Identifying molecular species on surfaces by scanning tunneling microscopy: Methyl pyruvate on Pd(111)

The structures of low coverages of methyl pyruvate on a Pd(111) surface at 120 K were studied using scanning tunneling microscopy in ultrahigh vacuum. The experimentally observed images were assigned to adsorbate structures using a combination of density functional theory calculations and by simulating the images using the Bardeen method. Two forms of methyl pyruvate were identified. The first, previously found using reflection-absorption infrared spectroscopy, was a flat-lying, keto form of cis-methyl pyruvate. It was characterized by elongated, two-lobed images with the long axes of the images oriented at ∼0 and ∼30 to the close-packed directions. The structure was simulated using clean, CO- and methyl-functionalized gold tips, and the simulated images agreed well with those found experimentally. The simulated structures were not strongly dependent on the tip structure or tip bias. This approach was used to identify the nature of the second species as the enol form of cis-methyl pyruvate with the carbonyl groups located over atop and bridge sites. Again, the orientation of the image with respect to the underlying Pd(111) lattice as well as the calculated image shape agreed well with the experimental images.