305 research outputs found
Anatectic melt trapped in garnet from migmatites of the Kerala Khondalite Belt, Southern India: evidence from crystallized and glassy melt inclusions.
No full textEffects of the mixing of charge transfer and molecular excitations on the resonance Raman properties of symmetric radical dimers
No full textThe mixing of charge transfer (CT) and molecular excitation (ME) states in a molecular radical dimer is investigated based on a model Hamiltonian for a dimer with two orbitals per site filled with two electrons. The CT/ME mixing brings about an intensity enhancement of intramolecular modes in the Raman spectra resonant with the CT absorption. The relevant parameter for this effect is the transfer integral tab for the hopping of one electron from orbital a in one molecule to orbital b in the other. This theoretical prediction is confirmed by Raman excitation profiles measured for the cation radical salt (TTF)2(W6O19)
Coupling of charge-transfer transitions to low-wavenumber phonons in quasi-one-dimensional radical ion salts: resonance Raman study of tetraethylammonium-DDQ
No full textLow-wavenumber resonance Raman spectra of the quasi-1-D anion radical salt tetratethylammoniumdichlorodicyano-p-benzoquinone are reported. By investigating the resonance behavior of the observed bands when the excitation wavelength is tuned from resonance with a localized electronic transition at 530 nm to the charge-transfer transition at 761 nm, three phonon modes at 148, 173, and 194 cm-1 coupled with the radical electrons through the modulation of their transfer integral are unambiguously identified. The Raman excitation profiles for these modes, measured in the 650–860 nm excitation wavelength range, were analyzed according to a Peierls–Hubbard dimer Hamiltonian using a time correlator approach. By combining the analysis of the excitation profiles with that of previously reported magnetic susceptibility data, it is possible to obtain values for the transfer integral, for the on-site electron correlation and for the electron–phonon coupling constants
Effects of the mixing of charge transfer and molecular excitations on the resonance Raman properties of symmetric radical dimers
No full text
Coupling of electrons to intermolecular phonons in molecular charge transfer dimers: a resonance Raman study
No full textWe report resonance Raman scattering (RRS) spectra and Raman excitation profiles (REP) of a system containing π dimers of identical molecular radical ions measured with laser excitation in resonance with the charge transfer (CT) transition. A Peierls–Hubbard (PH) Hamiltonian has been used to model the investigated system and to calculate its optical and RRS properties. Results are reported for two polyoxometallate salts of tetrathiafulvalene (TTF), namely (TTF)2(W6O19) and (TTF)2(Mo6O19) whose structures contain almost isolated (TTF+)2 dimers. The RRS spectra of (TTF)2(W6O19), measured in resonance with the CT absorption band centered at 832 nm, show three phonon modes located at 55, 90, and 116 cm−1 which are strongly resonance enhanced. These modes have been associated to the out‐of‐phase combinations of the translational motions of the two molecules composing the dimer. Such modes are effective in modulating the intradimer transfer integral, thus providing an efficient mechanism for coupling with the electronic system and for enhancement of the scattering intensity at resonance with the CT transition. The REP for the three strongly coupled modes of (TTF)2(W6O19) have been measured with laser excitation wavelengths ranging from 740 to 930 nm. Quantitative analysis of the REP data has been performed based on a perturbative solution of the PH model to second order in the electron‐molecular‐vibration (EMV) and electron‐intermolecular‐phonon (EIP) interactions. The CT absorption profile and the REP’s have been calculated using a time correlator technique and the model parameters have been optimized in order to fit the experimental REP data. Infrared vibronic absorptions of (TTF)2(W6O19), originated by the EMV coupling, have been measured and independent information on the electronic parameters of the PH model have been derived. This has made the choice of the fitting parameters used for the REP calculations rather unambiguous and has allowed us to obtain, for the first time, reliable experimental estimates of the EIP coupling constants
COUPLING OF ELECTRONS TO LOW-FREQUENCY PHONONS IN CATION-RADICAL SALTS OF TTF STUDIED BY RESONANCE RAMAN-SCATTERING
No full textFactors Affecting Calving Interval in Italian Holstein-Friesian Heifers
No full textEffect of rump conformation on calving interval of 520 Italian Holstein Friesian heifers was evaluated with simultaneous adjustment for herd, year of birth, calving month, age at first parturition, and milk production. All factors, except age at first parturition, were important sources of variation. Heifers with narrow rumps at the pins had the longest calving interval. There were significant differences among the herds. Cows born between 1979 and 1981 had shorter calving intervals, probably resulting from a national plan against bovine infertility initiated in 1981. Cows calving in July had better reproductive performance. There was positive relationship between milk production and calving interval
- …
