Vibrational spectroscopy photon-based techniques

First, surface-sensitive techniques that can operate under technologically relevant conditions, i.e., at least in the 1 —lOOOmbar pressure range, are required. In this respect, photon-based techniques such as sum frequency generation (SFG) and polarization-modulation IR reflection absorption spectroscopy (PM-IRAS) provide surface vibrational spectra of adsorbates from UHV up to atmospheric pressure. Although electron spectroscopies are typically limited to pressures <10 mbar, recent developments in XPS allow the determination of... 

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in using Raman for the vibrational characterization of surfaces arises from the fact that the technique can be used in situ under non-vacuum environments, and also because it follows selection rules that complement those of IR spectroscopy. 

Raman spectroscopy is based on the inelastic scattering of light. Similar to IR spectroscopy the Raman technique yields information about vibrational modes of molecular bonds. A Raman spectrum can yield complementary information about the vibration modes observed in IR spectroscopy. However, Raman spectroscopy is not directly equivalent to IR spectroscopy. The condition for a molecular bond to be Raman active is a change in the polarization of the electron cloud during the interaction with a photon. 

Raman spectroscopy comprises a family of spectral measurements based on inelastic optical scattering of photons at molecules or crystals. It involves vibrational measurements as well as rotational or electronic studies and nonlinear effects. Following, Raman will be used in the established but slightly inaccurate way as a synonym for the most important and most common technique of the family, linear vibrational Raman scattering. 

The harmonic forw field of benzene is the most studied force field of any molecule of similar size. Due to the high symmetry of benzene, its harmonic force field can be expressed with only 34 distinct parameters. The modest number of parameters makes it possible to collect enough experimental information to determine the complete force field. Experimentalists have obtained the required data by collecting information on almost all possible isotope-substituted species by various techniques, including non-tranditional vibrational methods based on one-photon and two-photon electron spectroscopy... 

There are three types of electron spectroscopy for the study of surfaces. The most common type, which is based on irradiation of the sample surface with monochromatic X-radiation, is called X-ray photoelectron spectroscopy (XPS). It is also termed electron spectroscopy for chemical analysis. Much of the material in this chapter is devoted to XPS. The primary beam for photoelectron spectroscopy can also consist of ultraviolet photons, in which case the technique is called ultraviolet photoelectron spectroscopy (UPS). Here, a monochromatic beam of ultraviolet radiation causes ejection of electrons from the analyte. This type of electron spectroscopy is not as common as the other two, and we shall not discuss it further. The second type of electron spectroscopy is called Auger (pronounced oh-ZHAY) electron spectroscopy (AES). Most commonly, Auger spectra are excited by a beam of electrons, although X-rays are also used. Auger spectroscopy is discussed in Section 21C-2. The third type of electron spectroscopy is electron energy-loss spectroscopy (EELS), in which a low-energy beam of electrons strikes the surface and excites vibrations. The resultant energy loss is then detected and related to the vibrations excited. We briefly describe EELS in Section 21C-3. 

Several other forms of nonlinear spectroscopy have been developed that are not strictly based on Raman-active vibrations or rotations. Degenerate four-wave mixing (DFWM) is a technique where all input and output frequencies are identical. Because it does not involve the generation of light at new frequencies, it can rely on non-local mechanisms other than the local electronic polarizability (e.g. electrostric-tion). The selection rules for DFWM are closely related to those of one-photon techniques (e.g. absorption). DFWM using infrared beams is therefore used to probe infrared absorbing transitions instead of Raman-active transitions. 

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