Raman diatomics

Figure Bl.2.2. Schematic representation of the polarizability of a diatomic molecule as a fimction of vibrational coordinate. Because the polarizability changes during vibration, Raman scatter will occur in addition to Rayleigh scattering.

Laser Raman diagnostic teclmiques offer remote, nonintnisive, nonperturbing measurements with high spatial and temporal resolution [158], This is particularly advantageous in the area of combustion chemistry. Physical probes for temperature and concentration measurements can be debatable in many combustion systems, such as furnaces, internal combustors etc., since they may disturb the medium or, even worse, not withstand the hostile enviromnents [159]. Laser Raman techniques are employed since two of the dominant molecules associated with air-fed combustion are O2 and N2. Flomonuclear diatomic molecules unable to have a nuclear coordinate-dependent dipole moment caimot be diagnosed by infrared spectroscopy. Other combustion species include CFl, CO2, FI2O and FI2 [160]. These molecules are probed by Raman spectroscopy to detenuine the temperature profile and species concentration m various combustion processes. 

This book, originally published in 1950, is the first of a classic tliree-volume set on molecular spectroscopy. A rather complete discussion of diatomic electronic spectroscopy is presented. Volumes 11 (1945) and 111 (1967) discuss infrared and Raman spectroscopy and polyatomic electronic spectroscopy, respectively. 

Rotational Raman spectra of diatomic and linear polyatomic molecules... 

In a diatomic or linear polyatomic molecule rotational Raman scattering obeys the selection rule... 

Figure 6.7 Rotational transitions accompanying a vibrational transition in (a) an infrared spectrum and (b) a Raman spectrum of a diatomic molecule...

The rotational selection rule for vibration-rotation Raman transitions in diatomic molecules is... 

As for a diatomic molecule, the general harmonic oscillator selection mle for infrared and Raman vibrational transitions is... 

It is important to realize that electronic spectroscopy provides the fifth method, for heteronuclear diatomic molecules, of obtaining the intemuclear distance in the ground electronic state. The other four arise through the techniques of rotational spectroscopy (microwave, millimetre wave or far-infrared, and Raman) and vibration-rotation spectroscopy (infrared and Raman). In homonuclear diatomics, only the Raman techniques may be used. However, if the molecule is short-lived, as is the case, for example, with CuH and C2, electronic spectroscopy, because of its high sensitivity, is often the only means of determining the ground state intemuclear distance. 

Different rules apply to Raman spectroscopy, so symmetric diatomic molecules do have Raman spectra (see Infrared technology and raman spectroscopy, RAMAN spectroscopy) (23,24). 

Infrared (ir) transmission depends on the vibrational characteristics of the atoms rather than the electrons (see Infrared and Raman spectroscopy). For a diatomic harmonic oscillator, the vibrational frequency is described by... 

Raman Spectroscopy. Raman spectroscopy is an excellent method for the analysis of deuterium containing mixtures, particularly for any of the diatomic H—D—T molecules. For these, it is possible to predict absolute light scattering intensities for the rotational Raman lines. Hence, absolute analyses are possible, at least in principle. The scattering intensities for the diatomic hydrogen isotope species is comparable to that of dinitrogen, N2, and thus easily observed. 

The Raman spectrum of aqueous mer-cury(I) nitrate has, in addition to lines characteristic of the N03 ion, a strong absorption at 171.7 cm which is not found in the spectra of other metal nitrates and is not active in the infrared it is therefore diagnostic of the Hg-Hg stretching vibration since homonuclear diatomic vibrations are Raman active not infrared active. Similar data have subsequently been produced for a number of other compounds in the solid state and in solution. 

Most values were taken from M. W. Chase, Jr., C. A. Davies, J. R. Downey, Jr.. D. J. Frurip, R. A. McDonald, and A. N. Syverud, "JANAF Thermochemical Tables, Third Edition", J. Phys. Chem. Ref. Data, 14, Supplement No. 1, 1985. A few (in parentheses) came from G. Hertzberg, Molecular Spectra and Molecular Structure, I. Spectra of Diatomic Molecules, and 11. In frared and Raman Spectra of Polyatomic Molecules, Van Nostrand Reinhold Co.. New York. 1950 and 1945. 

Bratos S., Marechal E. Raman study of liquids. I. Theory of Raman spectra of diatomic molecules in inert solutions, Phys. Rev. A4,1078-92 (1971). 

Le Duff Y., Holzer W. Raman band shapes of small diatomic molecules dissolved in inert liquids, Chem. Phys. Lett. 24, 212-16 (1974). 

Herzberg, G. Molecular Spectra and Molecular Structure, I. Spectra of Diatomic Molecules, 1950 II. Infrared and Raman Spectra of Polyatomic Molecules, 1945 III. Electronic Spectra and Electronic Structure of Polyatomic Molecules, Van Nostrand, Princeton, NJ, 1967. 

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