Al. Schwaner et al., METHYL FORMATE ON AG(111) - 1 - THERMAL ADSORPTION-DESORPTION CHARACTERISTICS AND ALIGNMENT IN MONOLAYERS, JOURNAL OF PHYSICAL CHEMISTRY B, 101(51), 1997, pp. 11112-11118
On Ag(111), there is no evidence that methyl formate dosed at or below
120 K dissociates during adsorption and subsequent thermal desorption
. Temperature-programmed desorption distinguishes monolayer (145 K) an
d multilayer (135 K) HCOOCH3 and DCOOCD3 desorption peaks, indicating
a weak adsorbate-substrate interaction (ca. 37.4 kJ mol(-1) desorption
activation energy) that is only slightly stronger than the adsorbate-
adsorbate interaction. The relative intensities of vibrational bands d
etected in reflection-absorption infrared spectra vary significantly w
ith coverage and are consistent with adsorption in a thermodynamically
stable cis form. HCOOCH3 and DCOOCD3 spectra for monolayer coverages
show no modes with a '' symmetry, as expected when the molecular symme
try plane lies perpendicular to the Ag(111) surface. The relative inte
nsities of modes with a' symmetry indicate that the carbonyl (C=O) axi
s is tilted from the surface plane, the CH3 group points toward the su
bstrate, and the C-O bond of the interior ester linkage is nearly para
llel to the substrate. Above ML coverage, a '' modes begin to appear,
and for 5 layers or more, the vibrational frequencies are similar to t
hose for gas-phase and matrix-isolated spectra.