Picosecond fluorescence of intact and dissolved PSI-LHCI crystals

Over the last years many crystal structures of photosynthetic pigment-protein complexes have been determined, and used extensively to model spectroscopic results obtained on the same proteins in solution. However, the crystal structure is not necessarily identical to the structure of the protein in solution. Here we studied picosecond fluorescence of Photosystem I-Light Harvesting Complex I (PSI-LHCI), a multisubunit pigment protein complex that catalyzes the first steps of photosynthesis. The ultrafast fluorescence of PSI-LHCI crystals is identical to that of dissolved crystals, but differs considerably from most kinetics presented in literature. In contrast to most studies, the present data can be modeled quantitatively with only 2 compartments: PSI core and LHCI. This yields the rate of charge separation from an equilibrated core (22.5+/-2.5 ps) and rates of excitation energy transfer from LHCI to core (kLC) and vice versa (kCL). The ratio R=kCL/kLC between these rates appears to be wavelength-dependent and scales with the ratio of the absorption spectra of LHCI and core, indicating the validity of a detailed balance relation between both compartments. kLC depends slightly but non systematically on detection wavelength, averaging (9.4+/-4.9 ps)(-1). R ranges from 0.5 (below 690 nm) to around 1.3 above 720 nm

Anisotropic interactions of a single spin and dark-spin spectroscopy in diamond

The nitrogen-vacancy (N-V) center in diamond is a promising atomic-scale system for solid-state quantum information processing. Its spin-dependent photoluminescence has enabled sensitive measurements on single N-V centers, such as: electron spin resonance, Rabi oscillations, single-shot spin readout and two-qubit operations with a nearby 13C nuclear spin. Furthermore, room temperature spin coherence times as long as 58 microseconds have been reported for N-V center ensembles. Here, we have developed an angle-resolved magneto-photoluminescence microscopy apparatus to investigate the anisotropic electron spin interactions of single N-V centers at room temperature. We observe negative peaks in the photoluminescence as a function of both magnetic field magnitude and angle that are explained by coherent spin precession and anisotropic relaxation at spin level anti-crossings. In addition, precise field alignment unmasks the resonant coupling to neighboring dark nitrogen spins that are not otherwise detected by photoluminescence. The latter results demonstrate a means of investigating small numbers of dark spins via a single bright spin under ambient conditions.Comment: 13 pages, 4 figure

Are Compact High-Velocity Clouds Extragalactic Objects?

Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the Local Group model and would have HI volume densities of order 0.0003/cm^3. Clouds with these volume densities and the observed neutral hydrogen column densities will be largely ionized, even if exposed only to the extragalactic ionizing radiation field. Here we examine the implications of this process for models of CHVCs. We have modeled the ionization structure of spherical clouds (with and without dark matter halos) for a large range of densities and sizes, appropriate to CHVCs over the range of suggested distances, exposed to the extragalactic ionizing photon flux. Constant-density cloud models in which the CHVCs are at Local Group distances have total (ionized plus neutral) gas masses roughly 20-30 times larger than the neutral gas masses, implying that the gas mass alone of the observed population of CHVCs is about 40 billion solar masses. With a realistic (10:1) dark matter to gas mass ratio, the total mass in such CHVCs is a significant fraction of the dynamical mass of the Local Group, and their line widths would exceed the observed FWHM. Models with dark matter halos fare even more poorly; they must lie within approximately 200 kpc of the Galaxy. We show that exponential neutral hydrogen column density profiles are a natural consequence of an external source of ionizing photons, and argue that these profiles cannot be used to derive model-independent distances to the CHVCs. These results argue strongly that the CHVCs are not cosmological objects, and are instead associated with the Galactic halo.Comment: 30 pages, 14 figures; to appear in The Astrophysical Journa

Vertical distribution of stars and gas in a galactic disk

We study the vertical density distribution of stars and gas (HI and H_2) in a galactic disk which is embedded in a dark matter halo. The new feature of this work is the inclusion of gas, and the gravitational coupling between stars and gas, which has led to a more realistic treatment of a multi-component galactic disk. The gas gravity is shown to be crucially important despite the low gas mass fraction. This approach physically explains the observed scaleheight distribution of all the three disk components, including the long-standing puzzle (Oort 1962) of a constant HI scaleheight observed in the inner Galaxy. The above model is applied to two external galaxies: NGC 891 and NGC 4565, and the stellar disk is shown to be not strictly flat as was long believed but rather it shows a moderate flaring of a factor of about 2 within the optical radius.Comment: 4 pages, 2 figures; to appear in the Proceedings of "Island Universes: Structure and evolution of disk galaxies" (Terschelling, The Netherlands, July 2005), ed. R. de Jon

Геомеханика разрушения и регламент тампонажного упрочнения пород вокруг наклонных стволов вязкопластическими растворами

Наведено підсумки шахтних досліджень руйнування порід навколо стволів вугільних шахт та обґрунтовано параметри їх зміцнення вязкопластичними розчинами.Research results are mine destruction of rocks around the shafts of coal mines and reasonable options to strengthen viscoplastic solutions