1,490 research outputs found
Composite infrared bolometers with Si_3N_4 micromesh absorbers
We report the design and performance of 300-mK composite bolometers that use micromesh absorbers and support structures patterned from thin films of low-stress silicon nitride. The small geometrical filling factor of the micromesh absorber provides 20Ă reduction in heat capacity and cosmic ray cross section relative to a solid absorber with no loss in IR-absorption efficiency. The support structure is mechanically robust and has a thermal conductance, G < 2 Ă 10^(â11) W/K, which is four times smaller than previously achieved at 300 mK. The temperature rise of the bolometer is measured with a neutron transmutation doped germanium thermistor attached to the absorbing mesh. The dispersion in electrical and thermal parameters of a sample of 12 bolometers optimized for the SunyaevâZelâdovich Infrared Experiment is ±7% in R (T), ±5% in optical efficiency, and ±4% in G
Role of tyrosine M210 in the initial charge separation of reaction centers of Rhodobacter sphaeroides
Femtosecond spectroscopy was used in combination with site-directed mutagenesis to study the
influence of tyrosine M210 (YM210) on the primary electron transfer in the reaction center of Rhodobacter
sphaeroides. The exchange of YM210 to phenylalanine caused the time constant of primary electron transfer
to increase from 3.5 f 0.4 ps to 16 f 6 ps while the exchange to leucine increased the time constant even
more to 22 f 8 ps. The results suggest that tyrosine M210 is important for the fast rate of the primary
electron transfer
Detailed studies of the subpicosecond kinetics in the primary electron transfer of reaction centers of Rhodopseudomonas viridis
The primary, light-induced charge separation in reaction centers of Rhodopseudomonas viridis is investigated with femtosecond time resolution. The absorption changes after direct excitation of the primary donor P at 955 nm are investigated in the time range from 100 fs to 600 ps. The experimental data, taken at various probing wavelengths, reveal one subpicosecond and two picosecond time constants: 0.65 ± 0.2 ps, 3.5 ± 0.4 ps, and 200 ± 20 ps. The previously undetected 0.65 ps kinetics can be observed clearly in the spectral range of the Qx and Qy transitions of the monomeric bacteriochlorophylls. The experimental data support the idea that the accessory bacteriochlorophyll B A participates in the electron-transfer process.
Reference
Energetics of the primary electron transfer reaction revealed by ultrafast spectroscopy on modified bacterial reaction centers
The modification of reaction centers from Rhodobacter sphaeroides by the introduction of pheophytins instead of bacteriopheophytins leads to interesting changes in the primary photosynthetic reaction: long-living populations of the excited electronic state of the special pair P* and the bacteriochlorophyll anion BâA show up. The data allow the determination of the energetics in the reaction center. The free energy of the first intermediate P+BâA, where the electron has reached the accessory bacteriochlorophyll BA lies â 450 cmâ1 below the initially excited special pair P*
Temperature dependence of the primary electron transfer in photosynthetic reaction centers from Rhodobacter sphaeroides
The primary electron transfer (ET) in reaction centers (RC) of Rhodobacter sphaeroides is investigated as a function of temperature with femtosecond time resolution. For temperatures from 300 to 25 K the ET to the bacteriopheophytin is characterized by a biphasic time dependence. The two time constants of Ï1=3.5±0.4 ps and Ï2=1.2±0.3 ps at T=300 K decrease continously with temperature to values of Ï1=1.4±0.3 ps and Ï2=0.3±0.15 ps at 25 K. The experimental results indicate that the ET is not thermally activated and that the same ET mechanisms are active at room and low temperatures. All observations are readily rationalized by a two-step ET model with the monomeric bacteriochlorophyll as a real electron carrier
Investigation of the crystallization process of CSD-ErBCO on IBAD-substrate via DSD approach
REBaCuO (REBCO, RE: rare earth, such as Y and Gd) compounds have been extensively studied as a superconducting layer in coated conductors. Although ErBCO potentially has better superconducting properties than YBCO and GdBCO, little research has been made on it, especially in chemical solution deposition (CSD). In this work, ErBCO films were deposited on IBAD (ion-beam-assisted-deposition) substrates by CSD with low-fluorine solutions. The crystallization process was optimized to achieve the highest self-field critical current density (J) at 77 K. Commonly, for the investigation of a CSD process involving numerous process factors, one factor is changed keeping the others constant, requiring much time and cost. For more efficient investigation, this study adopted a novel design-of-experiment technique, definitive screening design (DSD), for the first time in CSD process. Two different types of solutions containing Er-propionate or Er-acetate were used to make two types of samples, Er-P and Er-A, respectively. Within the investigated range, we found that crystallization temperature, dew point, and oxygen partial pressure play a key role in Er-P, while the former two factors are significant for Er-A. DSD revealed these significant factors among six process factors with only 14 trials. Moreover, the DSD approach allowed us to create models that predict J accurately. These models revealed the optimum conditions giving the highest J values of 3.6 MA/cm for Er-P and 3.0 MA/cm for Er-A. These results indicate that DSD is an attractive approach to optimize CSD process
Radio Sources in Galaxy Clusters at 28.5 GHz
We present serendipitous detections of radio sources at 28.5 GHz (1 cm),
which resulted from our program to image thermal Sunyaev-Zeldovich (SZ) effect
in 56 galaxy clusters. We find 64 radio sources with fluxes down to 0.4 mJy,
and within 250 arcseconds from the pointing centers. The spectral indices (S ~
\nu^-\alpha) of 54 sources with published low frequency flux densities range
from -0.6 to 2 with a mean of 0.77, and a median of 0.84. Extending low
frequency surveys of radio sources towards galaxy clusters CL 0016+16, Abell
665, and Abell 2218 to 28.5 GHz, and selecting sources with 1.4 GHz flux
density greater than 7 mJy to form an unbiased sample, we find a mean spectral
index of 0.71 and a median of 0.71. We find 4 to 7 times more sources predicted
from a low frequency survey in areas without galaxy clusters. This excess
cannot be accounted for by gravitational lensing of a background radio
population by cluster potentials, indicating most of the detected sources are
associated with galaxy clusters. For the cluster Abell 2218, the presence of
unsubtracted radio sources with 28.5 GHz flux densities less than 0.5 mJy, can
only contribute to temperature fluctuations at a level of 10 to 25 \muK. The
corresponding error due to radio point source contamination in the Hubble
constant derived through a combined analysis of 28.5 GHz SZ images and X-ray
emission observations ranges from 1% to 6%.Comment: 18 pages, 8 figures, to appear in April 1998 issue of A
Epitaxial growth and anisotropy of La(O,F)FeAs thin films deposited by Pulsed Laser Deposition
LaFeAsO1-xFx thin films were deposited successfully on (001)-oriented LaAlO3
and MgO substrates from stoichiometric LaFeAsO1-xFx polycrystalline targets
with fluorine concentrations up to x = 0.25 by PLD. Room temperature deposition
and post annealing of the films yield nearly phase pure films with a pronounced
c-axis texture and a strong biaxial in-plane orientation. Transport
measurements show metallic resistance and onset of superconductivity at 11 K.
Hc2(T) was determined by resistive measurements and yield Hc2 values of 3 T at
3.6 K for B||c and 6 T at 6.4 K for B||ab.Comment: 11 pages, 5 figure
- âŠ