Spectroscopy sensitivity

Before the molybdenum catalysts can be used in hydrotreating reactions, they have to be sulfided. Raman spectroscopy sensitively reveals this transition. The characteristic Mo-S frequencies of MoS2 are at 389 and 411 cm-1 [45,46], much lower... 

What is the nature of the excited state from which the reactions start (tt tt, n tt, singlet or triplet) and how is this state reached (spectroscopy, sensitization and quenching experiments) ... 

The setup for ESR spectroscopy is a cross between NMR and micro-wave techniques (Section 5.8). The source is a frequency-stabilized klystron, whose frequency is measured as in microwave spectroscopy. The microwave radiation is transmitted down a waveguide to a resonant cavity (a hollow metal enclosure), which contains the sample. The cavity is between the poles of an electromagnet, whose field is varied until resonance is achieved. Absorption of microwave power at resonance is observed using the same kind of crystal detector as in microwave spectroscopy. Sensitivity is enhanced, as in microwave spectroscopy, by the use of modulation The magnetic field applied to the sample is modulated at, say, 100 kHz, thus producing a 100-kHz signal at the crystal when an absorption is reached. The spectrum is recorded on chart paper. 

Impurities are undetectable by inductively coupled plasma emission spectroscopy, sensitivity 50 ppb.4 5... 

Before the molybdenum catalysts can be used in hydrotreating reactions, they must be sulfided, and Raman spectroscopy sensitively reveals this transition. The characteristic Mo-S frequencies of M0S2 are at 389 and 411 cm-1 [51], much lower than the Mo-O bands. This occurs because, first, sulfur is twice as heavy as oxygen and, second, the Mo-S bond is weaker than the Mo-O bond [see Eq. (8-3)]. The Raman spectra of a sulfided M0O3/AI2O3 also show a band at 529 cm-1, due to the (S-S)2 disulfide species, which is probably located at the edges of the... 

In chemical state relaxation, a spectroscopy sensitive to chemical binding state is used to follow the evolution of an initially random adsorbate distribution into a preferred chemical state. Such techniques have quite limited apphcation for surface diffusion because (a) two easily distinguished chemical states must exist, and (b) it must be estabhshed that the transformation is diffusively mediated. 

For studies of atherosclerosis, a method is needed to identify nonstenotic, lipid-rich coronary plaques that are likely to cause acute coronary events. NIR spectroscopy can provide information on the chemical composition of tissue. NIR spectroscopy can identify plaque composition and features associated with plaque vulnerability in human aortic atherosclerotic plaques obtained at the time of autopsy. In one study, 199 samples from 5 human aortic specimens were analyzed by NIR spectroscopy [35]. Features of plaque vulnerability were defined by histology as presence of lipid pool, thin fibrous cap (65 fim by ocular micrometry), and inflammatory cell infiltration. NIR spectroscopy sensitivity and specificity for histological features of plaque vulnerability were 90 and 93% for lipid pool, 77 and 93% for thin cap, and 84 and 89% for inflammatory cells, respectively. NIR spectroscopy can identify plaque composition and features associated with plaque vulnerability in postmortem human aortic specimens. These results support efforts to develop an NIR spectroscopy catheter system to detect vulnerable coronary plaques in living patients. 

IR spectroscopy Sensitive, provides structural information, good for small molecules, identifies organic functionality, use of the fingerprint region can provide compound ID for small molecules, moderate cost Not useful for protein ID... 

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

The principal use of Auger spectroscopy is in the determination of surface composition, although peak positions are secondarily sensitive to the valence state of the atom. See Refs. 2, 82, and 83 for reviews. 

RS Raman spectroscopy [210, 211] Scattered monochromatic visible light shows frequency shifts corresponding to vibrational states of surface material Can observe IR-forbidden absorptions low sensitivity... 

RRS Resonance Raman spectroscopy [212, 213] Incident light is of wave length corresponding to an absorption band Enhanced sensitivity... 

The following several sections deal with various theories or models for adsorption. It turns out that not only is the adsorption isotherm the most convenient form in which to obtain and plot experimental data, but it is also the form in which theoretical treatments are most easily developed. One of the first demands of a theory for adsorption then, is that it give an experimentally correct adsorption isotherm. Later, it is shown that this test is insufficient and that a more sensitive test of the various models requires a consideration of how the energy and entropy of adsorption vary with the amount adsorbed. Nowadays, a further expectation is that the model not violate the molecular picture revealed by surface diffraction, microscopy, and spectroscopy data, see Chapter VIII and Section XVIII-2 Steele [8] discusses this picture with particular reference to physical adsorption. 

Wliat does one actually observe in the experunental spectrum, when the levels are characterized by the set of quantum numbers n. Mj ) for the nonnal modes The most obvious spectral observation is simply the set of energies of the levels another important observable quantity is the intensities. The latter depend very sensitively on the type of probe of the molecule used to obtain the spectmm for example, the intensities in absorption spectroscopy are in general far different from those in Raman spectroscopy. From now on we will focus on the energy levels of the spectmm, although the intensities most certainly carry much additional infonnation about the molecule, and are extremely interesting from the point of view of theoretical dynamics. 

Electrons are extremely usefiil as surface probes because the distances that they travel within a solid before scattering are rather short. This implies that any electrons that are created deep within a sample do not escape into vacuum. Any technique that relies on measurements of low-energy electrons emitted from a solid therefore provides infonuation from just the outenuost few atomic layers. Because of this inlierent surface sensitivity, the various electron spectroscopies are probably the most usefid and popular teclmiques in surface science. 

Powell C J, Jablonski A, Tilinin I S, Tanuma S and Penn D R 1999 Surface sensitivity of Auger-electron spectroscopy and x-ray photoelectron spectroscopy J. Eiectron Spec. Reiat. Phenom. 98-9 1... 

Infrared and Raman spectroscopy each probe vibrational motion, but respond to a different manifestation of it. Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion, whereas Raman spectroscopy probes the change in polarizability as the molecule undergoes vibrations. Resonance Raman spectroscopy also couples to excited electronic states, and can yield fiirtlier infomiation regarding the identity of the vibration. Raman and IR spectroscopy are often complementary, both in the type of systems tliat can be studied, as well as the infomiation obtained. 

Time-resolved spectroscopy has become an important field from x-rays to the far-IR. Both IR and Raman spectroscopies have been adapted to time-resolved studies. There have been a large number of studies using time-resolved Raman [39], time-resolved resonance Raman [7] and higher order two-dimensional Raman spectroscopy (which can provide coupling infonuation analogous to two-dimensional NMR studies) [40]. Time-resolved IR has probed neutrals and ions in solution [41, 42], gas phase kmetics [42] and vibrational dynamics of molecules chemisorbed and physisorbed to surfaces [44]- Since vibrational frequencies are very sensitive to the chemical enviromnent, pump-probe studies with IR probe pulses allow stmctiiral changes to... 

Toleutaev B N, Tahara T and Hamaguchi H 1994 Broadband (1000 cm multiplex CARS spectroscopy application to polarization sensitive and time-resolved measurements Appl. Phys. 59 369-75... 

Oudar J-L, Smith R W and Shen Y R 1979 Polarization-sensitive coherent anti-Stokes Raman spectroscopy Appi. Rhys. Lett. 34 758-60... 

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