Inelastic-Tunneling Spectra of Organic Compounds

Inelastic electron tunneling has been studied in Al-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Pb tunnel junctions doped with a wide range of organic compounds. These include simple straight-chain hydrocarbons, amino acids, aromatic ring compounds, pyrimidine bases, cyclic saturated hydrocarbons, and other nonaromatic ring compounds. The observed inelastic-tunneling spectra have been analyzed in terms of the expected vibrational modes of these molecules and results show that the method has sufficient sensitivity to differentiate many of the characteristic modes in these compounds. Comparison with Raman and infrared spectra have been made and a detailed analysis of this comparison is included. In general, the tunneling spectra often show a better over-all agreement with Raman spectra than with infrared spectra, but important differences exist between the different methods in a number of cases. In most cases the identification of specific modes in the tunneling spectrum is in quite good agreement with both the Raman and infrared spectra. The tunneling spectra have been recorded at 4.2 K in most cases, but improved resolution can be obtained near 1 K and preliminary examples are included. The doping has been accomplished using a combination of vacuum chambers which allows doping of successive junctions with different organic compounds without cross contamination. The experiments demonstrate that the inelastic-tunneling technique can be applied to a complete range of organic compounds and may prove to be highly useful technique for the analysis of very small quantities of material.