High resolution inelastic tunneling spectroscopy of macromolecules and adsorbed species with liquid-phase doping

Abstract

The vibrational spectra of organic molecules can be measured with inelastic electron tunneling if the molecules can be deposited onto the insulating layer of a thin film metal-insulator-metal tunneling junction. A new technique, liquid-phase doping, simplifies that deposition. Further, it makes it possible to deposit macromolecules and other compounds that decompose before evaporating, minimizes contamination problems, and gives higher resolution spectra than the usual vapor-phase doping. Spectra of anthracene, a high symmetry molecule, show that both Raman and infrared active modes are present in inelastic tunneling spectra with comparable intensity. Spectra of amino acids, proteins, purine and pyrimidine bases, nucleotides, and nucleic acids illustrate the applicability of the technique to a wide range of compounds of biological interest. A detailed comparison of a tunneling spectrum of benzaldehyde to recent infrared and Raman spectra of aluminum benzoate and benzaldehyde adsorbed on alumina shows the potential of the technique for studying surface reactions and possible catalysis.