Helium cryostat

Besides thermolysis, the photochemical decomposition of solid trihalo-methylmercury compounds RHgCCli, CF3HgOCOCF3 and Hg(OCOCF3)2 has been studied (Scheme 2). The irradiation of samples placed in an evacuated quartz tube, which was connected to a helium cryostat, was carried out at -50 to +10°C. Thus, a desorption into the gas phase of the primary products of the photolysis occurred, and consequent low-temperature matrix stabilization of them was made. As a result, the formation of only the radicals CCI3 (1 3 898 cm" ) and CF3 (vi 1084, V2 702, P2+ V4 1205, r>3 1249 cm ) or of products of their secondary reactions was observed (Mal tsev et al., 1974, 1975, 1977b). 

Laboratory simulation experiments with a helium cryostat at temperatures of around 10 K and pressures of 10-8 mmHg, which have been carried out in particular by Majo Greenberg at the University of Leiden in Holland (Greenberg, 1983). 

A review and a performance comparison of liquid helium cryostats flown or planned for space flights is reported in ref. [40], A survey of low-power cryocooler suitable for space applications is done in ref. [41,42],... 

Hardie MJ, Kirschbaum K, Martin A, Pinkerton AA (1998) An open-flow helium cryostat for single-crystal X-ray diffraction experiments. J Appl Crystallogr 31 815-817... 

Hanson BL, Martin A, Harp JM, Parrish DA, Bunick CG, Kirschbaum K, Pinkerton AA, Bunick GJ (1999) Use of an open-flow helium cryostat for macromolecular cryocrystallography. J Appl Crystallogr 32 814—820... 

The TiC>2 samples, mounted in a helium cryostat (10-300 K), are lmm-thick (001) and (110)-cut single crystals to study the electronic response with the (THz) electric field perpendicular ( L) and parallel (//) to the c axis, respectively. Rutile is the most common and stable TiC>2 polymorph and has a tetragonal structure with a=4.6A and c=2.9A. The band gap of rutile is 2.9 eV at room temperature, increasing slightly at lower temperatures. 

The IR reflectivity measurements were performed on single crystals of 2 0.5 0.3 mm3 in size. A FT-IR Perkin-Elmer 1725X spectrometer equipped with microscope and a helium cryostat was used. Polarized reflectivity spectra (R(ro)) were measured from the conducting plane in two principal directions. Optical conductivity a(co) was obtained by Kramers-Kronig transformation. 

NHa, S0E, HCHQ and H sQs and measurements of these gases have been made under field conditions and in smog chamber experiments. A modified version of the system in which a liquid Na Dewar was substituted for the helium cryostat was flown successfully in and above the boundary layer over the eastern Pacific to measure N0a and HNOa. 

Normal-beam geometry. Since the sample rotates only around the vertical axis, it is very easy to attach a two- or three-stage cryorefrigerator, a standard reservoir-type liquid helium cryostat, or even a compact dilution cryostat, to the to cradle of the instrument. This is the simplest assembly, which also offers the lowest temperatures and, as we shall see later, is also most appropriate for LT + HP studies. Three-circle diffractometers are relatively common in neutron laboratories but a few x-ray diffractometers have also been built (R. Moret, unpublished, 1993 J. Gaultier, unpublished, 1993) and used for measurements on organic conductors. 

If we exclude marginal designs, such as the use of an unusually large Eulerian cradle able to internally hold a two-stage cryocooler (T. F. Koetzle and J. L. Henriques, unpublished, 1976) or the use of a very small full-circle Eulerian cradle mounted completely inside the sample room of a helium cryostat [82], the LT device must fit a commercial Eulerian cradle. At present, x shafts with inner diameters in the range 250 to 400 mm in diameter are common the plane of the x circle may either contain the axis of the circle or be offset. 

IR spectra were taken at room temperature (300 K) and liquid-helium temperatures (5-15 K), using a Bomem DAS Fourier transform infrared (FTIR) spectrometer and an InSb detector. For the low-temperature measurements, a Janis continuous-flow liquid-helium cryostat with wedged, IR-transparent windows was utilized. Hall-effect measurements, in the Van der Pauw geometry, were performed at room temperature using a system from MMR Technologies. Wires were attached to the ZnO using silver paint, which provided adequate Ohmic contacts for the electron concentrations (10 cm ) in these samples. 

Figure 14 An experimental setting for single-crystal X-ray diffraction study at 30 K, showing a Bruker three-circle diffractometer with a SMART 1000 CCD area detector, a laser device for crystal irradiation and a HehX open flow helium cryostat in action...

Figure I. Liquid helium cryostat adapted for photolysis experiments...

The vacuum chamber is interfaced to an ion implanter (200 kV) from which ions with energy up to 200 keV (400 keV for double ionizations) can be obtained. The IR spectrometer is positioned (by a moveable optical bench) in a way such that the IR beam is transmitted, through a hole of the sample holder, by the substrate. The sample holder is mounted at an angle of 45° both with the ion beam and with the incoming IR beam and spectra can be acquired in situ without tilting the sample. Samples can also be put in thermal contact with a closed-cycle helium cryostat whose temperature can be varied between 10 and 300 K. The spectra shown in this work have been taken with a resolution of 2cm and a sampling of 0.5cm . ... 

Standard liquid helium cryostats can be used at any temperature above the lambda point of helium, 1.5K, and specialised types of cryostat routinely operate as low as 0.05K. For most samples of chemical interest, there is no practical difference between spectra recorded at 4.2K and at 20K (unless phase transitions occur), thus the additional expense and complexity of very low temperatures cannot be justified. 

Time-resolved PL measurements were carried out using time-correlated singlephoton counting (TCSPC). Time-integrated PL and EL measurements were performed using the TCSPC apparatus in time-integrated mode. The temperature was controlled using an Oxford Instruments OptistatCF Helium cryostat. 

Mossbauer spectra have been recorded in a conventional transmission geometry in the constant acceleration mode. The temperature of the sample could be varied from 4.2 K up to room tenq>era-ture by means of a liquid helium cryostat, the source (57co in Rh) being kept at room temperature. The Mossbauer experiments were carried out in the "Laboratoire d Interactions Hyperfines at the CEN-Grenoble. 

Fig. 3.45 A high-pressure clamp with B4C anvils and a helium cryostat. (Reprinted with permission of the publisher.)...

Workstations for protein crystallography have been set up by the Departments of Chemistry and Biology at the Brookhaven National Laboratory. The optical system design for the Chemistry beam line is based on a spherical collimating mirror, followed by a two-crystal ( +, —) monochromator and finally by a bent cylindrical mirror (Hastings et al 1983). The instrument combines a liquid helium cryostat Huber diffractometer, for work on valence electron density studies of small molecules, with an oscillation camera for protein crystallography. The diffractometer may be upgraded to include a multichannel electronic area detector. 

If a large external magnetic field is to be applied to an absorber, this is usually done with a superconducting magnet installation which also requires a liquid helium cryostat for its operation [76]. Again, several commercial instruments are available. 

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