Experimental Tunneling Spectroscopy

The final chapters of this book review the progress of three recently developed techniques that provide information about the vibrational states of adsorbed molecules. Perhaps the most important of these techniques is electron energy loss spectroscopy that, despite its inherent low resolution, gives valuable information on vibrational modes that are either inactive in the IR, or inaccessible because of experimental difficulties. The applications of this technique are discussed in two chapters by Somorjai and Weinberg. The review of new experimental techniques concludes with presentations on inelastic electron tunneling spectroscopy and neutron scattering by Kirtley and Taub. 

The self-energy effect in the phonon feature of a superconducting point contact can be used, in principle, in the same way as the Rowell-McMillan program for determination of the EPI spectral function in tunneling spectroscopy of superconductors [16]. Two difficulties arise on this way. One is theoretical, since this program works well only for the one-band superconductor, and its application to the two-band case, like MgB2, encounters difficulties [17]. The other is experimental, since all other sources of I — V nonlinearities should be removed, and especially, the nonequilibrium effects in superconductor should be excluded. 

Small water clusters have been the subject of intense theoretical and experimental interest over the last decade [118,306], due primarily to the advent of far-infrared vibration-rotation tunneling spectroscopy [307-310]. 

In MWNT, the individual tubes are in such close proximity that electronic interactions come to bear. Indeed the behavior experimentally observed for macroscopic samples resembles that of the semimetallic graphite. When exairiming individual MWNT by means of scanning tunneling spectroscopy (STS), however, both metallic and semiconducting species were found, although it is not settled if maybe just the properties of the outermost single tube have been measured. 

This interpretation agrees perfectly with results originating from Scanning Tunneling Spectroscopy (STS) investigations. An experimental setup as sketched in Figure 4 is used to study the current (I)-voltage (U) behavior of individual nanoparticles. 

Electron tunneling spectroscopy applied in a different experimental configuration can yield the vibrational structure of adsorbates. For example, by adsorbing a monolayer of molecules at an aluminum oxide-lead interface, the vibrational spectrum of benzoic acid was obtained by plotting d V/dP, the second derivative of the applied voltage with respect to the tunnel current, versus the applied voltage V. The result is shown in Figure 5.19. The experiment was performed at 4.2 K. 

The distinct advantage of scanning tunneling spectroscopy (STS), compared to conventional experimental techniques like photoelectron spectroscopy and inverse photoemission, is the high lateral resolution. Additionally, electronic states can be investigated in a single measurement on both sides of the Fermi level which cannot be carried out in photoemission techniques photoemission allows to determine occupied states only, inverse photoemission empty states only. 

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