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Dynamik Vortrag: Mi., 12:10–12:30 M-V8
Combining neutron and helium spin echo: A powerful tool to clarify surface
effects in confined systems
Peter Fouquet 1 , Andrew P. Jardine 2 , Holly Hedgeland 2 , Gil
Alexandrowicz 2 , John Ellis 2 , William Allison 2
1 Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France – 2 Cavendish Laboratory,
Madingley Road, Cambridge CB3 0HE, U.K.
We present a new approach to gain a microscopic understanding of surface contributions
to the dynamics of molecules confined in mesoporous materials. In mesoporous
confinements a large fraction of molecules interacts with the wall of the matrix changing
the overall dynamics significantly [1]. At present, however, the understanding of the
surface contribution is very limited and almost exclusively based on volume measurements.
In our approach we combine neutron spin-echo spectroscopy (NSE) to measure
the diffusion of molecules confined in mesoporous matrices with increasing pore size
and helium spin-echo spectroscopy (HeSE) [2,3] to measure the diffusion of molecules
adsorbed on surfaces which are structurally similar to the walls of the confining matrices.
Both techniques work at typical diffusional ps to ns time scales and on molecular
lenth scales. Using additionally state-of-the-art molecular dynamics (MD) simulations
we get a detailed microscopic picture of the surface diffusion.
As a prototype study for our approach we have measured the diffusion of benzene
molecules on HOPG and exfoliated graphite surfaces as well as in activated carbon
fibers. The dynamics of the benzene molecules on the graphite surfaces differ strongly
from the widely assumed jump diffusion model (Fig. 1). With the aid of MD calculations
we were able to interpret the dynamics in terms of a 2d liquid with rather low
friction and we could clarify the important role of rotational activation.
[1] C. Alba-Simionesco et al., Eur. Phys. J. E 12 (2003) 19. [2] A.P. Jardine et al.,
Science 304 (2004) 1790. [3] P. Fouquet et al., Rev. Sci. Instrum. 76 (2005) 053109.