An Experimental Study of the Line Shape of Orbital Mediated Tunneling Bands Seen in Inelastic Electron Tunneling Spectroscopy K. W. Hipps* and Ursula Mazur Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: December 17, 1999; In Final Form: February 21, 2000 The line shape of inelastic transitions seen in inelastic electron tunneling spectroscopy (IETS) performed on metal-insulator-adsorbate-metal junctions is well understood. Recently, there have been reports of quasi- elastic processes leading to strong wide bands in what heretofore had been called IETS. It is demonstrated that these quasi-elastic ionizations, known as orbital mediated tunneling (OMT) bands, are actually differential in nature. However, because of the steeply rising elastic background in IETS, and because of the asymmetric shape of these bands, they often give the appearance of simple peaks. It is experimentally shown that the position and width of these false peaks can be used to reasonably estimate the actual maximum in the density of states associated with the orbital mediated transition. Introduction Inelastic electron tunneling spectroscopy is an all electron spectroscopy that has been extensively reviewed. 1-7 By measur- ing currents and voltages across a metal-insulator-adsorbate- metal (M-I-A-M′) device, one is able to extract vibrational and electronic spectroscopic information about the metals, the insulator, and the adsorbate. In its simplest form, an IET spectrum is a plot of d 2 I/dV 2 versus V. It turns out that using |d 2 I/dV 2 /(dI/dV)| [or |(dσ/dV)/ σ|] as the y axis provides spectra having flatter baselines and is most appropriate for high bias work. 8-11 These are called normalized tunneling intensities (NTI) or constant modulation spectroscopy intensities. Simple tunneling spectra are measured by applying a variable bias, V, and a small modulation component, V f , at frequency, f. A lock-in amplifier is used to detect the 2f signal that is proportional to d 2 I/dV 2 . The instrumentation required for obtaining normalized intensities, NTI, is a bit more complex. 8-10 In general, the bias voltage may be converted to the more conventional wavenumbers through the factor of 8066 cm -1 /V. The amplitude of the modulation determines the observed signal strength and resolu- tion, with the signal increasing as V f 2 while the line width is proportional to V f . 1,12 Until relatively recently, it was assumed that all the bands seen by this method were due to inelastic interactions between the tunneling electron and the barrier region. 1,13 Kirtley and Soven first considered the possibility of resonant scattering. 13 They argued that resonant scattering should be signaled by multiple overtones in the coupling vibrations. Since even single overtone bands are almost never seen in IETS, 6 they concluded that the role of resonant scattering was generally negligible. In the tunnel junction spectroscopy community, this idea was so strongly imbedded that the name, inelastic electron tunneling spectroscopy (IETS), became synonymous with all d 2 I/dV 2 vs V spectra. Although it did not receive much attention, a theoretical analysis presented by Persson and co-workers, showed that under certain conditions resonance tunneling via adsorbate electronic states might not produce multiple phonon bands. 14,16 These conditions, for a single active mode, are that the vibrational frequency and electronic-vibrational coupling parameter both be small compared to the width of the resonant electronic transition. As the number of active modes increases, the distortion per mode usually grows smaller, thereby reducing the coupling parameter for each mode. Thus, in the case of a complex molecular electronic transition, one might not observe multiphonon transitions. While resonance-like effects were first attributed to features in the IETS in 1991, 17 the first clear-cut evidence of elastic tunneling via adsorbate molecular orbitals in M-I-A-M′ diodes was provided in 1994. 18,19 Later, Mazur and Hipps proposed a simple model based on electrochemical redox potentials for predicting the position of these bands that they called orbital mediated tunneling (OMT) bands. 20 Since then, there has been progress in identifying features in the tunneling spectrum that are associated with quasi-resonant tunneling through adsorbate orbitals, both occupied and unoccupied. 21,22 In the STM realm, elastic tunneling via surface states has been well-known and discussed. 23-25 In this case peaks are observed in dI/dV (or σ) versus bias voltage curves. In the case of semiconductors and metals, these surface state variations are in the electrodes, but cases of adsorbate orbital mediated tun- neling in STM are known both in UHV 22,25 and in solution. 26-28 In UHV the orbital mediation may take the role of a direct resonance interaction (virtual state) or may be through a phonon- mediated resonance interaction or through simple hopping. In solution STM work, redox enhanced tunneling was predicted on theoretical grounds by Schmickler 29 for true resonance processes, and by Kuznetsov 30 in the case where there is relaxation of the intermediate ion before the electron is transferred to the opposite electrode. Experimentally, both mechanisms lead to Gaussian-like bands in dI/dV. 27 This issue of resonant tunneling versus electron hopping with the molecular ion being an intermediate state is still not resolved. It is presumed that all of these processes would fall under the general topic of orbital mediated tunneling spectroscopy (OMTS). Because of the confusion associated with which mechanism applies to a given process, we in general identify all OMT transitions as associated with quasi-resonance processes. 4707 J. Phys. Chem. B 2000, 104, 4707-4710 10.1021/jp994421i CCC: $19.00 © 2000 American Chemical Society Published on Web 04/26/2000