Simultaneous transition

Both absorption and emission may be observed in each region of the spectrum, but in practice only absorption spectra are studied extensively. Three techniques are important for analytical purposes visible and ultraviolet spectrometry (electronic), infrared spectrometry (vibrational) and nuclear magnetic resonance spectrometry (nuclear spin). The characteristic spectra associated with each of these techniques differ appreciably in their complexity and intensity. Changes in electronic energy are accompanied by simultaneous transitions between vibrational and rotational levels and result in broadband spectra. Vibrational spectra have somewhat broadened bands because of simultaneous changes in rotational energy, whilst nuclear magnetic resonance spectra are characterized by narrow bands. 

We note that the formalism presented here plays a major role in infrared spectroscopy. The process that gives rise to a fundamental line in the infrared spectrum of a molecule is the absorption of a photon whose frequency corresponds to that of one of the normal modes, and the simultaneous transition of this mode from the ground state (n = 0) to the first excited state. 

Simultaneous transitions involving two interacting molecules are also known at sums and differences of molecular or electronic transition frequencies. These are considered below, along with the single rotovibrational or electronic transitions where appropriate. Spectra of van der Waals molecules are in general closely associated with all of these. Furthermore,... 

Specifically, the eleven profiles include the obvious single transitions, i.e., the rotovibrational transitions in just one of the two colliding H2 molecules these are the Si(0), Si(l), and Qi (1) transitions in one of the two interacting molecules. Double transitions in both collisional partners are also taking place, such as the simultaneous transitions gi(l)+So(0) (which occur near the Si(0) transition frequency) and Q (1) + So(l) (near Si(l)), Fig. 3. 32. Intensities of all these lines are known from theory (classical multipole approximation, Chapter 6) their superposition reproduces the measurement closely, Fig. 3.33. 

In hydrogen-helium mixtures, the overtone spectra are so weak that they have not yet been recorded. But in mixtures of the heavier rare gases with hydrogen, it has been possible to record the H2 overtone spectra. In those cases, the simultaneous transitions are missing, a compelling confirmation of the conclusions concerning simultaneous transitions. 

Besides the isotropic overlap-induced dipole component familiar from the rare gas pairs, we will now in general have other significant induced dipole components if molecules are present, namely multipole-induced and distorted frame-induced dipole components, see Chapter 4 for details. Moreover, these anisotropic dipole components couple with the polarizability tensor and thus give rise to simultaneous transitions in two (or perhaps more) molecules. Furthermore, molecules in general interact with more or less anisotropic forces which to some extent does also affect the spectra of molecular systems. 

The 630 nm band, on the other hand, shows no significant intensity variation when the foreign-gas density is varied an oxygen density squared behavior is observed regardless of the presence and concentration of the admixtures. This band arises from simultaneous transitions of both molecules of the O2-O2 complex. In this case, an O2 molecule cannot be replaced by a foreign gas particle without losing the band, or perhaps shifting it to a different part of the spectrum. 

The significance of collision-induced absorption for the planetary sciences is well established (Chapter 7) reviews and updates appeared in recent years [115, 165, 166, 169-173]. Numerous efforts are known to model experimental and theoretical spectra of the various hydrogen bands for the astrophysical applications [170, 174-181]. More recently, important applications of colhsional absorption in astrophysics were discovered in the cool and extremely dense stellar atmospheres of white dwarf stars [14, 43, 182-184], at temperatures from roughly 3000 to 6000 K. Under such conditions, large populations of vibra-tionally excited H2 molecules exist and collision-induced absorption extends well into the visible region of the spectrum and beyond. Numerous hot bands, high H2 overtone bands, and H2 rotovibrational sum and difference spectral bands due to simultaneous transitions that were never measured in the laboratory must be expected. Ab initio calculations of the collisional absorption processes in the dense atmospheres of such stars have yet to be provided so that the actual stellar emission spectra may be obtained more accurately than presently known. 

Combination bands correspond to simultaneous transitions in two modes. For example, a molecule that possesses a carbonyl (u, = 1750 cm-1) and hydroxyl (v2 = 3600 cnr1) functional group in close proximity to each other can show a combination band at v1 + v2 = 5350 cm1. 

According a simplified theory (Alexandrov, 1976), a concerted reaction occurs as a result of the simultaneous transition (taking approximately 10 13 s) of a system of independent oscillators, with the mean displacement of nuclei ground state, to the activated state in which this displacement exceeds for each nuclei a certain critical value (q>cr). If activation energy of the concerted process Esyn > nRT, the theory gives the following expression for the synchronization factor which is the ratio of the pre-exponential factors of the synchronous and simple processes ... 

Considering the case where simultaneous transitions of two consecutive bonds occur, one obtains... 

An interesting CILS process involves simultaneous transitions in two (or more) interacting particles that appear at sums or differences of the rotovibrational transition frequencies of the molecules involved [243, 347]. 

C. Brodbeck and J.-P. Bouanich. Simultaneous Transitions in Compressed Gas Mixtures. In G. Birnbaum (ed.), Phenomena Induced by Intermolecular Interactions, Plenum Press, New York, 1985, pp. 169-192. 

M. O. Bulanin and A. P. Kouzov. Simultaneous transitions in light-scattering spectra induced by collisions between molecules. Optika i Spektr. (U.S.S.R.), 53 266-269 (1982). 

Attempts have been made to estimate quantitatively the various effects possible from the theoretical viewpoint on an electrochemical interface for superconductors have been made. For example, it was established [154, 156, 158] that the probability of the electron-pair tunneling is, in principle, always substantially lower than that for usual electrons (all other factors being equal), a result that implies the prediction of inhibition near 7. Kuznetsov [158] considered in detail the mechanisms of the processes with the participation of Cooper pairs. For instance, the energy barriers were estimated for a variety of mechanisms, including the transfer to one and the same particle (capable of multielectron transformation) to two spatially separated particles, and also the transfer of the pair to one particle with the simultaneous transition of one of the pair s electrons to the normal state. It was found that the properties of the system can vary substantially, depending on the relationship between the band gap, the medium reorganization energy, and the overpotential. 

Ketelaar et al. [797] found that the liquid mixture of CS2 and S2Br2, for example, exhibits IR spectra that show combination bands between the V3 of CS2 (1515 cm ) and a series of S2Br2 vibrations. Such simultaneous transitions seem to suggest the formation of an intermolecular complex, at least on the vibrational time scale. 

Figure 8. Schematic of the alamethicin mechanism in which the hinge region, H, provides a flexible linkage to allow alamethicin N-terminal (N) and C-terminal (C) helical segments to optimize their solvent and dipole interactions with the membrane in the presence and absence of a transmembrane potential. In the absence of a potential, helical segments could bury an optimal amount of hydrophobic surface in the bilayer while maintaining contact of the hydrophilic helix termini with the aqueous layer. A transmembrane potential could align helix dipoles with the field and facilitate movement of hydrophilic helix termini through the membrane. It is probable that a concerted mechanism of several aggregated alamethicin molecules that undergo simultaneous transitions could reduce the transition barrier.

Aim multipass cell for i.r. spectroscopy, utilizing two parallel concave mirrors, has allowed path lengths up to 150 m to be achieved. For a study of collision-induced simultaneous transitions in binary gas mixtures, a 2m sample cell has been constructed that allows pressure variation up to 1500 bar. A cell has been designed for pressures up to lOKbar and temperature variation over the range 10—300 A Pfund-type cell has been constructed for i.r. spectroscopy with... 

The transitions were obtained from the maximum in G" at about —25°C. The simultaneous transition at about —140°C is the y-transition of polyethylene. 

The 634-nm light emission Is due to a simultaneous transition involving a pair of 02 molecules, both In their Ag state ... 

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