J. Ludecke et al., THE STRUCTURE OF SULFUR ADSORPTION PHASES ON NI(111) STUDIED BY X-RAYSTANDING WAVE-FIELD ABSORPTION, Surface science, 366(2), 1996, pp. 260-274
The technique of normal-incidence X-ray standing wavefield (NIXSW) abs
orption has been applied to an investigation of several structural pha
ses of S on Ni(111), specifically the (2 x 2), (root 3 x root 3)R30 de
grees and (5 root 3 x 2)rect. phases, with a view in particular to cla
rifying recent controversy over the structure of the last of these. Ab
solute adsorbate site determination has been effected through real-spa
ce triangulation using both(lll) and (<(1)over bar 11>) Bragg reflecti
ons. For the (2 x 2) phase the results are consistent with a simple ov
erlayer structure with S atoms occupying the ''foc'' hollow sites (dir
ectly above third-layer Ni atoms), in agreement with earlier determina
tions by LEED, ion scattering and SEXAFS. The same local site is found
to be occupied in the (root 3 x root 3)R30 degrees phase, although th
e NISXW data indicate poorer local order. For the (5 root 3 x 2)rect.
phase, a simple undistorted missing-row model is found to be inconsist
ent with the data, as is any ideal ''coincidence lattice'' structure,
including a simple undistorted pseudo-(100) surface reconstruction. A
modification of this latter model, in which some S atoms penetrate the
pseudo-(100) layer to bond to the underlying (111) substrate does, ho
wever, contain the main ingredients needed to fit the data. By using n
ot only (111) and (<(1)over bar 11>) NIXSW but also (1(1) over bar1$)
and (11(1) over bar) absorption profiles, recorded from a near single-
domain structure formed on a miscut crystal surface, specific informat
ion on some of the structural parameters of this modified reconstructi
on model is obtained and compared with complementary data from other p
ublished studies.