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排序方式: 共有424条查询结果,搜索用时 31 毫秒
411.
412.
Tino P. Golub Dr. Christian Merten 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(11):2349-2353
Spectroscopic characterizations of key reaction intermediates are often considered the final confirmation of a reaction mechanism. This proof-of-principle study showcases the application of vibrational circular dichroism (VCD) spectroscopy for the characterization of in situ generated reaction intermediates using the key intermediates of enamine catalysis of Jørgensen–Hayashi-type prolinol ether catalysts as model system. By comparison of experimental and computed spectra, the enamines are shown to preferentially adopt an anti-conformation with E-configured C=C bond. For the parent prolinol catalyst, the structure and stereochemistry of the oxazolidine side product is determined as well. This study thus demonstrates that VCD spectra can provide insights into structural preferences of organocatalysts that utilize a covalent activation mechanism. Thereby it outlines new fields of applications for VCD spectroscopy and finally adds the technique to the toolbox of physical organic chemistry for in-depth mechanistic studies. 相似文献
413.
Ultrasonic attenuation measurement is a frequently used tool for non-destructive determination of dispersion characteristics. Useful information like particle or droplet size and their concentration can be obtained, if the relation between size and attenuation of the dispersion is known. In this work, the theoretical model by Faran for the intermediate sound wave regime (IWR) is presented in combination with experimental data. In the IWR, the acoustic behavior is governed by elastic scattering rather than by dissipative effects. Experiments with emulsion of droplet sizes greater than 10 mum were carried out. Silicone oil, sunflower oil and olive oil were selected for the disperse phase of the oil-in-water emulsions. First, emulsions having droplets in the micrometer range were created. Afterwords, attenuation measurements of different concentrated emulsion were carried out. Some adjustments reflecting concentration influence were performed to outline the agreement between calculations and measurements. The validity of the model can be confirmed, if the volume fraction of the disperse phase is considered as a variable. Finally, droplet size distributions from theoretical attenuation spectra could be calculated based on a log-normal distribution. 相似文献
414.
Rösener B Rattunde M Moser R Kaspar S Töpper T Manz C Köhler K Wagner J 《Optics letters》2011,36(3):319-321
We demonstrate an optically pumped semiconductor disk laser based on the (AlGaIn)(AsSb) material system, which operates at an emission wavelength of 2.8 μm. Up to 120 mW of output power were obtained in cw operation and more than 500 mW in pulsed mode. The performance of the present laser is discussed in comparison to shorter-wavelength semiconductor disk lasers based on the same materials system. 相似文献
415.
M. G. Tarallo N. Poli M. Schioppo D. Sutyrin G. M. Tino 《Applied physics. B, Lasers and optics》2011,103(1):17-25
We describe a frequency-stabilized diode laser at 698 nm used for high-resolution spectroscopy of the 1S0–3P0 strontium clock transition. For the laser stabilization we use state-of-the-art symmetrically suspended optical cavities
optimized for very low thermal noise at room temperature. Two-stage frequency stabilization to high-finesse optical cavities
results in measured laser frequency noise about a factor of three above the cavity thermal noise between 2 Hz and 11 Hz. With
this system, we demonstrate high-resolution remote spectroscopy on the 88Sr clock transition by transferring the laser output over a phase noise-compensated 200-m-long fiber link between two separated
laboratories. Our dedicated fiber link ensures a transfer of the optical carrier with frequency stability of 7×10−18 after 100 s integration time, which could enable the observation of the strontium clock transition with an atomic Q of 1014. Furthermore, with an eye toward the development of transportable optical clocks, we investigate how the complete laser system
(laser+optics+cavity) can be influenced by environmental disturbances in terms of both short- and long-term frequency stability. 相似文献
416.
M. Schmidt M. Prevedelli A. Giorgini G. M. Tino A. Peters 《Applied physics. B, Lasers and optics》2011,102(1):11-18
We present a modular rack-mounted laser system for the cooling and manipulation of neutral rubidium atoms which has been developed
for a portable gravimeter based on atom interferometry that will be capable of performing high-precision gravity measurements
directly at sites of geophysical interest. This laser system is constructed in a compact and mobile design so that it can
be transported to different locations, yet it still offers improvements over many conventional laboratory-based laser systems.
Our system is contained in a standard 19″ rack and emits light at five different frequencies simultaneously on up to 12 fibre
ports at a total output power of 800 mW. These frequencies can be changed and switched between ports in less than a microsecond.
The setup includes two phase-locked diode lasers with a phase noise spectral density of less than 1 μrad/Hz1/2 in the frequency range in which our gravimeter is most sensitive to noise. We characterise this laser system and evaluate
the performance limits it imposes on an interferometer. 相似文献
417.
We prove a multivariate Whitney type theorem for the local anisotropic polynomial approximation in Lp(Q) with 1≤p≤∞. Here Q is a d-parallelepiped in Rd with sides parallel to the coordinate axes. We consider the error of best approximation of a function f by algebraic polynomials of fixed degree at most ri−1 in variable , and relate it to a so-called total mixed modulus of smoothness appropriate to characterizing the convergence rate of the approximation error. This theorem is derived from a Johnen type theorem on equivalence between a certain K-functional and the total mixed modulus of smoothness which is proved in the present paper. 相似文献
418.
Morgenstern K 《J Phys Condens Matter》2011,23(48):484007
We have investigated vibrational spectra of nitrobenzene molecules adsorbed on Cu(111) by low temperature inelastic electron tunneling spectroscopy. This molecule, which should support 39 internal modes, only gives rise to seven peaks in the spectra. We outline a comparison with ensemble IR data and interpret the small number of vibrational peaks by the superposition of a multitude of almost isoenergetic vibrational modes. The non-detectability of further modes cannot be understood in terms of symmetry considerations. Additional modes in the spectra are attributed to external molecular-metal vibrations. 相似文献
419.
Lamporesi G Bertoldi A Cacciapuoti L Prevedelli M Tino GM 《Physical review letters》2008,100(5):050801
We present a new measurement of the Newtonian gravitational constant G based on cold-atom interferometry. Freely falling samples of laser-cooled rubidium atoms are used in a gravity gradiometer to probe the field generated by nearby source masses. In addition to its potential sensitivity, this method is intriguing as gravity is explored by a quantum system. We report a value of G = 6.667 x 10(-11) m(3) kg(-1) s(-2), estimating a statistical uncertainty of +/-0.011 x 10(-11) m(3) kg(-1) s(-2) and a systematic uncertainty of +/-0.003 x 10(-11) m(3) kg(-1) s(-2). The long-term stability of the instrument and the signal-to-noise ratio demonstrated here open interesting perspectives for pushing the measurement accuracy below the 100 ppm level. 相似文献
420.
Edyta?C?drowska Marek?Pruszynski Agnieszka?Majkowska-Pilip Sylwia?M?czyńska-Wielgosz Frank?Bruchertseifer Alfred?Morgenstern Aleksander?BilewiczEmail author 《Journal of nanoparticle research》2018,20(3):83
The 225Ac radioisotope exhibits very attractive nuclear properties for application in radionuclide therapy. Unfortunately, the major challenge for radioconjugates labelled with 225Ac is that traditional chelating moieties are unable to sequester the radioactive daughters in the bioconjugate which is critical to minimize toxicity to healthy, non-targeted tissues. In the present work, we propose to apply TiO2 nanoparticles (NPs) as carrier for 225Ac and its decay products. The surface of TiO2 nanoparticles with 25 nm diameter was modified with Substance P (5-11), a peptide fragment which targets NK1 receptors on the glioma cells, through the silan-PEG-NHS linker. Nanoparticles functionalized with Substance P (5-11) were synthesized with high yield in a two-step procedure, and the products were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS) and thermogravimetric analysis (TGA). The obtained results show that one TiO2-bioconjugate nanoparticle contains in average 80 peptide molecules on its surface. The synthesized TiO2-PEG-SP(5-11) conjugates were labelled with 225Ac by ion-exchange reaction on hydroxyl (OH) functional groups on the TiO2 surface. The labelled bioconjugates almost quantitatively retain 225Ac in phosphate-buffered saline (PBS), physiological salt and cerebrospinal fluid (CSF) for up to 10 days. The leaching of 221Fr, a first decay daughter of 225Ac, in an amount of 30% was observed only in CSF after 10 days. The synthesized 225Ac-TiO2-PEG-SP(5-11) has shown high cytotoxic effect in vitro in T98G glioma cells; therefore, it is a promising new radioconjugate for targeted radionuclide therapy of brain tumours. 相似文献