The initial electron transfer steps in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides have been investigated by femtosecond timeresolved spectroscopy. The experimental data taken at various wavelengths demonstrate the existence of at least four intermediate states within the first nanosecond. The difference spectra of the intermediates and transient photodichroism data are fully consistent with a sequential four-step model of the primary electron transfer: Light absorption by the special pair P leads to the state P*. From the excited primary donor P*, the electron is transferred within 3.5 ± 0.4 ps to the accessory bacteriochlorophyll B. State P+B-decays with a time constant of 0.9 ± 0.3 ps passing the electron to the bacteriopheophytin H. Finally, the electron is transferred from H-to the quinone QA within 220 ± 40 ps. According to present knowledge, an electron is transferred upon light absorption from the P along branch A to the quinone QA (8)(9)(10)(11)(12)(13) Materials and Experimental TechniquesRCs from two strains ofRb. sphaeroides, the wild-type strain (ATCC 17023) and the carotenoid-free strain (R 26.1), were isolated as described (39). The measurements were performed at room temperature (297 K) in cuvettes with a 1-mm path length with stirring. The concentration of the samples was adjusted to OD8w values between 0.5 and 1.0 mm' A synchronously pumped unidirectional ring dye laser (42) generated pulses with a duration of 60 fs at a wavelength of 860 nm. The energy of single pulses was increased to 20 lJ by a three-stage dye amplifier (repetition rate, 10 Hz). Each of these pulses was split into two parts. (i) The excitation pulse was focused to a 0.5-mm spot providing an energy density of not more than 100 uJ/cm2 in the sample. In the probed volume, -7% of the RCs were excited per pulse. This low excitation density prevents double excitation of the RC and related nonlinear processes. The exciting pulse, centered at 860 nm, had a bandwidth of <15 nm (full width at half-maximum). No spectral components of the exciting pulse were detected at wavelengths of <840 nm. Thus only the lowest electronic level ofP at 860 nm was excited. (ii) The probing pulse passed an adjustable delay line and was focused onto a 1-mm-thick jet of ethylene glycol to generate a femtosecond white-light pulse. A 10-to 20-nm-wide portion of this continuum was selected by means of a special dispersion compensating monochromator (43). The energy of the probing pulses was <7 p.J/cm2.

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The primary light-induced charge separation in reaction Centers of Rkfobacter sphueroides was investigated with femtosecond time resolution. The absorption changes in the time range 100 fs to 1 ns observed after direct excitation of the primary donor P at 860 mn could only be explained by a kinetic model which uses three time constants. This fmding supports the following reaction scheme: (i) the electronically excited primary donor P* decays with a time constant of 3.5 ps and populates a very short-lived intermediate involving a reduced accessory bacteriochlorophyll molecule; (ii) with a time constant of 0.9 ps the electron is transferred to the neighboring bacteriopheophytin molecule; and (iii) from there within 200 ps to the quinone.

The effect of pressure on atomic positions and lattice parameters is determined for trigonal Se under pressures from 0 to 86 kbar and for trigonal Te from 0 to 40 kbar by single-crystal x-ray diffraction using a diamond anvil high-pressure cell in a standard precession camera. The structural data confirm the latticedynamical homology that had been observed in the variation of the Raman frequencies at low pressures.However, the strongly nonlinear variations in the interatomic distances and the bond angles show deviations from a structural homology as the high-pressure phase transitions are approached.

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The equilibrium phase diagram of boron nitride thermodynamically calculated by Solozhenko in 1988 has been now refined on the basis of new experimental data on BN melting and extrapolation of heat capacities of BN polymorphs into high-temperature region using the adapted pseudo-Debye model. As compared with the above diagram, the hBN ⇆ cBN equilibrium line is displaced by 60 K toward higher temperatures. The hBN−cBN−L triple point has been calculated to be at 3480 ± 10 K and 5.9 ± 0.1 GPa, while the hBN−L−V triple point is at T = 3400 ± 20 K and p = 400 ± 20 Pa, which indicates that the region of thermodynamic stability of vapor in the BN phase diagram is extremely small. It has been found that the slope of the cBN melting curve is positive whereas the slope of hBN melting curve varies from positive between ambient pressure and 3.4 GPa to negative at higher pressures.

The interaction of the hydrogen atoms with nearest neighbor oxygen atoms in the hydrogen bonds of ice VII are approximated by two equivalent Morse potentials. With the inclusion of a repulsive oxygen—oxygen interaction, the model predicts a transition to symmetric hydrogen bonding in ice VII at a pressure between 350 and 800 kbar.

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