Spectroscopy energy-selective

Hyper-Raman spectroscopy is not a surface-specific technique while SFG vibrational spectroscopy can selectively probe surfaces and interfaces, although both methods are based on the second-order nonlinear process. The vibrational SFG is a combination process of IR absorption and Raman scattering and, hence, only accessible to IR/Raman-active modes, which appear only in non-centrosymmetric molecules. Conversely, the hyper-Raman process does not require such broken centrosymmetry. Energy diagrams for IR, Raman, hyper-Raman, and vibrational SFG processes are summarized in Figure 5.17. 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

Although similar transitional energy ranges occur in IR and Raman spectroscopies, different selection rules govern the intensities in Raman... 

Combination of PES with mass spectrometry in a coincidence system leads to an extremely powerful technique called photoelectron-photoion coincidence (PEPICO) spectroscopy, which renders it possible to examine dissociations of energy-selected molecular ions. 

Gradinaru CC, Pascal AA, van Mourik F, Robert B, Horton P, van Grondelle H and van Amerongen H (1998) Ultrafast evolution ofthe excited states in the chlorophyll a/b complex CP29 from green plants studied by energy-selective pump-probe spectroscopy. Biochemistry 37 1143-1149... 

Huffman GP, Mitra S, Huggins FE, Shah NS, Vaidya N, Lu F (1991) Quantitative analysis of all major forms of sulfur in coal by X-ray absorption fine structure spectroscopy. Energy Fuels 5 574-581 Huffman GP, Shah NS, Huggins FE, Stock LM, Chatterjee K, Kilbane JJ, Chou M, Buchanan DH (1995) Sulfur speciation of desulfurized coals by XANES spectroscopy. Fuel 74 549-555 Huggins FE, Huffman GP (1995) Chlorine in coal anEXAFS spectroscopic investigation. Fuel 74 556-569 Hundal LS, Carmo AM, Bleam WL, Thompson ML (2000) Sulfur in biosolids-derived fulvic acidic characterization by XANES spectroscopy and selective dissolution approaches. Environ Sci Technol 34 5184-5188... 

The discovery that in many cases the spectra of organic compounds at low temperature are inho-mogeneously broadened was the next and very important step in the understanding of the spectroscopy of complex molecules. In most cases, the broad spectra have an intrinsic line structure, which can be revealed by selective laser excitation. This new technique, laser fine-structure selective spectroscopy of complex molecules in frozen solutions is often called site-selection spectroscopy (SSS). A physically more realistic term is energy selection spectroscopy. By now the SSS technique is so far developed that inhomogeneous broadening of organic molecules at low temperature can be eliminated by selective laser excitation [2]. 

The high time resolution, achievable with pulsed or mode-locked lasers (Chap. 11) opens the possibility for studying the dynamics of collision processes and relaxation phenomena. The interesting questions of how and how fast the excitation energy, selectively pumped into a polyatomic molecule by absorption of laser photons, is redistributed among the various degrees of freedom by intermolecular or intramolecular energy transfer can be attacked by time-resolved laser spectroscopy. 

---