CH3Br Structures on Pt(111): Kinetically Controlled Self-Assembly of Weakly Adsorbed Dipolar Molecules

Methyl bromide is a rodlike molecule with a large 1.8 D dipole moment that physisorbs on Pt(111) with its Br end preferentially bound to the surface. The kinetically controlled self-assembly of several ordered CH3Br structures following molecular adsorption at a surface temperature of 20 K and various annealing procedures is revealed by scanning tunneling microscopy (STM). Thermal programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) provide complementary evidence for kinetic control over the CH3Br monolayer structures at temperatures under similar to 100 K. A well-ordered (6 x 3) CH3Br monolayer can be formed by dosing CH3Br multilayers at 20 K and slowly annealing the surface to 104 K over several minutes. The unit cell contains four CH3Br molecules assigned to adsorption at Pt(111) top sites and 3-fold hollow sites in equal measure. The saturation coverage is 0.22 ML with respect to the Pt(111) areal density. RAIRS shows that CH3Br within the (6 x 3) monolayer is aligned along the surface normal while at lower coverages some molecules are tilted. The RAIRS signature of the ordered (6 x 3) CH3Br monolayer is the disappearance of the v(5) asymmetric CH3 deformational mode near 1411 cm(-1) and the adsorption-site-induced splitting of the v(2) symmetric CH3 deformational mode into a doublet at 1271 and 1277 cm(-1) with full width at half maximums (fwhm's) of just 3 cm-1. Other ordered structures and molecular aggregates were observed by STM at submonolayer coverages following particular annealing procedures even though TPD shows evidence for only repulsive molecular interactions. The range of structures observed for the relatively simple CH3Br/ Pt(111) system suggest it may be a useful proving ground for theoretical treatments of the kinetics of self-assembly of dipolar molecules at surfaces.