| 1. |
- Adcox, K, et al.
(författare)
-
PHENIX detector overview
- 2003
-
Ingår i: Nuclear Instruments & Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment. - 0167-5087. ; 499:2-3, s. 469-479
-
Tidskriftsartikel (refereegranskat)abstract
- The PHENIX detector is designed to perform a broad study of A-A, p-A, and p-p collisions to investigate nuclear matter under extreme conditions. A wide variety of probes, sensitive to all timescales, are used to study systematic variations with species and energy as well as to measure the spin structure of the nucleon. Designing for the needs of the heavy-ion and polarized-proton programs has produced a detector with unparalleled capabilities. PHENIX measures electron and muon pairs, photons, and hadrons with excellent energy and momentum resolution. The detector consists of a large number of subsystems that are discussed in other papers in this volume. The overall design parameters of the detector are presented. (C) 2002 Elsevier Science B.V. All rights reserved.
|
|
| 2. |
- Lebedev, A.A., et al.
(författare)
-
Amplification of the signal in triode structures of ion detectors based on 6H-SIC epitaxial films
- 2001
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 79:26, s. 4447-4449
-
Tidskriftsartikel (refereegranskat)abstract
- The possibility of about 50 times the inneramplification of signals in SiC-based detectors of short-range ions is shown. The detector has an n-p-n+-like structure, where the p-type base was grown epitaxially on a 6H n+-SiC substrate. To complete the structure a Schottky barrier was made on top. Detector parameters were investigated in a "floating base" regime. Alpha particles from 244Cm were used and the augmentation of signal (E) with increasing applied voltage (U) was investigated. A superlinear increase of E was observed with a significant (tens of times) amplification of the introduced by the alpha particle nonequilibrium charge. It was also found that the nonuniformity of the diffusion-drift carrier transport parameters in the films does not exceed 10%. © 2001 American Institute of Physics.
|
|
| 3. |
- Donchev, V., et al.
(författare)
-
Photoluminescene line-shape analysis in quantum wells embedded in superlattices
- 2001
-
Ingår i: Materials science & engineering. C, biomimetic materials, sensors and systems. - 0928-4931 .- 1873-0191. ; 15:1-2, s. 75-77
-
Tidskriftsartikel (refereegranskat)abstract
- The temperature evolution of the main photoluminescence (PL) mechanisms, in GaAs quantum wells embedded in short-period AlAs/GaAs superlattices, is investigated. PL spectra are measured from 2 to 300 K. A detailed line-shape analysis of the PL peaks is performed by means of a statistical model, including both free exciton and free carrier recombination. The fits based on this model reproduce satisfactorily the experimental PL line shapes and allow to assess quantitatively the relative contributions of free excitons and free carriers to the radiative recombination at different temperatures. The results indicate the predominant role of free excitons in the radiative recombination up to room temperature, and are consistent with the mass action law. © 2001 Elsevier Science B.V. All rights reserved.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
- Sips, A. C. C., et al.
(författare)
-
An international database for the study of the formation of ITBs in tokamaks
- 2002
-
Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 44, s. A391-A398
-
Tidskriftsartikel (refereegranskat)abstract
- For the first time, data from eight,different tokamaks have been combined in an international database for internal transport barriers (ITBs). An analysis of the data for the formation of an ITB with dominant ion heating shows a clear dependence of the threshold power on the minor radius and line-averaged electron density for the formation of ion ITBs. The dependence of ITB formation on the toroidal magnetic field is weak. For the formation of ITBs with dominant electron heating, the database is smaller, but for the threshold power a strong increase with plasma size and a weak toroidal field dependence could also be identified. Based on these results, an expression for the power required to form an ITB is given using global variables only. These results give a basis for the analysis of the database using local values (like magnetic shear) and a detailed comparison with theory-based models.
|
|
| 7. |
- Strokan, N.B., et al.
(författare)
-
Detection of strongly and weakly ionizing radiation by triode structure based on SIC films
- 2003
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 93:9, s. 5714-5719
-
Tidskriftsartikel (refereegranskat)abstract
- The detection of strongly and weakly ionizing radiation by triode structure based on silicon carbide (SiC) films was discussed. The possibility of alpha particle spectrometry in spite of slow carrier transport via diffusion was demonstrated. Analysis showed that the signal generated by weakly ionizing radiation incident on a film had low amplitude.
|
|
| 8. |
- Strokan, N.B., et al.
(författare)
-
Radiation resistance of transistor- and diode-type SiC detectors irradiated with 8-MeV protons
- 2004
-
Ingår i: Semiconductors (Woodbury, N.Y.). - : Pleiades Publishing Ltd. - 1063-7826 .- 1090-6479. ; 38:7, s. 807-811
-
Tidskriftsartikel (refereegranskat)abstract
- Nuclear-particle detectors based on SiC with a structure composed of an n+-type substrate, a p-type epitaxial layer, and a Schottky barrier are studied. Structures with a ~10-µm-thick 6H-SiC layer exhibit transistor properties, whereas those with a ~30-µm-thick 4H-SiC layer exhibit diode properties. It is established that a more than tenfold amplification of the signal is observed in the transistor-type structure. The amplification is retained after irradiation with 8-MeV protons with a dose of at least 5 × 1013 cm-2, in this case, the resolution is =10%. Amplification of the signal was not observed in the structures of diode type. However, there were diode-type detectors with a resolution of ˜3%, which is acceptable for a number of applications, even after irradiation with the highest dose of 2 × 1014 cm-2. © 2004 MAIK "Nauka/Interperiodica".
|
|
| 9. |
- Strokan, N.B., et al.
(författare)
-
Silicon carbide transistor structures as detectors of weakly ionizing radiation
- 2003
-
Ingår i: Semiconductors (Woodbury, N.Y.). - : Pleiades Publishing Ltd. - 1063-7826 .- 1090-6479. ; 37:1, s. 65-69
-
Tidskriftsartikel (refereegranskat)abstract
- SiC-based nuclear radiation detectors figured prominently in the very first attempts of the 1960s to replace gas in ionization chambers with a more condensed semiconducting medium. However, the dynamics of improvement of SiC in those years was markedly inferior to the progress made in the development of competing materials. This study continues with the investigation of triode detector structures based on "pure" SiC films. It is established that for weakly ionizing radiation (as also in the case of strongly ionizing alpha particles) the signal is amplified by no less than a factor of several tens. This allows SiC films with a thickness of about 10 µm to be used to detect penetrating radiation, e.g., X-rays, since the effective thickness of the films is on the order of hundreds of micrometers. © 2003 MAIK "Nauka/Interperiodica".
|
|
| 10. |
|
|
| 11. |
|
|
| 12. |
- Donchev, V., et al.
(författare)
-
Photoluminescence study of AlAs/GaAs superlattices containing enlarged wells
- 2000
-
Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 364:1, s. 224-227
-
Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence (PL) spectra of MBE grown short-period AlAs/GaAs superlattices with one or two enlarged wells (5 and 12 nm) have been measured at 2 K. Sharp PL peaks corresponding to excitonic transitions between the lowest electron and heavy-hole states in the enlarged wells are observed. The excitonic transition energies are calculated by means of an envelope function based model, taking into account the exciton binding energies. The model incorporates a smooth potential at the interfaces, which is represented by a diffusion potential, the diffusion length being a parameter. The calculated and experimentally observed excitonic transition energies agree well if diffusion lengths of 3.5 and 4.5 monolayers are considered in the samples with and without a buffer layer, respectively. These values are consistent with the complicated nature of the growth kinetics and mechanisms of quantum heterostructures. The PL spectra reveal also complicated structures connected with the superlattice. Their qualitative discussion confirms the smooth potential model. Thus, an attempt is made to extend the analysis of complicated AlAs/GaAs heterostructures towards real interfaces, which is essential for advanced device fabrication.
|
|
| 13. |
- Ivanov, A G, et al.
(författare)
-
Iron stress restricts photosynthetic intersystem electron transport in Synechococcus sp, PCC 7942
- 2000
-
Ingår i: FEBS Letters. - 0014-5793 .- 1873-3468. ; 485:2-3, s. 173-177
-
Tidskriftsartikel (refereegranskat)abstract
- Although exposure of Synechococcus sp. PCC 7942 to iron stress induced the accumulation of the isiA gene product (CP43') compared with non-stressed controls, immunodetection of the N-terminus of cytochrome (Cyt) f indicated that iron stress not only reduced the content of the 40 kDa, heme-binding, Cyt f polypeptide by 32% but it also specifically induced the accumulation of a new, 23 kDa, non-heme-binding, putative Cyt f polypeptide, Concomitantly, iron stress restricted intersystem electron transport based on the in vivo reduction of P700(+), monitored as DeltaA(820)/A(820) in the presence and absence of electron transport inhibitors, as well as the inhibition of the Emerson enhancement effect on O-2 evolution. However, iron stress appeared to be associated with enhanced rates of PS I cyclic electron transport, low rates of PS I-driven photoreduction of NADP(+) but comparable rates for PS II+PS I photoreduction of NADP(+) relative to controls. We hypothesize that Synechococcus sp, PCC 7942 exhibits a dynamic capacity to uncouple PS II and PS I electron transport, which may allow for the higher than expected growth rates observed during iron stress. (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.
|
|
| 14. |
- Ivanov, A G, et al.
(författare)
-
Low-temperature modulation of the redox properties of the acceptor side of photosystem II : photoprotection through reaction centre quenching of excess energy
- 2003
-
Ingår i: Physiologia Plantarum. - 0031-9317 .- 1399-3054. ; 119:3, s. 376-383
-
Tidskriftsartikel (refereegranskat)abstract
- Although it has been well established that acclimation to low growth temperatures is strongly correlated with an increased proportion of reduced Q(A) in all photosynthetic groups, the precise mechanism controlling the redox state of Q(A) and its physiological significance in developing cold tolerance in photoautotrophs has not been fully elucidated. Our recent thermoluminescence (TL) measurements of the acceptor site of PSII have revealed that short-term exposure of the cyanobacterium Synechococcus sp. PCC 7942 to cold stress, overwintering of Scots pine (Pinus sylvestris L.), and acclimation of Arabidopsis plants to low growth temperatures, all caused a substantial shift in the characteristic T-M of S(2)Q(B)(-) recombination to lower temperatures. These changes were accompanied by much lower overall TL emission, restricted electron transfer between Q(A) and Q(B), and in Arabidopsis by a shift of the S(2)Q(A)(-)-related peak to higher temperatures. The shifts in recombination temperatures are indicative of a lower activation energy for the S(2)Q(B)(-) redox pair and a higher activation energy for the S(2)Q(A)(-) redox pair. This results in an increase in the free-energy gap between P680(+)Q(A)(-) and P680(+)Pheo(-) and a narrowing of the free energy gap between Q(A) and Q(B) electron acceptors. We propose that these effects result in an increased population of reduced Q(A) (Q(A)(-)), facilitating non-radiative P680(+)Q(A)(-) radical pair recombination within the PSII reaction centre. The proposed reaction centre quenching could be an important protective mechanism in cyanobacteria in which antenna and zeaxanthin cycle-dependent quenching are not present. In herbaceous plants, the enhanced capacity for dissipation of excess light energy via PSII reaction centre quenching following cold acclimation may complement their capacity for increased utilization of absorbed light through CO2 assimilation and carbon metabolism. During overwintering of evergreens, when photosynthesis is inhibited, PSII reaction centre quenching may complement non-photochemical quenching within the light-harvesting antenna when zeaxanthin cycle-dependent energy quenching is thermodynamically restricted by low temperatures. We suggest that PSII reaction centre quenching is a significant mechanism enabling cold-acclimated organisms to acquire increased resistance to high light.
|
|
| 15. |
- Ivanov, A G, et al.
(författare)
-
Photosynthetic electron transport adjustments in overwintering Scots pine (Pinus sylvestris L.)
- 2001
-
Ingår i: Planta. - 0032-0935 .- 1432-2048. ; 213:4, s. 575-585
-
Tidskriftsartikel (refereegranskat)abstract
- As shown before [C. Ottander et al. (1995) Planta 197:176-183], there is a severe inhibition of the photosystem (PS) II photochemical efficiency of Scots pine (Pinus sylvestris L.) during the winter. In contrast, the in vivo PSI photochemistry is less inhibited during winter as shown by in vivo measurements of DeltaA(820)/Delta (820) (P700(+)). There was also an enhanced cyclic electron transfer around PSI in winter-stressed needles as indicated by 4-fold faster reduction kinetics of P700(+). The differential functional stability of PSII and PSI was accompanied by a 3.7-fold higher intersystem electron pool size, and a 5-fold increase in the stromal electron pool available for P700(+) reduction. There was also a strong reduction of the QB band in the thermoluminescence glow curve and markedly slower Q-A re-oxidation in needles of winter pine, indicating an inhibition of electron transfer between QA and QB. The data presented indicate that the plastoquinone pool is largely reduced in winter pine, and that this reduced state is likely to be of metabolic rather than photochemical origin. The retention of PSI photochemistry, and the suggested metabolic reduction of the plastoquinone pool in winter stressed needles of Scots pine are discussed in terms of the need for enhanced photoprotection of the needles during the winter and the role of metabolically supplied energy for the recovery of photosynthesis from winter stress in evergreens.
|
|
| 16. |
- Ivanov, A G, et al.
(författare)
-
Seasonal responses of photosynthetic electron transport in Scots pine (Pinus sylvestris L.) studied by thermoluminescence
- 2002
-
Ingår i: Planta. - : Springer Science and Business Media LLC. - 0032-0935 .- 1432-2048. ; 215:3, s. 457-465
-
Tidskriftsartikel (refereegranskat)abstract
- The potential of photosynthesis to recover from winter stress was studied by following the thermoluminescence (TL) and chlorophyll fluorescence changes of winter pine needles during the exposure to room temperature (20 degreesC) and an irradiance of 100 mumol m(-2) s(-1). TL measurements of photosystem 11 (PSII) revealed that the S(2)Q(B)(-) charge recombinations (the B-band) were shifted to lower temperatures in winter pine needles, while the S(2)Q(A)(-) recombinations (the Q-band) remained close to 0 degreesC. This was accompanied by a drastically reduced (65%) PSII photochemical efficiency measured as F-v/F-m and a 20-fold faster rate of the fluorescence transient from F-o to F, as compared to summer pine. A strong positive correlation between the increase in the photochemical efficiency of PSII and the increase in the relative contribution of the B-band was found during the time course of the recovery process. The seasonal dynamics of TL in Scots pine needles studied under field conditions revealed that between November and April, the contribution of the Q- and B-bands to the overall TL emission was very low (less than 5%). During spring, the relative contribution of the Q- and B-bands, corresponding to charge recombination events between the acceptor and donor sides of PSII, rapidly increased, reaching maximal values in late July. A sharp decline of the B-band was observed in late summer, followed by a gradual decrease, reaching minimal values in November. Possible mechanisms of the seasonally induced changes in the redox properties Of S-2/S(3)Q(B)(-) recombinations are discussed. It is proposed that the lowered redox potential Of Q(B) in winter needles increases the population Of Q(A)(-). thus enhancing the probability for non-radiative P680(+) Q(A)(-) recombination. This is suggested to enhance the radiationless dissipation of excess light within the PSII reaction center during cold acclimation and during cold winter periods.
|
|
| 17. |
- Ivanov, A M, et al.
(författare)
-
High-resolution short range ion detectors based on 4H-SiC films
- 2004
-
Ingår i: Technical physics letters. - : Pleiades Publishing Ltd. - 1063-7850 .- 1090-6533. ; 30:7, s. 575-577
-
Tidskriftsartikel (refereegranskat)abstract
- The energy resolution of SiC detectors has been studied in application to the spectrometry of alpha particles with 5.1-5.5 MeV energies. The Schottky barrier structure of the detector was based on a CVD-grown epitaxial n-4H-SiC film with a thickness of 26 mum and an uncompensated donor concentration of (1-2) x 10(15) cm(-3). An energy resolution of 0.5% achieved for the first time with SiC detectors allows fine structure of the alpha particle spectrum to be revealed. The average energy of the electron-hole pair formation in 4H-SiC is estimated at 7.71 eV.
|
|
| 18. |
- Ivanov, Alexander V., et al.
(författare)
-
Binuclear cadmium dithiophosphate crystals: 13C, 31P, and 113Cd CP/MAS NMR studies
- 2003
-
Ingår i: Russian journal of coordination chemistry. - 1070-3284 .- 1608-3318. ; 29:5, s. 301-306
-
Tidskriftsartikel (refereegranskat)abstract
- 31P NMR signals for the terminal and bridging ligands of the complexes were differentiated. The experimental NMR spectra show 31P-111,113Cd and 113Cd-31 P spin-spin couplings only for the terminal ligands. The chemical shift anisotropy δ and the asymmetry parameter η were calculated for 31P and 113Cd NMR signals. It was found that the 31P chemical shifts for the terminal and bridging dithiophosphato groups differ in anisotropy character.
|
|
| 19. |
|
|
| 20. |
- Ivanov, Alexander V., et al.
(författare)
-
Structural organization of nickel(II) and copper(II) O,O'-dialkyl phosphorodithioate complexes as probed by X-ray diffraction, EPR, and CP/MAS 13C and 31P NMR
- 2004
-
Ingår i: Russian Journal of Inorganic Chemistry. - 0036-0236 .- 1531-8613. ; 49:3, s. 373-385
-
Tidskriftsartikel (refereegranskat)abstract
- The nickel(II) and copper(II) complexes [M{S2P(OR)(2)}(2)] (M = Ni, Cu-63, Cu-65; R = C2H5, C3H7, i-C3H7, C4H9, i-C4H9, s-C4H9, i-C5H11, c-C6H11) with eight symmetric O,O'-dialkyl phosphorodithioates were synthesized, and their structures and spectral properties were studied by EPR and CP/MAS C-13 and P-31 NMR. As determined by EPR, the [CuS4] chromophores in the lattice of nickel(II) complexes have predominantly a square-planar structure. The chromophores in complexes with phosphorodithioate ligands incorporating bulky alkyl substituents have somewhat distorted geometries. NMR shows that the Dtph groups in most nickel(II) complexes are structurally equivalent. An exception is [Ni{S2P(OC3H7)(2)}(2)], which gives rise to two (1 : 1) P-31 NMR signals. For [Ni(S2P(O-i-C4H9)(2))(2)], the alpha and beta modifications were obtained. In the alpha modification, the phosphorodithioate groups are nonequivalent, whereas the P modification has structurally equivalent ligands. The broadening of the P-31 NMR signal of the [Ni{S2P(O-s-C4H9)(2)}(2)] complex is rationalized by the existence of six optical isomers (due to two chiral centers in each of the ligands). For the Dtph groups acting as bidentate terminal ligands in the nickel(II) complexes, the P-31 chemical shift anisotropy delta(aniso) and asymmetry parameter eta were calculated from experimental NMR spectra. Single-crystal X-ray diffraction shows that bis(O,O'-dipropyl phosphorodithioato)nickel(II) exists in the form of two centrosymmetric molecules. In the structure of alpha-bis(O,O'-dipropyl phosphorodithioato)nickel(II), the Dtph ligands in the molecule are nonequivalent. The experimental P-31 NMR signals in the spectra of [Ni{S2P(OC3H7)(2)}(2)] and alpha-[Ni{S2P(O-i-C4H9)(2)}(2)] are assigned to the structural positions of phosphorus atoms in the corresponding molecular structures determined by X-ray diffraction.
|
|
| 21. |
|
|
| 22. |
|
|
| 23. |
|
|
| 24. |
- Krol, M, et al.
(författare)
-
Low growth temperature inhibition of photosynthesis in cotyledons of jack pine seedlings (Pinus banksiana) is due to impaired chloroplast development
- 2002
-
Ingår i: Canadian Journal of Botany. - 0008-4026 .- 1480-3305. ; 80:10, s. 1042-1051
-
Tidskriftsartikel (refereegranskat)abstract
- Cotyledons of jack pine seedlings (Pinus banksiana Lamb.) grown from seeds were expanded at low temperature (5degreesC), and total Chl content per unit area of cotyledons in these seedlings was only 57% of that observed for cotyledons on 20degreesC-grown controls. Chl a/b ratio of 5degreesC-grown jack pine was about 20% lower (2.3 +/- 0.1) than 20degreesC controls (2.8 +/- 0.3). Separation of Chl-protein complexes and SDS-PAGE indicated a significant reduction in the major Chl a containing complex of PSI (CP1) and PSII (CPa) relative to LHCII1 in 5degreesC compared to 20degreesC-grown seedlings. In addition, LHCII1/LHCII3 ratio increased from 3.8 in control (20degreesC) to 5.5 in 5degreesC-grown cotyledons. Ultrastructurally, 5degreesC-grown cotyledons had chloroplasts with swollen thylakoids as well as etiochloroplasts with distinct prolamellar bodies. Based on CO2-saturated O-2 evolution and in vivo Chl a fluorescence, cotyledons of 5degreesC jack pine exhibited an apparent photosynthetic efficiency that was 40% lower than 20degreesC controls. Seedlings grown at 5degreesC were photoinhibited more rapidly at 5degreesC and 1200 mumol.m(-2).s(-1) than controls grown at 20degreesC, although the final extent of photoinhibition was similar. Exposure to high light at 5degreesC stimulated the xanthophyll cycle in cotyledons of both controls and 5degreesC-grown seedlings. In contrast to winter cereals, we conclude that growth of jack pine at 5degreesC impairs normal chloroplast biogenesis, which leads to an inhibition of photosynthetic efficiency.
|
|
| 25. |
- Kyutt, R N, et al.
(författare)
-
X-ray diffraction determination of the interface structure of CdSe/BeTe superlattices
- 2003
-
Ingår i: Journal of Physics D. - : Iop Publishing Ltd. - 0022-3727 .- 1361-6463. ; 36:10A, s. A166-A171
-
Tidskriftsartikel (refereegranskat)abstract
- Structural study of CdSe/BeTe superlattices (SLs) grown by molecular beam epitaxy on GaAs substrate was performed by using double and triple crystal x-ray diffractometry. The period of the studied structures was about 5 nm, while the thickness of thin CdSe insertions varied from 0.4 to 1.5 monolayer. It is shown that new Be-Se bonds arise at the BeTe-CdSe interfaces in addition to the Be-Se bonds expected at the CdSe-BeTe interfaces. From the analysis of the diffraction curves of 002-reflection the complex composition of interfaces and thin insertions has been determined and contribution of all types of bonds in each SL period calculated. The diffraction curves of 004-reflection were used for the specification of the fine structure of the interfaces.
|
|
| 26. |
- Lebedev, A.A., et al.
(författare)
-
Radiation hardness of wide-gap semiconductors (using the example of silicon carbide)
- 2002
-
Ingår i: Semiconductors (Woodbury, N.Y.). - : Pleiades Publishing Ltd. - 1063-7826 .- 1090-6479. ; 36:11, s. 1270-1275
-
Tidskriftsartikel (refereegranskat)abstract
- Results obtained in studying the effect of ionizing radiation on epitaxial layers and devices based on silicon carbide (SiC) are considered. It is shown that, in investigations of wide-gap semiconductors (WGS), account should be taken of how the rate of removal of mobile charge carriers - the standard parameter in determining the radiation hardness of a material - depends on temperature. The use of data obtained only at room temperature may lead to an incorrect assessment of the radiation hardness of WGS. A conclusion is made that the WGS properties combine, on the one hand, high radiation hardness of high-temperature devices based on these semiconductors and, on the other, the possibility of effective radiation-induced doping (e.g., for obtaining semi-insulating local regions in a material at room temperature). © 2002 MAIK "Nauka/Interperiodica".
|
|
| 27. |
- Moskvin, O V, et al.
(författare)
-
Carbonic anhydrase activities in pea thylakoids. : A photosystem II core complex-associated carbonic anhydrase
- 2004
-
Ingår i: Photosynthesis Research. - 0166-8595. ; 79:1, s. 93-100
-
Tidskriftsartikel (refereegranskat)abstract
- Pea thylakoids with high carbonic anhydrase (CA) activity (average rates of 5000 µmol H+ (mg Chl)–1 h–1 at pH 7.0) were prepared. Western blot analysis using antibodies raised against the soluble stromal beta-CA from spinach clearly showed that this activity is not a result of contamination of the thylakoids with the stromal CA but is derived from a thylakoid membrane-associated CA. Increase of the CA activity after partial membrane disintegration by detergent treatment, freezing or sonication implies the location of the CA in the thylakoid interior. Salt treatment of thylakoids demonstrated that while one part of the initial enzyme activity is easily soluble, the rest of it appears to be tightly associated with the membrane. CA activity being measured as HCO3 – dehydration (dehydrase activity) in Photosystem II particles (BBY) was variable and usually low. The highest and most reproducible activities (approximately 2000 µmol H+ (mg Chl)–1 h–1) were observed in the presence of detergents (Triton X-100 or n-octyl-beta-D-glucopyranoside) in low concentrations. The dehydrase CA activity of BBY particles was more sensitive to the lipophilic CA inhibitor, ethoxyzolamide, than to the hydrophilic CA inhibitor, acetazolamide. CA activity was detected in PS II core complexes with average rate of 13,000 µmol H+ (mg Chl)–1 h–1 which was comparable to CA activity in BBY particles normalized on a PS II reaction center basis.
|
|
| 28. |
- Rojdestvenski, I, et al.
(författare)
-
Segregation of photosystems in thylakoid membranes as a critical phenomenon
- 2002
-
Ingår i: Biophysical Journal. - 0006-3495 .- 1542-0086. ; 82:4, s. 1719-1730
-
Tidskriftsartikel (refereegranskat)abstract
- The distribution of the two photosystems, PSI and PSII, in grana and stroma lamellae of the chloroplast membranes is not uniform. PSII are mainly concentrated in grana and PSI in stroma thylakoids. The dynamics and factors controlling the spatial segregation of PSI and PSII are generally not well understood, and here we address the segregation of photosystems in thylakoid membranes by means of a molecular dynamics method. The lateral segregation of photosystems was studied assuming a model comprising a two-dimensional (in-plane), two-component, many-body system with periodic boundary conditions and competing interactions between the photosystems in the thylakoid membrane. PSI and PSII are represented by particles with different values of negative charge. The pair interactions between particles include a screened Coulomb repulsive part and an exponentially decaying attractive part. The modeling results suggest a complicated phase behavior of the system, including quasi-crystalline phase of randomly distributed complexes of PSII and PSI at low ionic screening, well defined clustered state of segregated complexes at high screening, and in addition, an intermediate agglomerate phase where the photosystems tend to aggregate together without segregation. The calculations demonstrated that the ordering of photosystems within the membrane was the result of interplay between electrostatic and lipid-mediated interactions. At some values of the model parameters the segregation can be represented visually as well as by analyzing the correlation functions of the configuration.
|
|
| 29. |
- Sane, P V, et al.
(författare)
-
A transient exchange of the photosystem II reaction center protein D1 : 1 with D1 : 2 during low temperature stress of Synechococcus sp PCC 7942 in the light lowers the redox potential of Q(B)
- 2002
-
Ingår i: Journal of Biological Chemistry. - 0021-9258 .- 1083-351X. ; 277:36, s. 32739-32745
-
Tidskriftsartikel (refereegranskat)abstract
- Upon exposure to low temperature under constant light conditions, the cyanobacterium Synechococcus sp. PCC 7942 exchanges the photosystem II reaction center D1 protein form I (D1:1) with D1 protein form 2 (D1:2). This exchange is only transient, and after acclimation to low temperature the cells revert back to D1:1, which is the preferred form in acclimated cells (Campbell, D., Zhou, G., Gustafsson, P., Oquist, G., and Clarke, A. K. (1995) EMBO J. 14, 5457-5466). In the present work we use thermoluminescence to study charge recombination events between the acceptor and donor sides of photosystem II in relation to D1 replacement. The data indicate that in cold-stressed cells exhibiting D1:2, the redox potential of Q(B) becomes lower approaching that of Q(A). This was confirmed by examining the Synechococcus sp. PCC 7942 inactivation mutants R2S2C3 and R2K1, which possess only D1:1 or D1:2, respectively. In contrast, the recombination of Q(A)(-) with the S-2 and S-3 states did not show any change in their redox characteristics upon the shift from D1:1 to D1:2. We suggest that the change in redox properties of Q(B) results in altered charge equilibrium in favor of Q(A). This would significantly increase the probability of Q(A)(-) and P680(+) recombination. The resulting non-radiative energy dissipation within the reaction center of PSII may serve as a highly effective protective mechanism against photodamage upon excessive excitation. The proposed reaction center quenching is an important protective mechanism because antenna and zeaxanthin cycle-dependent quenching are not present in cyanobacteria. We suggest that lowering the redox potential of Q(B) by exchanging D1:1 for D1:2 imparts the increased resistance to high excitation pressure induced by exposure to either low temperature or high light.
|
|
| 30. |
- Sane, P V, et al.
(författare)
-
Changes in the redox potential of primary and secondary electron-accepting quinones in photosystem II confer increased resistance to photoinhibition in low-temperature-acclimated arabidopsis
- 2003
-
Ingår i: Plant Physiology. - : Oxford University Press (OUP). - 0032-0889 .- 1532-2548. ; 132:4, s. 2144-2151
-
Tidskriftsartikel (refereegranskat)abstract
- Exposure of control (non-hardened) Arabidopsis leaves for 2 h at high irradiance at 5 degreesC resulted in a 55% decrease in photosystem II (PSII) photochemical efficiency as indicated by FcFm. In contrast, cold-acclimated leaves exposed to the same conditions showed only a 22degreesC decrease in FupsilonFm. Thermoluminescence was used to assess the possible role(s) of PSII recombination events in this differential resistance to photoinhibition. Thermoluminescence measurements of PSH revealed that S(2)QA(-) recombination was shifted to higher temperatures, whereas the characteristic temperature of the S(2)Q(B)(-) recombination was shifted to lower temperatures in cold-acclimated plants. These shifts in recombination temperatures indicate higher activation energy for the S(2)Q(A)(-) redox, pair and lower activation energy for the S(2)Q(B) redoxpair. This results in an increase in the free-energy gap between P680(+)Q(A)(-) and P680(+)Pheo(-) and a narrowing of the free energy gap between primary and secondary electron-accepting quinones in PSH electron acceptors. We propose that these effects result in an increased population of reduced primary electron-accepting quinone in PSII, facilitating non-radiative P680(+)QA(-) radical pair recombination. Enhanced reaction center quenching was confirmed using in vivo chlorophyll fluorescence-quenching analysis. The enhanced dissipation of excess light energy within the reaction center of PSII, in part, accounts for the observed increase in resistance to high-light stress in cold-acclimated Arabidopsis plants.
|
|
| 31. |
- Savitch, L V, et al.
(författare)
-
Cold acclimation of Arabidopsis thaliana results in incomplete recovery of photosynthetic capacity, associated with an increased reduction of the chloroplast stroma
- 2001
-
Ingår i: Planta. - 0032-0935 .- 1432-2048. ; 214:2, s. 295-303
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of short-term cold stress and long-term cold acclimation on the light reactions of photosynthesis were examined in vivo to assess their contributions to photosynthetic acclimation to low temperature in Arabidopsis thaliana (L.) Heynh.. All photosynthetic measurements were made at the temperature of exposure: 23 degreesC for non-acclimated plants and 5 degreesC for cold-stressed and cold-acclimated plants. Three-day cold-stress treatments at 5 degreesC inhibited light-saturated rates of CO2 assimilation and O-2 evolution by approximately 75%. The 3-day exposure to 5 degreesC also increased the proportion of reduced QA by 50%, decreased the yield of PSII electron transport by 65% and decreased PSI activity by 31%. In contrast, long-term cold acclimation resulted in a strong but incomplete recovery of light-saturated photosynthesis at 5 degreesC. The rates of light-saturated CO2 and O-2 gas exchange and the in vivo yield of PSII activity under light-saturating conditions were only 35-40% lower, and the relative redox state of QA only 20% lower, at 5 degreesC after cold acclimation than in controls at 23 degreesC. PSI activity showed full recovery during long-term cold acclimation. Neither short-term cold stress nor long-term cold acclimation of Arabidopsis was associated with a limitation in ATP, and both treatments resulted in an increase in the ATP/NADPH ratio. This increase in ATP/NADPH was associated with an inhibition of PSI cyclic electron transport but there was no apparent change in the Mehler reaction activity in either cold-stressed or cold-acclimated leaves. Cold acclimation also resulted in an increase in the reduction state of the stroma, as indicated by an increased total activity and activation state of NADP-dependent malate dehydrogenase, and increased light-dependent activities of the major regulatory enzymes of the oxidative pentose-phosphate pathway. We suggest that the photosynthetic capacity during cold stress as well as cold acclimation is altered by limitations at the level of consumption of reducing power in carbon metabolism.
|
|
| 32. |
- Shtinkov, N., et al.
(författare)
-
Effect of non-abrupt interfaces in AlAs/GaAs superlattices with embedded GaAs quantum wells
- 2000
-
Ingår i: Vacuum. - 0042-207X .- 1879-2715. ; 58:2, s. 561-567
-
Tidskriftsartikel (refereegranskat)abstract
- In the present paper, we investigate the effect of the non-abrupt interfaces on the electronic and optical properties of short-period AlAs/GaAs superlattices with embedded GaAs quantum wells. The lateral disorder and the component interdiffusion at the interfaces are averaged over the layer planes and are effectively represented by a diffusion concentration profile in the growth direction. The diffusion length LD is used as a parameter characterizing the degree of interface broadening. The electronic structure calculations are made using the sp3s* spin-dependent empirical tight-binding Hamiltonian, the virtual crystal approximation, and the surface Green function matching method. The dependencies of the lowest electron (E1), heavy hole (HH1), and light hole (LH1) bound states on the diffusion length are calculated for LD from 0 to 4 monolayers. It is found that the energies of the transitions (E1-HH1) and (E1-LH1) increase as LD increases. The results obtained are compared with photoluminescence data for MBE-grown samples. It is found that the degree of interface broadening depends on the growth temperature and on the sample geometry. The diffusion lengths calculated from the experimental data follow the expected trends, revealing a good qualitative agreement between theory and experiment.
|
|
| 33. |
|
|
| 34. |
- Shubina, Tatiana, et al.
(författare)
-
Optical properties of GaN/AlGaN quantum wells with inversion domains
- 2003
-
Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 195:3, s. 537-542
-
Tidskriftsartikel (refereegranskat)abstract
- Two-band photoluminescence (PL) and respective absorption and reflection features are observed in GaN/AlGaN MBE-grown quantum well (QW) structures of dominant N polarity with inversion domains (IDs). The PL bands are related to transitions in the regions of different polarity, characterized by different strain and electric fields. A micro-PL study reveals sharp and narrow (1.5-2.5 meV) PL lines placed between the bands, which are tentatively attributed to recombination at localization sites associated with intersections of the QWs with the domains. Additionally, we demonstrate that the ID formation decreases the overall strength of the intrinsic electric fields in the QW structures.
|
|
| 35. |
- Tkachman, M.G., et al.
(författare)
-
Phonon-assisted exciton luminescence in GaN layers grown by MBE and chloride-hydride VPE
- 2003
-
Ingår i: Semiconductors (Woodbury, N.Y.). - : Pleiades Publishing Ltd. - 1063-7826 .- 1090-6479. ; 37:5, s. 532-536
-
Tidskriftsartikel (refereegranskat)abstract
- Optical properties of GaN layers grown by the molecular-beam epitaxy (MBE) and chloride-hydride vapor-phase epitaxy (CHVPE) have been studied, and the quality of two types of samples has been compared. The photoluminescence spectra have an excitonic nature for both types of layers. To determine precisely the nature of exciton transitions, the reflectance spectra were studied. A key point was the investigation of phonon-assisted exciton luminescence, which provides information on the density distribution of the exciton states. Temperature dependences of the exciton transition energy and the ratio of intensities of one- and two-phpnon replicas were studied. The high quality of both types of layers has been confirmed, though the concentration of acceptors in MBE-grown samples is higher than in CHVPE samples. © 2003 MAIK "Nauka/Interperiodica".
|
|
| 36. |
- Toropov, A.A., et al.
(författare)
-
Excitons as a probe of interface morphology in Cd(Zn)Se/ZnSe heterostructures
- 2000
-
Ingår i: Applied Surface Science. - 0169-4332 .- 1873-5584. ; 166:1, s. 278-283
-
Tidskriftsartikel (refereegranskat)abstract
- We present studies of the excitonic spectrum in superlattices (SLs) of CdSe insertions in a ZnSe matrix aimed at elucidating the CdSe/ZnSe interface morphology. The experimental photoluminescence excitation spectra are compared with the results of variational exciton calculations performed within the effective mass approximation. The shape of the average vertical (along the SL growth axis) distribution of CdSe within each insertion, used in the calculations, was obtained from a theoretical simulation of X-ray diffraction (XRD) rocking curves measured in the same samples. The results indicate that the thinnest layers are graded composition ZnCdSe quantum wells (QWs), generally homogeneous in the layer planes, whereas flat islands enriched by Cd appear at the CdSe nominal thickness larger than 0.5-0.6 monolayer (ML).
|
|
| 37. |
- Tzolov, M, et al.
(författare)
-
Modification of the structure of ZnO : Al films by control of the plasma parameters
- 2001
-
Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 396:1-2, s. 274-279
-
Tidskriftsartikel (refereegranskat)abstract
- ZnO:Al films were deposited by RF magnetron sputtering in triode configuration applying an external DC electric field to the substrates. Reflection high-energy electron diffraction measurements characterized the different films as consisting of randomly-oriented zinc blende crystallites or randomly and texture-oriented wurtzite crystallites, as well as of the amorphous phase. The non-resonant Raman spectra are strongly influenced by the presence of a built-in electric field at the grain boundaries and they do not depend on the symmetry of the microcrystallites. The Raman spectra taken at resonant excitation are more sensitive to the presence of the amorphous phase in the films. (C) 2001 Elsevier Science B.V. All rights reserved.
|
|
| 38. |
- Tzolov, M., et al.
(författare)
-
Vibrational properties and structure of undoped and Al-doped ZnO films deposited by RF magnetron sputtering
- 2000
-
Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 379:1-2, s. 28-36
-
Tidskriftsartikel (refereegranskat)abstract
- Highly conductive and transparent in the visible range Al-doped ZnO (ZnO:Al) and undoped ZnO films have been deposited by RF magnetron sputtering. Reflection high-energy electron diffraction observations characterized them as textured. The habitus of the microcrystallites forming the texture depends on the Al doping. The layer texture of undoped ZnO films has texture axis parallel to the substrate. The ZnO:Al films, instead, show a columnar texture with texture axis perpendicular to the substrate. The Raman spectra of the films obtained by non-resonant excitation are completely different from those of the target material which is polycrystalline ZnO. For the interpretation of the different bands in the Raman spectra the existence of a depletion region near the grain boundaries has been assumed. The most intensive band in the Raman spectra at approximately 570 cm-1 has been assigned to electric field-induced Raman scattering on longitudinal optical phonons. The built-in electric field in the depletion region induces the Raman activity of the B2 modes and a band at 276 cm-1 appears in the spectra. Phonon modes highly localized near the grain boundaries have been detected at 516 cm-1 and 468 cm-1 which are well pronounced in the Raman spectra for the doped samples. Localized modes were observed also in the infrared reflection spectra of the doped films. Surface enhanced Raman scattering has been applied and the band in the range 830-920 cm-1 has been interpreted as due to adsorbates from the ambient air. It has been shown that the non-resonant Raman scattering can be used for qualitative study of some details of the microstructure of the zinc oxide films like the built-in electric field and the adsorbates in the films.
|
|
| 39. |
- Valakh, M.Ya., et al.
(författare)
-
Optical investigation of CdSe/ZnSe quantum nanostructures
- 2002
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 17:2, s. 173-177
-
Tidskriftsartikel (refereegranskat)abstract
- We have used resonant and non-resonant Raman scattering as well as photoluminescence and cathodoluminescence experiments to study the structure and composition properties of CdxZn1-xSe formed by migration enhanced epitaxy of CdSe layers on ZnSe buffers. The spectral change of the photoluminescence maximum correlates with the increase of the Cd content, depending on the nominal CdSe thickness in the 1.5-3.0 ML range. The inhomogeneous broadening of the photoluminescence band is caused by the composition difference between the two-dimensional mixed CdxZn1-xSe layer and the inserted islands with larger Cd concentration. This is confirmed by phonon frequency changes in resonant Raman scattering for samples with different nominal CdSe thicknesses as well as in Stokes and anti-Stokes frequency changes observed in the 1.5 ML sample. Attention is paid to the role of defects on Raman scattering and photoluminescence for the 3.15 ML sample.
|
|
