Vibrational Transitions in Molecules

For the purposes of basic understanding of this branch of optical spectroscopy, molecules can be visualized as a set of weights (the atoms) joined together by springs (the chemical bonds). The atoms can vibrate toward and away from each other, or they may bend at various angles to each other as 

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Infrared (IR) 1-5 xm and 8-20 xm contains information on vibrational transitions in molecules and functional groups in solids. 

For purposes of determining the force constant for a particular bond, the energy required for the v = 0 to u = 1 transition is used (this is assumed to be v0). This procedure is illustrated in Example 14.9. Vibrational transitions in molecules typically require energies that correspond to the infrared region of the electromagnetic spectrum. The data are often represented in wave numbers a wave number is the reciprocal of the wavelength (in cm) required to cause the vibrational transition. 

Changes in rotational and vibrational transitions of molecules under the influence of strong electric fields are known as the Stark effect. The effect of strong electric fields was found first on atomic spectra and extended by Condon to vibrational transitions in molecules in 1932 [161]. 

Ro-vibrational transitions in molecules provide an enormous number of narrow absorption lines in the infrared region. By using overtones of these transitions a large number of visible and near visible absorption lines could be provided. However, the weakness of these lines require very sensitive spectroscopic techniques. An example is the NICE OHMS technique [58, 67] where a gas sample of C2HD contained in a high finesse cavity absorbs at the (+ 3Vj) overtone at a 1064 nm. In beat experiments between this system and a iodine-stabilized YAG-laser, stabilities of below lO in 800 s have been observed. Especially interesting, is the demonstration of how this technique could be combined with the selective cold-atom technique to obtain line widths as narrow as 20 kHz. 

Devices for the highly sensitive detection of biological analytes using surface-enhanced Raman scattering (SERS) spectroscopy. SERS is a highly sensitive optical detection technique in which lasers are used to excite vibrational transitions in molecule adsorbed on a metal nanoparticle surface. As a result of large optical fields, the Raman cross section for a molecule on a surface is enhanced by factors of lO -lO. ... 

Vibrational transitions in molecules cause absorption in the infrared region of the electromagnetic spectrum. They may also be studied using the technique of Raman spectrometry, where they scatter exciting radiation with an accompanying shift in its wavelength. 

Vibrational circular dichroism (VCD) is defined as circular dichroism (CD) in vibrational transitions in molecules. These transitions typically occur in the infrared (IR) region of the spectrum and hence a VCD spectrometer is an infrared spectrometer that can measure the circular dichroism associated with infrared vibrational absorption bands. CD is defined as the difference in the absorption of a sample for left versus right circularly polarized radiation. This difference is zero unless the sample possesses molecular chirality, either through its constituent chiral molecules or through a chiral spatial arrangement of non-chiral molecules. 

The radiative lifetime ranges from 10 to 10 s for atomic transitions and from 10 to 0.1 s for vibrational transitions in molecules. Therefore, radiative decay is entirely negligible in comparison with the other pathways on metal and semiconductor surfaces. However, it can play a role in relaxation of electronically excited states of adsorbates on dielectrics. 

Plenary 9. J W Nibler et al, e-mail address niblerj chem.orst.edu (CARS and SRS). High resolution studies of high lymg vibration-rotational transitions in molecules excited in electrical discharges and low density monomers and clusters in free jet expansions. Ionization detected (REMPI) SRS or IDSRS. Detect Raman... 

The selection rules for AK depend on the nature of the vibrational transition, in particular, on the component of itrans along the molecule-fixed axes. For the second 3-j symbol to not vanish, one must have... 

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

Although we have been able to see on inspection which vibrational fundamentals of water and acetylene are infrared active, in general this is not the case. It is also not the case for vibrational overtone and combination tone transitions. To be able to obtain selection mles for all infrared vibrational transitions in any polyatomic molecule we must resort to symmetry arguments. 

This behaviour is very similar to that in a 77 — A vibrational transition in a linear polyatomic molecule (Section 6.2.4.1) in which the splitting is known as f-type doubling. Quantitatively, though, H-type doubling is often a much larger effect. 

The range of photon energies (160 to 0.12 kJ/mol (38-0.03 kcal/mol)) within the infrared region corresponds to the energies of vibrational and rotational transitions of individual molecules, of electronic transitions in many semiconductors, and of vibrational transitions in crystalline lattices. Semiconductor electronics and crystal lattice transitions are beyond the scope of this article. 

I 5 Real Time Monitoring of Molecular Structure at Solid/Liquid Interfaces Pirnj, near a vibrational transition in a molecule is given by [38]... 

Electronic and vibrational transitions in a gaseous diatomic molecule. 

In general, though, Raman spectroscopy is concerned with vibrational transitions (in a manner akin to infrared spectroscopy), since shifts of these Raman bands can be related to molecular structure and geometry. Because the energies of Raman frequency shifts are associated with transitions between different rotational and vibrational quantum states, Raman frequencies are equivalent to infrared frequencies within the molecule causing the scattering. 

Vibrational fine structure is absent from the excimer emission because the Franck-Condon transition is to the unstable dissociative state where the molecule dissociates before it is able to undergo a vibrational transition. In the case of the monomer emission, all electronic transitions are from the v = 0 vibrational level of M to the quantised vibrational levels of M, resulting in the appearance of vibrational fine structure. 

In the near-UV, visible and near-IR regions of the spectrum, emission is due to electronic transitions in excited atoms and molecules, while in the near- and mid-IR it is due to vibrational transitions within molecules. 

Using this same analysis of the collision problem, it is dear that the action of a colliding dectron on a molecule in exciting vibration transitions without dectron excitation, the fourth type of coupling in the list, is a consequence of the non-vanishing of the same matrix components as those which measure the probability of vibration transitions in infra-red, vibration-rotation bands. The corrdation is, however, not a sharp one, for in the collision process tte electric moments of higher order of the molecule may be active. 

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