Energy-resolved spectroscopy

Spectroscopic Basics of Nas C. In the late 1980s special interest was focused on the C(2) E" state (in Dsh symmetry) of Nas. Energy-resolved spectroscopy allowed the observation of lower vibrational levels of this electronic state by means of TPI, whereas the upper levels require the use of DS to probe dissociative states [369, 374, 393]. The spectrum of the C state is characterized by a vibrational band structure with pseudorotational features, as shown in Fig. 4.3. These investigations confirmed the C state to be partially predissociated. Therefore, the dissociation channel was proposed to be the main relaxation process for states higher in energy than the C state. This could also be demonstrated for the D state by the depletion technique with a few nanoseconds time resolution [375], as well as for Rydberg states close to the ionization limit [124]. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

The chemical composition of the IF phase deviates only very slightly, if at all, from the composition of the bulk layered compound. Deviations from stoichiometry can only occur in the cap of the nanotube. In fact, even the most modem analytical techniques, like scanning probe techniques and high (spatial) resolution electron energy loss spectroscopy, are unable to resolve such a tiny deviation from the stoichiometry, like the excess or absence of a single Mo (W) or S (Se) atom in the nanotube cap. 

The role of the conditions in which these phenomena are observed is now well understood [40, 45], The chromophore should be solvatofluorochromic, that is, its fluorescence spectra should respond to changes in interaction energy with its environment by significant shifts. This environment should be relatively polar, but rigid or highly viscous, so that the relaxation times of its dipoles, tr, are comparable or longer than the fluorescence lifetime tf (in the case of recording the steady-state spectra) or on the time scale of observation (in time-resolved spectroscopy). Thus, these effects are coupled with molecular dynamics in condensed media. 

Flash Photolysis. Time-resolved spectroscopy techniques are a powerful means of studying materials, giving information about the nature of the excitations, energy transfer, molecular motion, and molecular environment, information that is not available from steady-state measurements. It is... 

We demonstrate by using ultrafast time resolved spectroscopy that the photoconversion from dihydroazulene (DHA) to vinylheptafulvene (VHF) is governed by two mechanisms The ring opening proceeds on the excited energy surface on the picosecond time scale. It is followed by an internal conversion to the VHF ground state that is accelerated by the presence of a conical intersection in the case of cyclopenta-DHA. This conical intersection hinders the photoinduced back reaction from the final VHF products. However, we successfully photo-converted the cyanophenyl-VHF-cis back to the DHA in an experiment with two delayed pulses. This opens the route to the development of bistable dihydroazulene switches. 

Gu, H., Shinoda, Y., and Wakai, F., Detection of boron segregation to grain boundaries in silicon carbide by spatially resolved electron energy-loss spectroscopy , J. Am. Ceram. Soc., 1999, 82, 469-72. 

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