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Study of inelastic electron tunneling spectroscopy (IETS) spectra obtained from junctions exposed to dilute solutions of organic molecules in both liquid and vapor phases. The results indicate that it is possible in principle to detect the presence and to measure the concentration of at least some organic molecules in dilute aqueous solution by means of the IETS technique. Some fine points pertaining to the application of this technique are discussed, and it is pointed out that through appropriate refinements IETS may become a valuable tool for analytical water chemistry.

Inelastic electron tunneling spectroscopy has been used to probe the irreversible chemisorption of C_6H_5OH and 1,4-C_6H_4(OH)_2 on a thin amorphous film of Al_2O_3. This spectroscopy yields a representation of the vibronic excitations of the chemical bonds in the adspecies, i.e., the tunneling spectra are analogous to ir spectra. The C_6H_5OH is found to adsorb predominately as C_6H_5O^− with a small amount of adsorption as C_6H_5OH. The 1,4-C_6H_4(OH)_2 is found to adsorb predominately as C_6H_4(OH)O^− and perhaps to a smaller extent as C_6H_4O^(2-)_2. Extensive hydrogen bonding is observed among adsorbates and between adsorbates and the hydroxylated alumina surface.

We wish to discuss the application of IETS to the observation of electronic, rather than vibrational transitions. This extension of the method seemed interesting for several reasons.

An experimental and theoretical study has been carried out on the process of inelastic tunneling between metal films. The experimental results show that this process gives rise to the observation of vibrational spectra for molecules contained in the junction region. The phenomena have been studied for a number of molecular species and at temperatures ranging from 1 to 300\ifmmode^\circ\else\textdegree\fi{}K. The principal effect of temperature is to broaden the observed spectral lines which can be accounted for quantitatively by considering the thermal smearing of the electron distribution in metals. Two detailed interaction mechanisms are discussed which can give rise to infrared dipole or Raman selection rules. None of the experiments carried out thus far can distinguish between these two processes, because of the low symmetry of the molecules used. In addition to the vibrational spectra, it appears that it should be possible to observe electronic transitions as well.

The inelastic tunneling of electrons in a metal-insulator-metal junction has been shown to be a spectroscopic method for studying the vibrational modes of the whole system. In the present paper we consider the possibility of deducing precise information from this spectroscopy. The low-voltage part of the spectrum (i.e., the $\frac{{d}^{2}I}{d{V}^{2}}\ensuremath{-}\mathrm{v}\mathrm{s}\ensuremath{-}V$ characteristic) gives information about the phonons of the electrodes. The phonon density, which is deduced for a Mg electrode, is critically compared with the density deduced from neutron scattering. The range of this phonon probe is then studied by tunneling into multilayer electrodes. The 40-90-meV range of the characteristic of a Mg-Pb junction exhibits a specific structure due to the lattice vibrations of the insulator. This structure is compared with the infrared spectrum and the phonon density of states of MgO, as well as with a theoretical calculation of the tunneling current in the transfer-Hamiltonian formalism. From the fit obtained, it is deduced that the 30-\AA{}-thick insulator, grown on Mg, is an oxide, in contrast with the insulator grown on Al, which was previously deduced to be a hydroxide. At higher energies (100-500 meV), the vibrational spectrum of molecules contained in the insulator region is observed. The identification of the lines is shown to be accurate and it gives precise information on these molecules, especially about their chemical binding with the insulator. This last point could be important in the future for studying the problem of adsorption on solid surfaces.

The conductance of metal-metal oxide-metal tunneling junctions has been observed to increase at certain characteristic bias voltages. These voltages are identified with vibrational frequencies of molecules contained in the barrier.

Inelastic electron tunneling spectroscopy has been used to probe the irreversible chemisorption at 295 K of phenol, catechol, resorcinol and hydroquinone on thin amorphous alumina films in Al/Al2O3/Pb sandwich structure tunneling junctions. This spectroscopy yields a representation of the vibronic excitations of the chemical bonds in the adspecies. Phenol chemisorbs predominantly as C6H5O- on Al+3 sites with a very small amount of associative chemisorption at high coverages. Catechol and resorcinol both adsorb predominently as the di-ion C6H4O2-2, whereas hydroquinone chemisorbs both as the di-ion and as the single ion 1, 4-C6H4(OH)O- at high surface coverages. Hydrogen bonding is observed among the adsorbates and between the adsorbates and the OH groups on the partially hydroxylated Al2O3 surface. The experimental results are explicable in terms of the geometry and chemical properties of the adsorbates and the oxide surface.