University of Illinois at Urbana-Champaign researchers have developed a powerful new tool for probing molecular structure on surfaces, combining the chemical selectivity of optical absorption spectroscopy with the atomic-scale resolution of scanning tunneling microscopy.

“First, the sample molecule is placed on a transparent silicon substrate,” said Joseph Lyding, a professor of electrical and computer engineering and a researcher at the Beckman Institute. “Laser light will either be absorbed by the sample or will pass through the substrate with little or no heating effect. Second, the tip-sample junction is illuminated through the rear face of the substrate, significantly reducing tip heating.”

Modulating the laser light with a mechanical chopper further reduces heating, Lyding said. A lock-in amplifier, which switches on and off at the same rate as the laser, filters out mechanical and electronic noise. As a result, the absorbed energy causes a change of shape in the electron density of the sample molecule, and the scanning tunneling microscope then measures that change of shape.

Source: University of Illinois at Urbana-Champaign news release