Expansion coefficient liquid helium

Cathodoluminescence microscopy and spectroscopy techniques are powerful tools for analyzing the spatial uniformity of stresses in mismatched heterostructures, such as GaAs/Si and GaAs/InP. The stresses in such systems are due to the difference in thermal expansion coefficients between the epitaxial layer and the substrate. The presence of stress in the epitaxial layer leads to the modification of the band structure, and thus affects its electronic properties it also can cause the migration of dislocations, which may lead to the degradation of optoelectronic devices based on such mismatched heterostructures. This application employs low-temperature (preferably liquid-helium) CL microscopy and spectroscopy in conjunction with the known behavior of the optical transitions in the presence of stress to analyze the spatial uniformity of stress in GaAs epitaxial layers. This analysis can reveal,... 

This setup has the following advantages (i) It is insensitive to vibrations, which occur in the cryostat (ii) the full fight path passes through suprasil and is not affected by immersion in liquid helium (iii) Suprasil has a small expansion coefficient, which does not change the optical properties of the objective at low and high temperatures and (iv) the shrinking of the optical components leaves the ratios of all dimensions invariant and does not alter its functionality. 

ABSTRACT. Three new techniques are presented which allow rate coefficient measurements for ion molecule reactions in the temperature range of interstellar cloudy. With an ion trap and a static drift tube cryogenic cooling by liquid helium is used. These techniques are therefore restricted to molecular hydrogen as neutral reactant. With the CRESU method the low temperature is obtained by supersonic expansion. This last technique has a high chemical versatility. 

The low temperature refractive properties of the He gas have not been studied extensively. However, the second virial Kerr coefficient can be related to the zeroth moment of the polarized Raman spectrum, and thus deduced from the Raman experiment. For the helium gas at the liquid nitrogen temperature the experiment gives 1.46 a.u.416, the full quantum calculation 1.45328, while the classical result computed according to Eq. (1-260) gives 1.63 328. This shows that also for the Kerr effect the quantum corrections are important. A systematic study of these corrections and of the convergence of the semiclassical expansion has not been reported thus far, even though all necessary expressions are derived328. 

The liquefaction of helium by a controlled expansion process necessitates preliminary cooling because its Joule-Thomson coefficient is negative (spontaneous expansion heats the gas) down to an inversion temperature of 40 All the gases have C /C ratios very close to 5/3, the theoretical value for an ideal monatomic gas. The elements are liquid over very small temperature ranges. Plelium can be solidified only under pressure under 26 atmospheres it solidifies at 0.9 °K. 

---