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51.
A short,highly efficient synthesis of coenzyme Q(10) 总被引:5,自引:0,他引:5
Lipshutz BH Mollard P Pfeiffer SS Chrisman W 《Journal of the American Chemical Society》2002,124(48):14282-14283
The most efficient synthesis reported to date of ubiquinone (CoQ10) is described. A sequence consisting of six operations is involved which leads to crystalline material in an overall yield of >64%. 相似文献
52.
Reilly DJ Buehler TM O'Brien JL Hamilton AR Dzurak AS Clark RG Kane BE Pfeiffer LN West KW 《Physical review letters》2002,89(24):246801
There is controversy as to whether a one-dimensional (1D) electron gas can spin polarize in the absence of a magnetic field. Together with a simple model, we present conductance measurements on ultra-low-disorder quantum wires supportive of a spin polarization at B=0. A spin energy gap is indicated by the presence of a feature in the range (0.5-0.7)x2e(2)/h in conductance data. Importantly, it appears that the spin gap is not constant but a function of the electron density. Data obtained using a bias spectroscopy technique are consistent with the spin gap widening further as the Fermi level is increased. 相似文献
53.
We observe density-induced 90 degrees rotations of the anisotropy axes in transport measurements at half-filled high Landau levels in the two dimensional electron system, where stripe states are proposed ( nu = 9/2, 11/2, etc.). Using a field effect transistor, we find the transition density to be 2.9x10(11) cm(-2) at nu = 9/2. Hysteresis is observed in the vicinity of the transition. We construct a phase boundary in the filling factor magnetic field plane in the regime 4.4相似文献
54.
Kolbitsch C Lorenz IH Hörmann C Hinteregger M Löckinger A Moser PL Kremser C Schocke M Felber S Pfeiffer KP Benzer A 《Magnetic resonance imaging》2002,20(7):535-541
Conflicting results reported on the effects of hyperoxia on cerebral hemodynamics have been attributed mainly to methodical and species differences. In the present study contrast-enhanced magnetic resonance imaging (MRI) perfusion measurement was used to analyze the influence of hyperoxia (fraction of inspired oxygen (FiO2) = 1.0) on regional cerebral blood flow (rCBF) and regional cerebral blood volume (rCBV) in awake, normoventilating volunteers (n = 19). Furthermore, the experiment was repeated in 20 volunteers for transcranial Doppler sonography (TCD) measurement of cerebral blood flow velocity in the middle cerebral artery (CBFV(MCA)). When compared to normoxia (FiO2 = 0.21), hyperoxia heterogeneously influenced rCBV (4.95 +/- 0.02 to 12.87 +/- 0.08 mL/100g (FiO2 = 0.21) vs. 4.50 +/- 0.02 to 13.09 +/- 0.09 mL/100g (FiO2 = 1.0). In contrast, hyperoxia diminished rCBF in all regions (68.08 +/- 0.38 to 199.58 +/- 1.58 mL/100g/min (FiO2 = 0.21) vs. 58.63 +/- 0.32 to 175.16 +/- 1.51 mL/100g/min (FiO2 = 1.0)) except in parietal and left frontal gray matter. CBFV(MCA) remained unchanged regardless of the inspired oxygen fraction (62 +/- 9 cm/s (FiO2 = 0.21) vs. 64 +/- 8 cm/s (FiO2 = 1.0)). Finding CBFV(MCA) unchanged during hyperoxia is consistent with the present study's unchanged rCBF in parietal and left frontal gray matter. In these fronto-parietal regions predominantly fed by the middle cerebral artery, the vasoconstrictor effect of oxygen was probably counteracted by increased perfusion of foci of neuronal activity controlling general behavior and arousal. 相似文献
55.
56.
Coherent anti-Stokes Raman scattering (CARS) spectra of excited molecules as well as Shpolskii spectra provide information about geometry changes between ground and excited states. Vibrational frequencies and relative intensities from recently obtained CARS spectra of the chrysene S1 and T1 state and earlier observed Shpolskii spectra are interpreted in terms of molecular geometry and force-field changes by means of quantum-chemical consistent force field (QCFF) and Franck-Condon factor calculations. The comparison of observed and calculated relative intensities indicates a coupling between the S1 and S2 state enhancing some of the vibrational radiative singlet transitions both in absorption and fluorescence spectra whereas within the phosphorescence spectra proportionality to calculated Franck-Condon factors is obeyed. The T1 state is the more loosely bound state and its geometry change is different from that of the S1 state. The resonance CARS transitions in the S1 state are assigned to totally symmetric vibrations getting their intensity by a coupling scheme analogous to the A term of the resonance Raman effect: the relative intensity of a transition is shown to be proportional to the Franck-Condon factor to the higher excited state and to the squared vibrational frequency. Using this relation this state can be identified by means of its finger-print-like intensity pattern. 相似文献
57.
With a high pressure gas target muonic F and S X-ray intensities in SF6 and H2+SF6 have been measured. It was found that the distribution of muons between sulphur and fluorine is not at all equal to the ratio of the atomic numbers Z. 相似文献
58.
For a gaussian broadband radiation field (multimode laser without phase-correlation) the bleaching behaviour of a homogeneously broadened two level system is investigated in dependence on the spectral width (δυ) of the field. Taking into consideration the coherent mode-interaction, originating from the nonstationary reaction of population inversion on the fluctuations of the radiation field, a decrease of the bleaching effect results as soon as the spectral width δυ becomes larger than ⊥1 = 1/T1 with T1 the longitudinal relaxation time of the molecular transition concerned. Neglecting this effect in the analysis of broadband bleaching measurements can result in the unjustified assumption of an inhomogeneous broadening of the spectral band under consideration, which has consequences on the magnitude of the molecular absorption cross-section derived from these measurements. 相似文献
59.
60.
M.?BiasiniEmail author R. D.?Gann L. N.?Pfeiffer K. W.?West X. P.A.?Gao B. C.D.?Williams J. A.?Yarmoff A. P.?MillsJr 《The European Physical Journal B - Condensed Matter and Complex Systems》2005,47(2):305-314
The conductivity of an Al0.24Ga0.76As/GaAs
quantum well was studied as a function of the surface charge
generated by electron bombardment of the sample in the absence of
an externally applied surface electric field. Under a suitable
rate of electron irradiation, it was possible to completely shut
off the conductive channel, implying a surface density
. Light illumination quenches the
increase of the resistivity, apparently due to photoemission from
the metastable surface states. Upon turning off the electron
bombardment the surface charge on adsorbed layers of xenon and
water at 8 K decays in room temperature darkness with a lifetime
τ= 0.30 ±0.02 s. The average charging efficiency, is
μ0 ≃0.001. Surface charging is shown to be an effective
method for contactless gating of field effect devices. 相似文献