Rare vibrational frequencies

In addition to the dependence of the intennolecular potential energy surface on monomer vibrational level, the red-shifting of the monomer absorption as a fiinction of the number of rare gas atoms in the cluster has been studied. The band origin for the Vppp = 1 -t— 0 vibration in a series of clusters Ar -HF, with 0 < n < 5, was measured and compared to the HF vibrational frequency in an Ar matrix (n = oo). The monomer vibrational frequency Vp p red shifts monotonically, but highly nonlinearly, towards the matrix value as sequential Ar atoms are added. Indeed, roughly 50% of the shift is already accounted for by n = 3. 

It is interesting to note that the emission spectra of the terbium chlorides solvated with H20 and D20 show no discernible differences. Since the rare-earth chlorides solvated with D20 are isostructural with the chlorides solvated with H20 and since the emission spectra are essentially identical, Freeman et al believe that the variations in lifetime are not brought about by changes in the radiative-transition probabilities, but are a consequence only of changes in radiationless quenching efficiencies. They speculate that the decreased efficiency upon substitution of D20 for H20 must be related to the large changes in vibrational frequencies associated with substitution of the H atoms by the D atoms. 

Raman spectroscopy or far-IR spectroscopy can determine the fundamental vibration frequencies of the host. However, these methods give information about the whole glass matrix and do not account for the local nature of electron-phonon interactions. So, the fundamental frequencies are preferably determined by recording the phonon-side bands (PSB) of rare-earth transitions or by studying the temperature-dependence of multiphonon relaxations [42,43]. The phonon energies determined by PSB spectroscopy, which is the most direct method, are usually lower (400 cm-1 in ZBLAN) than those measured by other methods ( 500 cm-1) suggesting that weak M—F bonds are coupled to the rare-earth [43]. 

Almost all of the es erimental data on these two classes of molecules has been obtained in rare-gas matrices. It is, in general, a reliable source since these matrices do not produce significant perturbations (23). Vibrational frequencies and electronic transitions can be observed, but no knowledge of bond distances can be obtained. Most of the electronic and magnetic... 

Reference to Fig. 1 shows that if vibrational frequencies in ground and excited states are similar, the normal fluorescence spectrum will be a mirror image (on a frequency scale) of the absorption. This condition is not always met. Because of the high density of states in complex polyatmic molecules, resolution of individual vibron-ic bands in absorption and fluorescence is rarely achieved, spectra being usually broad and relatively featureless. In some molecules of high symmetry where electronic transitions are mmetry forbidden, vibrational structure is ob rved however. 

As a result of different bonding properties (which arise from different interionic separations in these electronic states) in the ground and excited states of an impurity ion in a crystal, they may have different geometries, what is revealed in the shift of the potential energy surfaces of the considered electron states and their different curvature. The latter is defined by the differences of the vibrational frequencies in these states, and, since this difference rarely exceeds few percents, can be readily neglected. In order to perform a qualitative analysis of this phenomenon, we use the effective Hamiltonian Hyiq, which describes the interaction of the electron states with the lattice normal modes in the form... 

Alteration of the vibrational frequencies and the appearance of new ones in those rare cases when the vibrational structures of the spectrum remains. 

Vibrational frequencies for example result from the appUcation of infrared, laser-induced fluorescence, Raman, and Raman resonance spectroscopy. Spectroscopy in the visible and near-UV regions yields information on electronic transitions. Electron spin resonance spectroscopy is used in determining the geometric and electronic structure. These methods were applied to study the gaseous species trapped at low temperatures in a solid inert rare gas matrix (matrix isolation technique) as well as in the free state. 

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