Metal cryostats

Bartelt A, Close J D, Federmann F, Quaas N and Toennies J P 1996 Cold metal clusters helium droplets as a nanoscale cryostat Phys. Rev. Lett. 77 3525... 

Figure 3.9 shows a schematic sectional drawing of a liquid-helium Mbssbauer cryostat with a superconducting magnet optionally included. The layout is kept generic to highlight a few issues that are essential for applications in transition metal chemistry ... 

As regards the heat conduction through the solid parts of a cryostat, in the choice of the structural materials a compromise is sought for a low thermal conductivity and suitable mechanical properties. When possible, disordered materials are used in the case of metals, low-conductivity alloys are used as Cu-Ni or stainless steel, in the form of thin-walled tubes. In the evaluation of the heat conduction, the most useful data are the thermal conductivity integrals shown in Fig. 5.2 for some structural materials. The thermal conductivity integral between two temperatures TL and rH is defined as ... 

Because of their brittleness, glass dewars have nowadays only an historical interest [3], We shall hereafter describe only metal dewars (or cryostat). 

Figure 5.3 shows a metallic cryostat with LN2 shield used for optical measurements in the far infrared (an optical filter in this case is inserted on the 77K to block the visible radiation). 

Fig. 5.3. Scheme of a metallic cryostat with LN2 shield for optical measurements (dimensions are in mm). 

An advantage of this technique, instead, is the good thermal contact with the (metallic) radioactive material, and, more important, the good thermal coupling between lattice and nuclear spins. Last, the j ray energy is usually quite high, and no special window in the cryostat is needed. Nuclear orientation thermometer is a primary thermometer. 

Standard optical metal or metal-glass cryostats are commonly used in low-temperature studies of TSL and TSC. Less expensive facilities for liquid N2 temperatures have been designed, and closed cycle cryotips may be employed as well. TSR requires dynamic temperature cycles. Therefore, it is generally advantageous to design the sample holder-dewar arrangement as small as possible to enable quick turnaround times. 

Raman spectra were measured on fresh, chemically etched surfaces in quasi-backscattering configuration using a triple DILOR XY spectrometer, a liquid nitrogen cooled CCD detector, and a 514.5-nm Ar-ion laser. The laser beam of power level 20 mW was focused on an area of 0.1 mm2 on the mirror-like plane (it was the (ab) plane of the single crystals). The measurements were performed in a cryostat with a helium gas atmosphere in the temperature range 5-295 K below temperature of metal-insulator phase transition. 

The formation of molecular radical ions by electron transfer reactions between alkali metals and a wide variety of aromatic and other organic compounds in polar solvents is well established. A very large number of radical anions have been prepared by this method and extensive studies of their e.s.r. and optical spectra have been made (Bowers, 1965 Gerson, 1967 Kaiser and Kevan, 1968). In solution the electron transfer reaction will be facilitated by the subsequent solvation of the two ions (or ion pair) by the polar solvent molecules. However, we have observed that similar electron transfer reactions occur readily when alkali metal atoms are deposited on a variety of relatively non polar substances at 77°K in the rotating cryostat. In most cases the parent compound acts as the matrix, though for some radical ions an inert matrix of a non-polar hydrocarbon has been used successfully. It is perhaps surprising that the reactions occur so readily as the energy of solvation of the ions must be quite small in most of these systems as compared with that in the polar liquids. 

The rotating cryostat has been used to prepare trapped electrons by chemical reaction between alkali metal atoms and ice or solid alcohols at 77°K (Bennett et ah, 1967b, 1967d). The advantage of this technique... 

Dalton TP, Li Q, Bittel D, Liang L, Andrews GK (1996) Oxidative stress activates metal-responsive transcription factor-1 binding activity. Occupancy in vivo of metal response elements in the metallothionein-I gene promoter. J Biol Chem 271 26233-26241 Danscher G, Howell G, Perez-Clausell J, Hertel N (1985) The dithizone, Timm s sulphide silver and the selenium methods demonstrate a chelatable pool of zinc in CNS. A proton activation (PIXE) analysis of carbon tetrachloride extracts from rat brains and spinal cords intravitally treated with dithizone. Histochemistry 83 419 22 Danscher G, Jensen KB, Frederickson CJ, Kemp K, Andreasen A, Juhl S, Stoltenberg M, Ravid R (1997) Increased amount of zinc in the hippocampus and amygdala of Alzheimer s diseased brains a proton-induced X-ray emission spectroscopic analysis of cryostat sections from autopsy material. J Neurosci Methods 76 53-59... 

These low molecular concentrations in the vapor, especially above transition metals, are the major reason why the investigation of various other properties of metal molecules have proven to be such a challenge. Experimental methods had to be devised, such as controlled synthesis of metal molecules in inert gas matrices at cryostatic temperatures, or the enhancement of relative molecular concentrations through nozzle expansion followed by subsequent optical spectroscopic or other physical-chemical e cperlmental studies. The success of such methods and the resulting physical property measurements will be the subject of several presentations in this symposium. 

Figure 4 Schematic of an XAS beamline. Cryostat is placed between lo and ft ionization detectors to allow for absorption measurements. In the cryostat, samples can be place at a variety of angles to allow for absorption or fluorescence measurements. A fluorescence detector is placed tangential to the X-ray path. A metal foil is placed between ft and L for energy calibration...

Usually for the observation of dHvA oscillations in organic metals temperatures below a few K down to the mK range are necessary. Therefore, the detection unit is inserted into a He cryostat or a He/ He dilution refrigerator. Often the sample is directly immersed in the liquid and can be rotated in situ around one or even two axes. Thus, the FS can be mapped out more easily without remounting of the sample. 

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