instrumental techniques applied to mineralogy and geochemistry

More documents

Recommendations

Info

Raman, Conventional Infrared and Synchrotron Infrared Spectroscopy in Mineralogy and Geochemistry: Basics and Applications 61 electrons are comparatively light. This process is regarded as elastic scattering and is the dominant process (Fig. 2). For molecules it is called Rayleigh scattering. (Blue light, e.g., is more efficiently scattered than red light, by approx. a factor of n 4 , and Rayleigh scattering is responsible for the blue color of the sky.) However, if nuclear motion is induced during the scattering process, energy will be transferred either from the incident photon to the molecule or from the molecule to the scattered photon (Fig. 2). In these cases the process is inelastic and the energy of the scattered photon is different from that of the incident photon by an amount equal to a vibrational transition, ±hn 1 . This is Raman scattering. The oscillating electric field of the light interacts with the molecule and polarizes (distorts) the electron cloud around the nuclei to form a short-lived state called a “virtual state” (Fig. 2). This unstable state is not necessarily a true quantum state of the molecule and the photon is quickly re-radiated. Although it is inherently much weaker process than Rayleigh scattering (by a factor of 10 -3 to 10 -6 ), it is still possible to observe the Raman scattering by using a strong exciting source. Energy units According to its wave nature, EM radiation is characterized by two quantities: the wavelength l and the frequency n. In the IR spectral region the wavelength is usually given in units of [mm]. The frequency is given in units of [s -1 ] or Hertz [Hz]. The product of l and n is the velocity of light (c = ln), which is approx. 310 10 cm s -1 in vacuum. Since the vibrational frequencies of molecules are so high, units such as Hz are not convenient. An additional parameter, which is commonly used in Raman and IR spectroscopy instead of the frequency, is the wavenumber, , defined as the reciprocal of the wavelength in cm [cm -1 ]. In general, the energy of the EM radiation is proportional to its frequency and its wave number, but inverse to the wavelength: E = hv = hc = hc / l. The following relation exists between the wavelength l, the frequency n, the wavenumber and the velocity of light c: [cm -1 ] = 10 4 / l [μm] = v [s -1 ] / c [cm s -1 ]. Although the dimensions of n and are different, they are often used interchangeably. Thus, an almost slang expression such as "a frequency shift of 5 cm -1 " is very common.
62 Biliana Gasharova Even though Raman spectroscopy probes vibrational transitions indirectly by light scattering, the Raman shift has the same energy range as IR absorption. Selection rules Different vibrational modes have different relative intensities in IR and Raman spectra – some modes are active in one and not the other, and some modes are not observed at all. The IR and Raman activities of particular modes are determined by the quantum mechanical selection rules for the vibrational transitions, and by the mode symmetry. In a simple model, the selection rules can be rationalized by considering the interaction between the oscillating electric field vector of the light beam and a changing molecular dipole moment associated with the vibration. In an IR experiment, the light interacts directly with an oscillating molecular dipole, so for a vibrational mode to be IR active, it must be associated with a changing dipole moment. In general, asymmetric vibrations tend to give stronger IR absorption than symmetric species, since they are associated with larger dipole moment changes. Similarly, highly polar (“more ionic”) molecules and crystals have stronger IR spectra than non-polar samples. In Raman scattering, the light beam induces an instantaneous dipole moment in the molecule by deforming its electronic wave function. The atomic nuclei tend to follow the deformed electron positions, and if the nuclear displacement pattern corresponds to that of a molecular vibration, the mode is Raman active. The magnitude of the induced dipole moment is related to the ease with which the electron cloud may be deformed, described by the molecular polarizability a. The classification of the vibrational quantum states and the description of the spectroscopic interaction are greatly simplified by exploiting the symmetry of the vibrating atomic groups. The mathematical framework of the group theory is the basis of the quantitative description of the symmetry relations possessed by the vibrating groups, finally giving rise to the formulation of the symmetry-based selection rules. As the symmetry of the atomic group increases, the number of different energy levels decreases. The degeneracy, i.e. the number of vibrational states, which have the same energy increases with increasing symmetry. The more symmetric the atomic group, the fewer different energy levels it has, and the greater the degeneracy of those levels. The symmetry must be compatible in order that the molecule may absorb light and the
Page 2 and 3:
SEMINARIOS DE LA SOCIEDAD ESPAÑOLA
Page 4 and 5:
FOREWORD On the occasion of the Spa
Page 6 and 7:
INDEX Electron back-scattered diffr
Page 8 and 9:
8 Elisabetta Mariani et al Introduc
Page 10 and 11: 10 Elisabetta Mariani et al success
Page 12 and 13: 12 Elisabetta Mariani et al number
Page 14 and 15: 14 Elisabetta Mariani et al FIGURE
Page 16 and 17: 16 Elisabetta Mariani et al FIGURE
Page 18 and 19: 18 Elisabetta Mariani et al In-situ
Page 20 and 21: TEM in Geology. Basics and applicat
Page 42 and 43: X-ray Absorption Spectroscopy in Mi
Page 56 and 57: Raman, Conventional Infrared and Sy
Page 82 and 83: 84 Clemente Recio Isotope Terminolo
Page 84 and 85: 86 Clemente Recio collector gas sou
Page 86 and 87: 88 Clemente Recio Something similar
Page 88 and 89: 90 Clemente Recio beers. Some brand
Page 90 and 91: 92 Clemente Recio The source of the
Page 92 and 93: 94 Clemente Recio By using GC-C-IRM
Page 94 and 95: 96 Clemente Recio about 25% of tota
Page 96 and 97: 98 Clemente Recio such as blood and
Page 98 and 99: 100 Clemente Recio relatively simpl
Page 100 and 101: 102 Clemente Recio following admini
Page 102 and 103: 104 Clemente Recio and high N in dr
Page 104 and 105: 106 Clemente Recio although 15 N d
Page 106 and 107: 108 Clemente Recio spectrometers th
Page 108 and 109: 110 Clemente Recio O’Brien, D.M.
Page 110 and 111:
112 Kjell Bilström time in old geo
Page 112 and 113:
114 Kjell Bilström ratios ( 204 Pb
Page 114 and 115:
116 Kjell Bilström Certain samples
Page 116 and 117:
118 Kjell Bilström Molybdenite is
Page 118 and 119:
120 Kjell Bilström analyzed with r
Page 120 and 121:
122 Kjell Bilström radiogenic sign
Page 122 and 123:
124 Kjell Bilström plants inherits
Page 124 and 125:
126 Kjell Bilström in some cases a
Page 126 and 127:
128 Kjell Bilström Besides, minera
Page 128 and 129:
130 Kjell Bilström References Bear
Page 130 and 131:
Analytical techniques applied to fl
Page 132 and 133:
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141:
Page 142 and 143:
Page 144 and 145:
Page 146 and 147:
Page 148 and 149:
Page 150 and 151:
show all

instrumental techniques applied to mineralogy and geochemistry

Create successful ePaper yourself

Delete template?

Save as template?