Example 2 : HCN adsorption on a Pt Surface

The RAIRS spectra shown below were observed during HCN adsorption on Pt at sub-ambient temperatures; the surface species which are generated give rise to much weaker absorptions than NO, and signal:noise considerations become much more important. These spectra also illustrate the effect of the surface normal selection rule for metallic surfaces.

(a) 0.15 L HCN at 100 K

The HCN molecules are weakly coordinated to the surface in a linear end-on fashion via the nitrogen; the ν(H-CN) mode is seen at 3302 cm^-1 but the ν(C-N) mode is too weak to be seen and the δ(HCN) mode expected at ca. 850 cm^-1 is forbidden by the surface selection rule. The overtone of the bending mode, 2δ(HCN), is however allowed and is evident at ca. 1580 cm^-1.

(b) 1.50 L HCN at 100 K

Higher exposures lead to the physisorption of HCN molecules into a second layer. These molecules are inclined to the surface normal and the HCN bending mode (∼ 820 cm^-1) of these second layer molecules is no longer symmetry forbidden. Hydrogen bonding between molecules in the first and second layers also leads to a noticeable broadening of the ν(H-CN) band to lower wavenumbers.

(c) 30 L HCN at 200 K

At the higher temperature of 200 K only a small amount of molecular HCN remains bound in an end-on fashion to the surface. The relatively strong band at 2084 cm^-1 suggests that some dissociation has also occurred to give adsorbed CN groups, which give rise to a markedly more intense ν(C-N) band than the HCN molecule itself.