ISSN:
1572-9567
Keywords:
concentration fluctuations
;
critical broadening
;
high pressure
;
line shape
;
line shift
;
local composition
;
mixtures
;
nitrogen-helium
;
Raman spectroscopy
Source:
Springer Online Journal Archives 1860-2000
Topics:
Physics
Notes:
Abstract Vibrational Raman spectroscopy is a very usful technique for studying the dynamical behavior of a system, in particular in the critical region. In a mixture, composition fluctuations play an important role and on the basis of the results for pure substances one would expect an increase of the linewidth in approaching the critical point. However, in previous experiments such an effect was not detected. The reasons for this failure will be discussed. Recently, we have observed a considerable broadening of the linewidth in approaching the critical point in the system helium-nitrogen under high pressure. However, it is well-known that broadening may occur through various other mechanisms such as changes in density, composition fluctuations far from the critical point, aggregation, etc. On the basis of the helium-nitrogen data the pitfalls in the experimental determination of critical broadening are discussed. The data analysis takes into account the contribution of noncritical effects. Consequences are drawn for the interpretation of spectroscopic data in terms of concentration of species in supercritical solvents. Moreover, we discuss the possibility of a difference between the local composition and the bulk composition and demonstrate from our line shift data that this is not a critical effect.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01438844
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