Low-temperature Absorption Research Articles

Glass Transition Process of Amorphous Quinidine Studied by Using Brillouin Light Scattering

The acoustic properties of amorphous quinidine were investigated by using Brillouin spectroscopy as a function of temperature. The longitudinal sound velocity and the acoustic absorption coefficient showed drastic changes in their slopes at the glass transition temperature of 328 K, which was nearly the same as the thermodynamic and the dielectric glass transition temperatures. The clear difference between the crystalline state and the amorphous state of quinidine, as well as the crystallization process, could be revealed by observing the change in the Brillouin spectrum. Analysis of the lowtemperature absorption coefficient showed that the excess acoustic damping could be attributed to intramolecular relaxation motions, such as side-group rotations.

Crystal growth, low-temperature spectroscopy and multi-watt laser operation of Yb:Ca3NbGa3Si2O14

We report on the crystal growth, structure, room- and low-temperature (6K) absorption and emission spectroscopy of a 3at% Yb3+-doped trigonal ordered langasite-type silicate crystal, Ca3NbGa3Si2O14 (Yb: CNGS). The Rietveld refinement data are presented. Absorption, stimulated-emission and gain cross-sections are determined for the Yb3+ ion in CNGS for π and σ polarizations. The maximum stimulated emission cross-section amounts to σSE = 0.97 × 10−20cm−2 at 1014nm (σ-polarization). The Stark splitting of the Yb3+ multiplets is resolved. Microchip laser operation is obtained with both a-cut and c-cut 3at% Yb: CNGS samples under end-pumping at 976nm by a Volume Bragg Grating-stabilized diode laser. For the latter cut, the maximum output power reaches 7.61W at 1048–1060nm with a slope efficiency of 59%. By varying the output coupling, multi-watt emission over a 50nm-broad spectral range is achieved.

The deep red state of photosystem II in Cyanidioschyzon merolae

We identified and characterised the deep red state (DRS), an optically-absorbing charge transfer state of PSII, which lies at lower energy than P680, in the red algae Cyanidioschyzon merolae by means of low temperature absorption and magnetic circular dichroism spectroscopies. The photoactive DRS has been previously studied in PSII of the higher plant Spinacia oleracea, and in the cyanobacterium Thermosynechococcus vulcanus. We found the DRS in PSII of C. merolae has similar spectral properties. Treatment of PSII with dithionite leads to reduction of cytochrome (cyt) b559 and the PsbV-based cyt c550 as well as the disassembly of the oxygen-evolving complex. Whereas the overall visible absorption spectrum of PSII was little affected, the DRS absorption in the reduced sample was no longer seen. This bleaching of the DRS is discussed in terms of a corresponding lack of a DRS feature in D1D2/cyt b559 reaction centre preparations of PSII.

Yellow laser potential of cubic Ca3(Nb,Ga)5O12:Dy3+ and Ca3(Li,Nb,Ga)5O12:Dy3+ single crystals

Spectroscopic investigation of Dy3+ doped Ca3(Nb,Ga)5O12 (CNGG) and Ca3(Li,Nb,Ga)5O12 (CLNGG) single crystals were performed in order to assess their potential as laser materials for yellow emission. Dy:CNGG and Dy:CLNGG single crystals were grown by the Czochralski method and investigated by high-resolution spectroscopic measurements for the first time. The Judd-Ofelt intensity parameters for the f-f transitions of Dy3+ in CNGG and CLNGG single crystals, were used to determine radiative transition rates Ar, branching ratios β, and radiative lifetime τr of the fluorescent Dy3+ levels. Based on low temperature absorption and emission spectra, partial energy levels diagram of Dy3+ doped CNGG and CLNGG single crystals were obtained. The emission cross-sections for the 4F9/2 → 6H13/2 transition of special interest for laser application were determined. The room temperature decay curves were analyzed through the framework of Inokuti–Hirayama (I-H) model and the results shows that electric dipole-dipole interaction are responsible for the energy transfer processes between Dy3+ ions in CNGG and CLNGG single crystals.

Spectroscopic identification of active sites for the oxygen evolution reaction on iron-cobalt oxides

The emergence of disordered metal oxides as electrocatalysts for the oxygen evolution reaction and reports of amorphization of crystalline materials during electrocatalysis reveal a need for robust structural models for this class of materials. Here we apply a combination of low-temperature X-ray absorption spectroscopy and time-resolved in situ X-ray absorption spectroelectrochemistry to analyze the structure and electrochemical properties of a series of disordered iron-cobalt oxides. We identify a composition-dependent distribution of di-μ-oxo bridged cobalt–cobalt, di-μ-oxo bridged cobalt–iron and corner-sharing cobalt structural motifs in the composition series. Comparison of the structural model with (spectro)electrochemical data reveals relationships across the composition series that enable unprecedented assignment of voltammetric redox processes to specific structural motifs. We confirm that oxygen evolution occurs at two distinct reaction sites, di-μ-oxo bridged cobalt–cobalt and di-μ-oxo bridged iron–cobalt sites, and identify direct and indirect modes-of-action for iron ions in the mixed-metal compositions.

Low temperature absorption edge and photoluminescence study in TlIn(Se1−xSx)2 layered mixed crystals

Transmission on TlIn(Se1−xSx)2 mixed crystals (0.25 ≤ x ≤ 1) were carried out in the 400–800 nm wavelength range at T = 10 K. Band gap energies of the studied crystals were obtained using the derivative spectra of transmittance. The compositional dependence of direct band gap energy at T = 10 K revealed that as sulfur composition is increased in the mixed crystals, the direct band gap energy rises from 2.26 eV (x = 0.25) to 2.56 eV (x = 1).Photoluminescence spectra of TlIn(Se1−xSx)2 mixed crystals were studied in the wavelength region of 400–620 nm at T = 10 K. The observed bands were attributed to the transitions of electrons from shallow donor levels to the valence band. The shift of the PL bands to higher energies with elevating sulfur content was revealed. Moreover, the composition ratio of the mixed crystals was obtained from the energy dispersive spectroscopy measurements.

Absorption chillers as a contribution to a climate-friendly refrigeration supply regime: Factors of influence on their further diffusion

Thermally-driven absorption refrigeration technology is currently being discussed as possibly contributing to a more climate-friendly refrigeration supply. However, factors beyond technological and economic aspects influencing a broader diffusion of this technology have not yet been considered. This article presents results of a socio-technical constellation analysis identifying barriers against and drivers of a fortified dissemination of innovative low-temperature absorption chillers in a more comprehensive way. Results show that future perspectives of absorption refrigeration arise from a coincidence of mutually influencing factors, also including (lacking) know-how, experience, standard solutions, developed supplier structures and political framework conditions. Due to interdependencies with developments in the heat and electricity sector, targeted intervention requires integrated approaches. Future promotion of absorption refrigeration should be part of an optimised overall energy system perspective (electricity, heat, cold).

Selection and evaluation of technologies for oil rim preparation of Zapolyarnoye oilfield

The article deals with various technologies for the oil rims preparation of Zapolyarnoye oil and gas condensate field. The following circuits were considered for the selection of the technology of preparation of extracted fluid: low-temperature separation (STC); low-temperature condensation (LTC); a low-temperature rectification (NTR); low-temperature absorption (LTA); membrane technology (Mt). Diagrams of these processes were collected and tested in the system HYSYS process simulation. Efficiency analysis of hydrocarbon raw materials preparation on the basis of functions, selection of components and narrow fractions.

Intensities and spectral features of the – potential laser transition of Er3+ centers in CaF2–CeF3 disordered crystal**Project supported by Shanghai Engineering Research Center for Sapphire Crystals, China (Grant No. 14DZ2252500), the Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics Chinese Academy of Sciences (Grant No. 2008DP17301), the Fundamental Research Funds for the Central Universities of China, the National

A CaF2–CeF3 disordered crystal containing 1.06% of Er3+ ions was grown by the temperature gradient technique. Optical absorption and emission spectra recorded at room temperature and at 10 K, luminescence decay curve recorded at room temperature, and extended x-ray-absorption fine structure spectra were analyzed with an intention to assess the laser potential related to the transition of Er3+. In addition, the thermal diffusivity of the crystal was measured at room temperature. The analysis of room-temperature spectra revealed that the emission is long-lived with a radiative lifetime value of 5.5 ms, peak emission cross section of , and large spectral width pointing at the tunability of the emission wavelength in the region stretching from approximately 1480 nm to 1630 nm. The energies of the crystal field components for the ground and excited multiplets determined from low-temperature absorption and emission spectra made it possible to predict successfully the spectral position and shape of the room-temperature emission band. Based on the correlation of the optical spectra and dynamics of the luminescence decay, it was concluded that in contrast to Yb3+ ions in heavily doped CaF2 erbium ions in the CaF2–CeF3 crystal reside in numerous sites with dissimilar relaxation rates.

Low-temperature hydrogen absorption in metallic nanocontacts studied by point-contact spectroscopy measurements

We report on hydrogen (H) and deuterium (D) atoms absorption below T = 20 K in metallic palladium (Pd) via quantum tunnelling (QT). When a small bias voltage is applied between Pd nanocontacts that are immersed in liquid H2 (D2), the differential conductance spectra measured by point-contact spectroscopy change enormously. The results indicate H (D) absorption in Pd nanocontacts at the temperature where H (D) absorption due to thermal hopping process is not expected, and can be explained by QT. The QT occurs when the energy level of the potential well trapping the H (D) atom coincides with those not trapping the H (D) atom, and is assisted by phonons induced by ballistic electrons.

Photo-thermal conversion characteristics of MWCNT-H2O nanofluids for direct solar thermal energy absorption applications

Stable water-based multi-walled carbon nanotube (MWCNT) nanofluids with mass fractions ranging from 0.0015% to 0.1% were prepared. To investigate the photo-absorption properties and photo-thermal conversion performance of MWCNT-H2O nanofluids, heating and light-irradiation cycling tests were experimentally performed at temperatures below 90°C. Heat treatment is beneficial to improve the optical absorption capability of MWCNT-H2O nanofluids, and there existed an optimal concentration of about 0.01wt% with respect to the spectral transmittance and extinction coefficient. Extinction coefficients of MWCNT-H2O nanofluids increased linearly with the MWCNT concentration in a range of 0–0.01wt% at wavelengths of 500–900nm. Compared to DI water, the temperature of MWCNT-H2O nanofluid at the optimal mass fraction of 0.01% was increased by about 14.8°C (or 22.7%) after a light irradiation time of 45min. The photo-thermal conversion performance was enhanced with increasing the light irradiation cycle at appropriate concentrations of nanofluid, and the probable mechanism caused by the CNT-fiber agglomeration was qualitatively analyzed. The receiver efficiency decreased with increasing the light irradiation time, and the highest value was up to 96.4% at 0.01% mass fraction. The prospects for possible applications of MWCNT-H2O nanofluids in low-temperature direct solar thermal energy absorption were presented.

Anomalies and peculiarities of radiation-induced light absorption in pure silica optical fibers at different temperatures

Undoped-silica-core F-doped-silica-cladding optical fibers (“undoped fibers”) are an important fiber type for applications requiring resistance to ionizing radiation (e.g., the nuclear industry, space, and military applications), the most important fundamental radiation-induced color centers arising in such fibers being self-trapped holes (STH). Despite the previous in-depth STH investigations, there have remained a few not-fully understood issues, such as the relationship between the radiation-induced absorption (RIA) bands due to STH in undoped fibers, on the one hand, and in bulk silica samples, on the other, the role of strain in the silica network in the STH occurrence, and possible peculiarities of short-lived STH-like radiation-induced color centers at temperatures above RT. To address these issues, we investigate the RIA spectra in undoped fibers with different frozen-in strain in their silica network immediately in the process of γ-irradiation to a dose of 1 kGy, the irradiation temperature being in the range ±60 °C or liquid nitrogen temperature (LNT). Gaussian decomposition of the RIA spectra measured at LNT has yielded STH bands at 2.6 and 2.16 eV together with the “classical” STH bands at 1.88 and 1.63 eV observed in fibers more frequently than the former bands. Based on this observation, it is proposed that all the STH bands observable in fibers fall into two classes: those inherent in silica and those strain-assisted, which can adjoin each other in the fiber silica network. The inherent STH include the well-known low-temperature infrared absorption and the bands at 2.6 and 2.16 eV; the strain assisted STH, the 1.88- and 1.63-eV bands. The 1.88-eV band is argued to be due to STH1, the 1.63-eV one, due to STH2. Anomalously high RIA at T = 0 and +60 °C is revealed and explained for the first time. The former effect is found to be caused by extreme compression of silica at T ∼ 0 °C enhancing the strain-assisted STH bands. The anomaly at T = +60 °C is found to be due to a previously unknown broad RIA band at ∼1.08 eV, which is likely to be associated with STH or self-trapped electrons and to result from either large network strain at the compression phases of enhanced thermal oscillation, or large expansion at the opposite phases. The RIA enhancement at T = +60 °C observed in this paper for the first time can influence fiber applications in the nuclear industry associated with high temperatures and high dose rates.

Conformational Complexity in the LH2 Antenna of the Purple Sulfur Bacterium Allochromatium vinosum Revealed by Hole-Burning Spectroscopy.

This work discusses the protein conformational complexity of the B800-850 LH2 complexes from the purple sulfur bacterium Allochromatium vinosum, focusing on the spectral characteristics of the B850 chromophores. Low-temperature B850 absorption and the split B800 band shift blue and red, respectively, at elevated temperatures, revealing isosbestic points. The latter indicates the presence of two (unresolved) conformations of B850 bacteriochlorophylls (BChls), referred to as conformations 1 and 2, and two conformations of B800 BChls, denoted as B800R and B800B. The energy differences between average site energies of conformations 1 and 2, and B800R and B800B are similar (∼200 cm-1), suggesting weak and strong hydrogen bonds linking two major subpopulations of BChls and the protein scaffolding. Although conformations 1 and 2 of the B850 chromophores, and B800R and B800B, exist in the ground state, selective excitation leads to 1 → 2 and B800R → B800B phototransformations. Different static inhomogeneous broadening is revealed for the lowest energy exciton states of B850 (fwhm ∼195 cm-1) and B800R (fwhm ∼140 cm-1). To describe the 5 K absorption spectrum and the above-mentioned conformations, we employ an exciton model with dichotomous protein conformation disorder. We show that both experimental data and the modeling study support a two-site model with strongly and weakly hydrogen-bonded B850 and B800 BChls, which under illumination undergo conformational changes, most likely caused by proton dynamics.

Luminescence and energy transfer phenomena in Gd3 (Al,Ga)5O12 crystals single doped with thulium and co-doped with thulium and holmium

Single crystals of Gd3Al2.5Ga2.5O12 (GAGG) doped with Tm3+ ions and co-doped with Ho3+ and Tm3+ ions were grown by the Czochralski technique and investigated employing methods of optical spectroscopy. The room temperature absorption spectra of GAGG:Tm3+ were analysed in the framework of the Judd-Ofelt theory. Obtained phenomenological parameters Ω2 = 0.62 × 10−20cm2, Ω4 = 1.41 × 10−20cm2 and Ω6 = 0.80 × 10−20cm2 were used to determine rates of radiative transitions, branching ratio values and radiative lifetimes for excited multiples of incorporated Tm3+ ions. Effect of Tm3+ concentration on decay curves of Tm3+ luminescence was examined and spectroscopic parameters characterizing the donor-acceptor interaction responsible for self-quenching of the emission from the 1D2, 1G4 and 3H4 multiplets were evaluated applying the Inokuti-Hirayama model. The crystal field splitting of Tm3+ multiplets were determined based on low temperature absorption and emission spectra. The emission cross section for Tm3+: 3F4 → 3H6 luminescence was determined to be 0.31 × 10−20cm2 at maximum and corresponding experimental lifetime amounts to 4.3ms resulting in quantum efficiency η = 0.79. The calculated ratio of Tm-Ho to back-transfer processes (CTm-Ho/CHo-Tm) is 5.18.

Frederick Yi-Tung Cho (1939–2011)

We present here a Tribute to Frederick Yi-Tung Cho (1939-2011), an innovative and ingenious biophysicist and an entrepreneur. He was one of the 4 earliest PhD students [see: Cederstrand (1965)-Carl Nelson Cederstrand; coadvisor: Eugene Rabinowitch; Papageorgiou (1968)-George C. Papageorgiou (coauthor of this paper); and Munday (1968)-John C. Munday Jr. (also a coauthor of this paper)] of one of us (Govindjee) in Biophysics at the University of Illinois at Urbana-Champaign (UIUC) during the late 1960s (1963-1968). Fred was best known, in the photosynthesis circle for his pioneering work on low temperature (down to liquid helium temperature, 4K) absorption and fluorescence spectroscopy of photosynthetic systems; he showed temperature independence of excitation energy transfer from (i) chlorophyll (Chl) b to Chl a and (ii) from Chl a 670 to Chl a 678; and temperature dependence of energy transfer from the phycobilins to Chl a and from Chl a 678 to its suggested trap. After doing research in biophysics of photosynthesis, Fred shifted to do research in solid-state physics/engineering in the Government Electronics Division (Group) of the Motorola Company, Scottsdale, Arizona, from where he published research papers in that area and had several patents granted. We focus mainly on his days at the UIUC in context of the laboratory in which he worked. We also list some of his papers and most of his patents in engineering physics. His friends and colleagues have correctly described him as an innovator and an ingenious scientist of the highest order. On the personal side, he was a very easy-going and amiable individual.

Experimental study of photothermal specifications and stability of graphene oxide nanoplatelets nanofluid as working fluid for low-temperature Direct Absorption Solar Collectors (DASCs)

The use of nanofluids as the working fluid in systems for converting solar to thermal energy has led to remarkable progresses. This study has investigated the thermo-optical characteristics of nanofluids containing Graphene oxide nanoplatelets/deionized water as the working fluid for low-temperature direct absorption solar collectors (DASCs). The prepared samples of nanofluid contain graphene oxide nanoplatelets based on deionized water with weight percentages of 0.001, 0.005, 0.015 and 0.045. Results show that the applied nanofluid has suitable ability compared to the base fluid in absorbing solar energy, ranging from 200 to 2500nm. Also, results from calculation of absorbed energy's fraction suggest that the minimum height or thickness of the nanofluid layer having the ability of full sun's energy, is for the nanofluid with weight percentage of 0.045 and height of 3cm that has the ability of absorbing 99.6% of energy. The thermal conductivity coefficient of prepared samples was measured at temperature ranging 25–50. Results showed that increasing the weight percentage of nanofluid along with increase in temperature would improve nanofluid's thermal properties in comparison with the base fluid. Finally, by investigating the impact of weight percentage of the nanofluid's temperature on radiative properties and thermal conductivity, this nanofluid with strong absorption band in the range of 280–350nm (nanometers) was introduced and proposed as the appropriate environment for using direct absorption solar collectors.

Probing the excitonic landscape of the Chlorobaculum tepidum Fenna-Matthews-Olson (FMO) complex: a mutagenesis approach

In this paper we report the steady-state optical properties of a series of site-directed mutants in the Fenna-Matthews-Olson (FMO) complex of Chlorobaculum tepidum, a photosynthetic green sulfur bacterium. The FMO antenna complex has historically been used as a model system for energy transfer due to the water-soluble nature of the protein, its stability at room temperature, as well as the availability of high-resolution structural data. Eight FMO mutants were constructed with changes in the environment of each of the bacteriochlorophyll a pigments found within each monomer of the homotrimeric FMO complex. Our results reveal multiple changes in low temperature absorption, as well as room temperature CD in each mutant compared to the wild-type FMO complex. These datasets were subsequently used to model the site energies of each pigment in the FMO complex by employing three different Hamiltonians from the literature. This enabled a basic approximation of the site energy shifts imparted on each pigment by the changed amino acid residue. These simulations suggest that, while the three Hamiltonians used in this work provide good fits to the wild-type FMO absorption spectrum, further efforts are required to obtain good fits to the mutant minus wild-type absorption difference spectra. This demonstrates that the use of FMO mutants can be a valuable tool to refine and iterate the current models of energy transfer in this system.

Reduced graphene oxide dispersed nanofluids with improved photo-thermal conversion performance for direct absorption solar collectors

Reduced-graphene oxide (RGO) dispersed nanofluids were prepared by irradiating a graphene oxide (GO)/water one under UV light for different times. X-ray photoelectron spectroscopy (XPS) analyses verified the reduction from GO to RGO under the UV irradiation and indicated that the reduction degree increased with the time. Zeta potential measurements suggested that the RGO/water nanofluids obtained under the irradiation time of 340s or less can keep stable at elevated temperatures. The transmittance of the RGO/water nanofluids was quite less than that of the GO/water one, indicating that the change from GO to RGO greatly enhanced optical absorption. The RGO/water nanofluid obtained under the irradiation of 340s possessed higher thermal conductivity and less specific heat than the GO/water one. The good stability along with the high optical absorption and thermal conductivity make the RGO/water nanofluid exhibited superior photo-thermal conversion efficiency to the GO/water and graphene (GE)/water ones at the same loading, which reached 96.93% at 30°C and 52% at 75°C. It is revealed that the RGO/water nanofluid prepared from the GO/water one shows greater promise for use as the working fluid in low-temperature direct absorption solar collectors.

Quantitative analysis of electronic absorption of phosphorus donors in diamond

In this work, phosphorus donors in diamond have been investigated by means of Fourier Transform Infra-Red (FTIR) transmission spectroscopy on a series of fully-characterized homoepitaxial layers. Low-temperature FTIR absorption spectra have been obtained for concentrations of donors in the range ND=3.7×1016–3.5×1018cm−3 and compensating acceptor impurities in the range NA=2.3×1016–3.0×1018cm−3. The absorption spectra is shown to exhibit two peaks at 4220cm−1 and 4540cm−1 corresponding to electronic transitions from the ground state to the 2p0 and 2p± excited states, respectively, of phosphorus donors in the neutral state of charge. The intensity of the most intense peak at 4540cm−1 is found proportional to ND-NA over two decades. The absorption cross-section of the corresponding transition is deduced providing a calibration law for the determination of ND-NA in phosphorus-doped diamond. Furthermore, the linewidth of the peak exhibits a linear variation with the compensating acceptor concentration, interpreted as the effect of the distribution of ionized impurities. The use of the non-destructive and contact-less FTIR optical method is finally discussed to determine the compensation ratio in phosphorus-doped diamond.

Performance of CMORPH, TMPA, and PERSIANN rainfall datasets over plain, mountainous, and glacial regions of Pakistan

The present study aims at the assessment of six satellite rainfall estimates (SREs) in Pakistan. For each assessed products, both real-time (RT) and post adjusted (Adj) versions are considered to highlight their potential benefits in the rainfall estimation at annual, monthly, and daily temporal scales. Three geomorphological climatic zones, i.e., plain, mountainous, and glacial are taken under considerations for the determination of relative potentials of these SREs over Pakistan at global and regional scales. All SREs, in general, have well captured the annual north-south rainfall decreasing patterns and rainfall amounts over the typical arid regions of the country. Regarding the zonal approach, the performance of all SREs has remained good over mountainous region comparative to arid regions. This poor performance in accurate rainfall estimation of all the six SREs over arid regions has made their use questionable in these regions. Over glacier region, all SREs have highly overestimated the rainfall. One possible cause of this overestimation may be due to the low surface temperature and radiation absorption over snow and ice cover, resulting in their misidentification with rainy clouds as daily false alarm ratio has increased from mountainous to glacial regions. Among RT products, CMORPH-RT is the most biased product. The Bias was almost removed on CMORPH-Adj thanks to the gauge adjustment. On a general way, all Adj versions outperformed their respective RT versions at all considered temporal scales and have confirmed the positive effects of gauge adjustment. CMORPH-Adj and TMPA-Adj have shown the best agreement with in situ data in terms of Bias, RMSE, and CC over the entire study area.