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Small angle scattering from liquids: Van der Waals forces in argon and collective mode in Na

Matthai, Clarence Cherian and March, N. H. 1982. Small angle scattering from liquids: Van der Waals forces in argon and collective mode in Na. Physics and Chemistry of Liquids 11 (3) , pp. 207-217. 10.1080/00319108208080743

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Because of discrepancies between recent estimates of van der Waals forces in liquid metals, we have examined available experimental data on the small angle scattering of X-rays from liquid Na, and have compared it with neutron scattering data on liquid argon. In the latter case, it follows from first principles that the liquid structure factor S(k) has the small k expansion. Therefore we have plotted (S(k)–S(0))/k 2 from available data for both Na and Ar, and they are immediately found to be qualitatively different. For a given state of argon, a 2 and a 3 can be extracted and are quite consistent with theoretical estimates. For Na, the small angle scattering does not have the form (i) and it is demonstrated that there is a term proportional to k at small k. This means that one of the leading terms determining the approach of the radial distribution function g(r) to its asymptotic value of unity at large r is proportional to r −4. It is shown that this behaviour arises from the dispersion of a collective mode, in which the density fluctuations oscillate independently with a well defined dispersion relation ω(k). This relation has the form, with vs the velocity of sound, We have estimated the dispersion for Na, K and Rb from an approximate theory and have compared it with the experimentally observed collective mode for liquid Rb. Finally, after extracting the k term discussed above, the magnitude of the k 3 term is estimated from experimental data on Na. It is quite consistent with the data to take the coefficient of k 3 as zero and possible reasons for this are discussed.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Publisher: Taylor & Francis
ISSN: 00319104
Last Modified: 04 Jun 2017 06:50

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