Liquid-nitrogen-cooled helium

Figure 17.5. Laboratory apparatus for TIRTS. A jet of liquid-nitrogen-cooled helium is directed onto the sample disk within, or just before, the field of view of the spectrometer. A jet of heated nitrogen returns the sample surface to its original temperature. A heat gun warms the back of the sample disk for experiments with elevated sample temperatures. (Reproduced from [8], by permission of the American Chemical Society copyright 1990.)...

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

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. 

As mentioned above, the choice of substrate is crucial for tuning the lateral mobility. This problem might also be addressed in part by temperature control. Cooling can lower mobility so that the 2D gas aggregates into 2D crystals. If mobility on a particular substrate is not sufficient, it can be increased by heating. However, this could raise the probability of decomposition or desorption. Low temperatures, on the contrary, put constrain on experimental control that comes with cooling with liquid nitrogen or helium. 

The circular atom microwave spectroscopy experimental set-up is sketched on Fig. 1-a. A thermal beam of Li atoms crosses three sections of the apparatus the excitation, the microwave interaction region and the detection zone. The whole set-up is protected from room temperature thermal radiation by a liquid nitrogen cooled shield (which can be replaced in a later stage of the experiment by a liquid helium cooled one). 

The pump, shown in Figs. 1 and 2, is made up of four basic components (1) one dense helium-cooled cold plate (2) two interconnected, liquid-nitrogen-cooled chevron panels (3) one gas thermometer for display of helium panel discharge gas temperature and (4) one feedthrough plate, an external bulkhead, providing connections to the helium and liquid nitrogen circuits as well as to the gas thermometer. 

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