METHYL FORMATE ON AG(111) - 1 - THERMAL ADSORPTION-DESORPTION CHARACTERISTICS AND ALIGNMENT IN MONOLAYERS

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.