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SUMMARY:The dynamics of thermally activated motion of cyclopentadienyl and
  pyrrole on Cu(111) - Barbara Lechner\, SMF Group\, Cavendish Laboratory
DTSTART:20101014T153000Z
DTEND:20101014T160000Z
UID:TALK26685@talks.cam.ac.uk
CONTACT:Stephen Walley
DESCRIPTION:Atomic-scale measurements of the dynamics of cyclopentadienyl 
 (Cp-)\, C5H5-\, and pyrrole\, C4H4NH\, on Cu(111) have been carried out us
 ing helium spin-echo spectroscopy (HeSE) which provides a unique means of 
 studying surface-dynamic processes on Ångström length scales and picosec
 ond time scales.\n\nThe cyclopentadiene (C5H6\, CpH) molecule adsorbs diss
 ociatively on a Cu(111) surface resulting in a predominantly ionically bou
 nd aromatic species of high anionicity (Cp-). Our HeSE data shows the char
 acteristic lineshapes predicted for single jumps between different sites w
 ithin the basis of the underlying lattice\, in our case energetically equi
 valent fcc and hcp hollow sites. Temperature-dependent measurements allow 
 us to identify an activation barrier for translation of 41±1 meV\, indica
 ting that Cp- is highly mobile. Using molecular dynamics (MD) simulations 
 we determine an effective potential energy landscape and a remarkably high
  adsorbate-substrate friction parameter for the system similar to that obs
 erved for another molecular adsorbate system\, benzene on graphite. Contra
 ry to what one would expect from highly charged adsorbates\, no evidence f
 or strong repulsive interactions could be found in the HeSE data.\n\nIn co
 ntrast to the negatively charged Cp-\, pyrrole adsorbs as a neutral specie
 s on the Cu(111) surface.  Surprisingly\, however\, the HeSE data shows si
 gnificant evidence for strong repulsive interactions between the pyrrole m
 olecules. In addition to activated jump diffusion between lattice sites th
 e dynamics measurements show evidence for possible rotational motion of th
 e molecule.\n
LOCATION:Mott Seminar Room\, Cavendish Laboratory\, Department of Physics
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