Frozen excitation spectra

In contrast with previous studies on He2Cl2 cluster, in the present work localized structures are determined for the lower He2Br2 vdW states. Traditional models based on a He2Cl2 tetrahedron frozen stucture have failed to reproduce the experimental absorption spectrum, suggesting a quite delocalized structure for its vibrationally ground state. Here, based on ab initio calculations we propose different structural models, like linear or police-nightstick , in order to fit the rotationally resolved excitation spectrum of He2Cl2 or similar species. 

Fig. 31 The emission (right) and excitation (left) spectra of frozen 6.0 mM solutions of [(C6HiiNC)2Aui](PF6) in various solvents at 77K. The wavelength used to monitor the excitation profile is given below each excitation spectrum, and the wavelength used for excitation is shown below each emission spectrum. From [38]...

Figure 11. (a) The experimental rotational excitation spectrum, (b) The rotation excitation spectrum calculated with J K independent quantum yield, (c) The rotational excitation spectrum calculated with Coriolis coupling, (d) The rotational excitation spectrum calculated with Coriolis coupling and the J" = 0, K" = 0 state frozen in at 20%. 

While the classical models can reproduce and sometimes predict some structural properties, they are unable to inform about the electronic characteristics of insulators, because they assume that the electrons are frozen around the ionic cores. The next step consists in finding the microscopic origin of the forbidden gap present in the electronic excitation spectrum, which is the defining property of the insulating state. 

A major limitation of CW double resonance methods is the sensitivity of the intensities of the transitions to the relative rates of spin relaxation processes. For that reason the peak intensities often convey little quantitative information about the numbers of spins involved and, in extreme cases, may be undetectable. This limitation can be especially severe for liquid samples where several relaxation pathways may have about the same rates. The situation is somewhat better in solids, especially at low temperatures, where some pathways are effectively frozen out. Fortunately, fewer limitations occur when pulsed radio and microwave fields are employed. In that case one can better adapt the excitation and detection timing to the rates of relaxation that are intrinsic to the sample.50 There are now several versions of pulsed ENDOR and other double resonance methods. Some of these methods also make it possible to separate in the time domain overlapping transitions that have different relaxation behavior, thereby improving the resolution of the spectrum. 

Figure 3.27 shows the Mossbauer spectrum that results from splitting of the 57Fe excited state, a quadrupole doublet, for a sample containing randomly oriented molecules such as found in polycrystalline solids or frozen solutions. The two doublets are separated in energy by the quadrupole splitting, A Eq, defined by the... 

The X-ray photoelectron spectrum of the core ionization of an atom in a molecule consists of peaks and bands corresponding to transitions to various excited states. None of these transitions corresponds to the formation of the Koopmans theorem frozen-orbital ionic state, which is a completely hypothetical state. However, the center of gravity of the various peaks and bands lies at the energy corresponding... 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

In order to determine whether energy migration makes a significant contribution to the photophysical behavior of P2VN and PS in dilute miscible blends, it is instructive to calculate the expected exdmer-to-monomer fluorescence quantum yield ratio in the absence of energy migration. To do so, it is first necessary to assume that intermolecular and non-adjacent intramolecular EFS are absent. In addition, the adjacent intramolecular EFS are assumed to be frozen into the aryl vinyl polymer and must be excited by direct absorption of a photon. Since the absorption spectrum of an EFS is no different from that of non-EFS chromophores, then the calculated fraction of rings within EFS is sufficient to determine the fluorescence ratio. 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

Mossbauer spectroscopy of the 57Fe nucleus has been extensively used to investigate aspects of spin equilibria in the solid state and in frozen solutions. A rigid medium is of course required in order to achieve the Mossbauer effect. The dynamics of spin equilibria can be investigated by the Mossbauer experiment because the lifetime of the excited state of the 57Fe nucleus which is involved in the emission and absorption of the y radiation is 1 x 10 7 second. This is just of the order of the lifetimes of the spin states of iron complexes involved in spin equilibria. Furthermore, the Mossbauer spectra of high-spin and low-spin complexes are characterized by different isomer shifts and quad-rupole coupling constants. Consequently, the Mossbauer spectrum can be used to classify the dynamic properties of a spin-equilibrium iron complex. 

The increase in luminescence intensity and the strong blue shift are suggestive of an interlayer region that is devoid of free water or the water molecules are either frozen or immobile and cannot effectively solvate the initially produced MLCT excited state of the metal complex. The red shift of the absorption spectrum and the... 

Figure 2-28 The vi (A symmetry) band of SO in K2SO4 and Na2S04 frozen solutions. Both spectra were measured with 488-nm excitation from an Ar-ion laser at a resolution of 5 cm 1. A-B is the Raman difference spectrum of K2SO4 minus Na2S04. (Reproduced with permission from Ref. 78.)...

In good solvents at ambient temperature, the excited state (67 ) will quickly relax to the planar form, so that only the 0-0 emission from Si is detected in steady-state emission. If the same experiment is performed at low temperature and in a viscous solvent, the molecular torsion of 67 in attaining its planar form is hampered by the medium, and planarization is slow on the timescale of the fluorescence lifetime (355 ps). Emission will not only occur from the potential minimum of the lowest excited state, but from virtually all frozen ro-tamers resulting in a broad and blue-shifted spectrum. Only after planarization of 67 is complete narrow emission from the lowest excited-state conformation will reoccur. Consequently, planarization of the excited state rather than energy migration is likely to govern the emission behavior in PPEs such as 12. 

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