Transitions in Molecules

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 SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

Shorter-wavelength radiation promotes transitions between electronic orbitals in atoms and molecules. Valence electrons are excited in the near-uv or visible. At higher energies, in the vacuum uv (vuv), inner-shell transitions begin to occur. Both regions are important to laboratory spectroscopy, but strong absorption by make the vuv unsuitable for atmospheric monitoring. Electronic transitions in molecules are accompanied by stmcture... 

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

M. R. Philpott, Effect of surface plasmons on transitions in molecules, J. Chem. Phys. 62, 1812-1817 (1975). 

Regularities in analogous transitions in molecules having the same united atom limit... 

Ab initio calculations of solvent effects on ECD spectra are less abundant than those on OR. An ab initio study of the solvent effects on the ECD spectra were carried out by Pecul et al. [76] using the IEF-PCM method [44,45,47] at the DFT/B3LYP level using LAOs. The rotatory strengths were shown to be strongly influenced by a change of solvent, and for certain transitions in molecules such as methyloxirane, even... 

Af,Af-dimethylaniline, which becomes yellow in air or oxygen but colorless again when the oxygen is removed. Such weak charge-transfer complexes make certain electronic transitions in molecules more intense they are also a plausible first stage in photooxidations. 

Periodic oscillations in this dipole can act as a source term in the generation of new optical frequencies. Here a is the linear polarizability discussed in Exps. 29 and 35 on dipole moments and Raman spectra, while fi and x are the second- and third-order dielectric susceptibilities, respectively. The quantity fi is also called the hyperpolarizability and is the material property responsible for second-harmonic generation. Note that, since E cos cot, the S term can be expressed as -j(l + cos 2 wt). The next higher nonlinear term x is especially important in generating sum and difference frequencies when more than one laser frequency is incident on the sample. In the case of coherent anti-Stokes Raman scattering (CARS), X gives useful information about vibrational and rotational transitions in molecules. 

The majority of electronic transitions in molecules occur in the UV region of the spectrum because the energy separations of electron states typically correspond to the energies of photons in the UV region. However, some molecules have electronic energy separations that correspond to radiation in the visible region (see Fig. 12.3). One such class of compounds includes the coordination compounds that contain transition metal ions (see Section 20.6). 

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]. 

Similar considerations apply to electronic transitions in molecules. Thus, a 7T 7t transition in a carbonyl group may be accompanied by a displacement of electron charge from the O atom to the C atom, and hence provides an interaction mechanism with the electromagnetic radiation. On the other hand, an n(cr) n transition is essentially a rotational migration of charge, with no dipolar effect. 

Miklavc, A. (1980) On the semi-classical theory of coUision-induced vibrational-rotational transitions in molecules. J.Chem. Phys. 72, 3805-3808. 

We notice that singlet-triplet transitions in molecules are forbidden by spin selection rules and the lifetime of triplet states in crystals exceeds by many orders... 

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. 

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