Summary: Photoinduced Fragmentation of Multilayer CH3Br on Cu/Ru(001)
Tsachi Livneh*, and Micha Asscher
Department of Physical Chemistry, Nuclear Research Center, NegeV, P.O. Box 9001 Beer-SheVa 84190, Israel,
and Department of Physical Chemistry and the Farkas Center for Light Induced Processes,
The Hebrew UniVersity, Jerusalem 91904, Israel
ReceiVed: December 30, 2002; In Final Form: July 14, 2003
The broadband UV (230-420 nm) photoinduced chemistry of CH3Br adsorbed on Cu(2ML)/Ru(001) in the
coverage range of 1-50 ML was studied by monitoring the desorption products (p-TPD mode) in combination
with post irradiation work function change measurements before and during surface heating ( -PD mode).
measurements enabled us to follow multilayer restructure and desorption of parent molecules and
photochemical reaction products in the temperature range of 80-700 K. Methyl radicals accumulated on the
surface are the precursor for the thermal formation of methane and ethylene at 450 K. Dehydrogenation of
the methyl group is the rate-limiting step of the surface reaction resulting in the formation of these molecules.
Based on work function change measurements, an estimate of the adsorbed methyl dipole moment is µ0 )
0.48 D. Dissociative electron attachment (DEA) driven CH3Br dissociation produced CH3 and CH2 fragments
within the parent molecules multilayer matrix. At the multilayer coverage range, increases by up to 1.1
eV after 10 min UV irradiation. Model calculations qualitatively describe the post irradiation work function
changes induced by the embedded photofragments (mostly Br- ions) inside the CH3Br dielectric film.
Comparison of the -TPD spectra on Cu(2ML)/Ru(001) to those on clean Ru(001) indicates that the nature
of the molecule-surface interaction and the structure of the first few layers strongly influence the resulting