Resolution, IETS

An example of how Raman, IR, and IETS complement each other is given in Fig. 4, which is drawn using data originally published in [49], While the resolution afforded by IETS (even at 4 K) is not as good as that provided by IR spectroscopy, it is not much worse than seen in Raman and is sufficiently good to identify individual normal modes. Of most interest is the fact that many more fundamentals are seen in IETS than in the IR or Raman spectra of this D3h symmetry ion. 

The width of the peaks in LETS depends upon the sharpness of the onset of the inelastic process, which in turn depends upon the thermal distribution of electron energies about EP. Thus, the IETS line width depends strongly on temperature and as shown by (3) [75]. Because of this, vibrational IETS provides infrared-quality resolution only when performed below 5 K. Electronic transitions are usually much broader than vibrational transitions therefore, electronic IETS is usually performed at liquid nitrogen temperature and slightly above (>77 K). An example of a system showing both vibrational and electronic IETS is presented in Fig. 5 [19]. 

The invariance of IETS in an M-A-M junction vs an M-I-A-M device is exceptionally well demonstrated by the work of Reed [30], Figure 7 shows the Au-alkanedithiol-Au structure he used to create a single barrier tunnel diode. The IET spectra obtained from this device were stable and repeatable upon successive bias sweeps. The spectrum at 4.2 K is characterized by three pronounced peaks in the 0-200 mV region at 33,133, and 158 mV. From comparison with previously reported IR, Raman, and high-resolution electron energy-loss (HREEL) spectra of... 

IETS is spectroscopically similar in resolution to the infrared method except for differences in the pattern of band intensities. Unfortunately, it is limited to samples held at liquid-helium temperatures. However, in the future, in conjunction with scanning tunneling microscopy, it might yield spectra from single adsorbed molecules on particular sites (JO). 

The above discussion is meant to point out specific possible application of surface vibrational spectroscopy to new areas of catalysis. Certainly there are many others and brevity prevents further discussion of such a large subject. Reflection IR, IETS and perhaps Raman, which is rapidly developing in useful directions, would appear to have a good future as high resolution techniques for studies of the chemisorption of organic molecules on a variety of substrates. 

Table 1. Gross comparison between IETS and ISTS The entries are illustrative of the differences between two techniques. IETS is more sensitive due to averaging for larger time window and larger number of molecules therefore, it may reach also better energy resolution. ISTS reaches higher spatial resolution by detection of signal on a single molecule, and employing a shorter tunneling barrier. Data for IETS are representative parameters, from [2].

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