible for most photomorphogenetic responses, are investigated by picosecond intensity-dependent transmission measurements. At sufficient high excitation intensity (starting at photon flux densities of about 1′10²⁵ cm^?2 s^?l) a moderate bleaching of the absorption is observed. For the quantitative analysis of the experimental data a rate equation system approach for the energy levels responsible for the ultrafast transformation P_r→ lumi-R and the photon transport through the sample was utilized. The following was found for the excited state parameters of the phytochrome P_r: (1) an excited state absorption of the same order of magnitude as the ground state absorption (cross section: S?_exc= 4′10^?16 cm^?2 at 650 nm), (2) a fast relaxation of the higher excited singlet state on the femtosecond time scale.     

Instantaneous Fluorescence Quantum Yield—A New Quantity with Characteristic “Fingerprints” for Excited-State Processes

The instantaneous fluorescence quantum yield _ins—a new quantity for fluorescence studies defined as the ratio of the fluorescence intensity to the optical density, both measured at the moment of the maximum of the exciting pulse—proves to be a very sensitive function for excited-state processes. Dependent on the excitation intensity _ins exhibits characteristic features (maxima/minima) indicating, for example, excited-state absorptions and annihilation processes. _ins is therefore more informative as the intensity dependence of the usually utilized fluorescence yield, the information content of which is restricted because this function is hardly structured. In the paper the influences of specific molecular parameters (excited-state absorption cross section, annihilation constant) on _ins are given, problems of the experimental accessibility of _ins are discussed, and an experimental setup for determination of this new quantity is presented. The application of the method is demonstrated for identification of excited-state absorptions of organic molecules in solution.     

Influence of laser pulse width on absolute EUV-yield from Xe-clusters

Using 50 fs ( ∼ 2×10¹⁸ W/cm²) and 2 ps ( ∼ 5×10¹⁶ W/cm²) pulses from a Ti:Sa multi-TW laser at 800 nm wavelength large Xe-clusters ( 10⁵...10⁶ atoms per cluster) have been excited. Absolute yield measurements of EUV-emission in a wavelength range between 10 nm and 15 nm in combination with cluster target variation were carried out. The ps-laser pulse has resulted in about 30% enhanced and spatially more uniform EUV-emission compared to fs-laser excitation. Circularly polarized laser light instead of linear polarization results in enhanced emission which is probably caused by electrons gaining higher energies by the polarization dependent optical field ionization process. An absolute emission efficiency at 13.4 nm of up to 0.8% in 2π sr and 2.2% bandwidth has been obtained. Received 11 January 2001 and Received in final form 27 March 2001     

Performance improvement of a Kα source by a high-resolution thin-layer-graphite spectrometer and a polycapillary lens

A tabletop, short-pulse laser-based hard X-ray (Kα) source equipped with an advanced X-ray optics and dedicated for high-resolution spectroscopy and time-resolved diffraction is described. Operation of the source together with a high-resolution spectrometer containing a large-aperture highly annealed pyrolytic graphite gave a resolution E/ΔE of ~1,800 for the spectral range around Kα line of Cu. The estimated total flux of the 8.05-keV photons was equal to 5.9 × 10¹⁰ ph/s in 4π sr. Performance boost of the source caused by X-ray optics relied on the significant increase in the Cu-Kα photon flux on both, the sample (4.7 × 10⁶ ph/s) and the detector (3.4 × 10³ ph/s). A spectral brightness of 1.4 × 10⁷ ph/s/mm²/mrad² was derived from the source parameters for the Kα line. Better performance due to high collecting power and reflectivity of the spectrometer enabled application of the cross-correlation technique with an Ni foil. An upper bound of emission duration of 323 ± 47 fs was obtained in this measurement. X-ray absorption near-edge spectroscopy on an Ni sample with an acquisition time of only 15 min confirmed the increased capability of the setup also for continuous spectrum (bremsstrahlung).     

Two-photon excitation of alkyl-substituted magnesium phthalocyanine: radical formation via higher excited states

The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t₄-PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr₄). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ_exc=670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q_x-band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.     

One- and Two-Photon-Induced Photochemistry of Modified Palladium Porphyrazines Involving Molecular Oxygen

Abstract— Octaphenyltetraanthraporphyrazinato palladium undergoes a self-sensitized photoreaction in the presence of oxygen to form a substituted palladium phthalocyanine with four endoperoxide bridges. This compound exhibits photophysical behavior similiar to palladium tetra-tert- butyl-phthalocyanine. The phthalocyanine-palladium complex with four endoperoxide bridges ejects molecular oxygen when excited by consecutive two-photon absorption in the Q-band region at 662 nm. This photocyclo-reversion, which produces palladium porphyrazines bearing a diminished number of endoperoxide bridges, can occur up to four times per initial molecule. Irradiation of these photoproducts in the presence of oxygen produces substituted palladium phthalocyanine containing four endoperoxide groups.     

Photophysical consequences of coupling bacteriochlorophyll a with serine and its resulting solubility in water

We investigated the dependence on solvents of optical absorption and emission of the bacteriochlorophyll a-serine (BChl-ser), a water soluble bacteriochlorophyll (BChl) derivative. Comparison between the experimental data and those collected for BChl in nonaqueous solvents shows that only a minor interaction takes place between serine and the macrocycle's pi-electron system. Nevertheless, the coupling with serine results in a small enhancement of the nonradiative relaxation rate from the first excited singlet state S1. In buffered aqueous solution (pH = 7.4), the Stokes shift of the BChl-ser fluorescence and its nonradiative relaxation rate are enhanced compared with those in nonaqueous solutions (Scherz, A., S. Katz, Y. Vakrat, V. Brumfeld, E. Gabelmann, D. Leupold, J. R. Norris, H. Scheer and Y. Salomon (1998) Photosynthesis: Mechanisms and Effects, Vol. V (Edited by G. Garab), pp. 4207-4212. Kluwer Academic, Dordrecht.), probably as a result of a hydrogen bonding between the BChl macrocycle and the water molecules. In aprotic solvents, without hydrogen bonds, the permanent dipole moment of the first excited singlet state in both BChl and BChl-ser is increased compared with the ground state by at least 2.5 Debye.