R. G. Della Valle

Anharmonic processes in molecular crystals. Calculation of the anharmonic shifts, bandwidths and energy decay processes in crystalline naphthalene

Abstract The anharmonic frequencies and the bandwidths of the lattice phonons of naphthalene at 4 K have been calculated using an intermolecular potential which includes atom-atom and quadrupole-quadrupole contributions. The calculated bandwidths agree well with the available experimental data. The contribution of the cubic and quartic terms of the crystal hamiltonian to the anharmonic shifts is discussed in terms of phonon diagrams. The calculated quartic shifts are found to be positive whereas the cubic shifts are smaller and negative. The total anharmonic shifts are then positive and it is shown that this is in agreement with the observed dependence of the phonon frequencies with temperature. The mechanism of energy transfer between the optical lattice phonons and the two-phonon manifold of the crystal is discussed in terms of phonon-phonon coupling processes. Up and down conversions are analyzed and their relative efficiency is evaluated. The temperature dependence of both the anharmonic shifts and bandwidths is studied in the range 4–68 K at constant volume. Comparison with the available experimental data at constant pressure is discussed.

Raman phonon spectra of pentacene polymorphs

We report for the first time lattice phonon Raman spectra of pentacene measured by means of a Raman microprobe technique. We experimentally prove the existence of two polymorphs, as expected from recent structural studies. A comparison with Quasi Harmonic Lattice Dynamics calculations, previously performed starting from the available X-ray data, help us in identifying the phase to which each crystal belongs.

HIGH DIMENSIONAL ANHARMONIC POTENTIAL ENERGY SURFACES : THE CASE OF METHANE

The overtone vibrational spectra of all Td symmetry isotopomers of methane have been analyzed simultaneously. A Hamiltonian expressed in internal curvilinear coordinates expanded to the fourth order has been employed, with a nine-dimensional basis of harmonic oscillator wave functions in symmetry coordinates. Near-resonant anharmonic interactions are treated to first order, while weaker interactions are handled as second order perturbations. A set of optimized Born–Oppenheimer force constants is obtained, which reproduces the observations up to 9500 cm−1 and shows an excellent agreement with the results of ab initio calculations.

Pressure-induced phase transition in pentacene

We have recently studied two solid phases of bulk pentacene (polymorphs H and C) by means of lattice phonon Raman spectroscopy. The assignment, previously based on lattice dynamics calculations alone, is now verified by X-ray diffraction measurements, conclusively confirming the existence of both polymorphs. Furthermore, Raman phonon spectra indicate a pressure-induced phase transition where the polymorph C (lower density phase) transforms to the H form (higher density phase). The onset pressure for the phase transition is only 0.2 GPa. The phase change is irreversible.

An improved representation for the high-density structure of Lennard-Jones systems : from liquid toward glass

We show that a modified hypernetted‐chain (MHNC) integral equation with a properly chosen hard sphere bridge function can yield an excellent representation for the high‐density structure and thermodynamics of monatomic Lennard‐Jones (LJ) systems, continuously from fluid to supercooled liquid and glassy states. In particular, the theory is able to reproduce the gradual development of the second peak splitting in the radial distribution function. The LJ bridge function is approximated with a hard sphere bridge function calculated through a slight modification of a formula due to Malijevsky and Labik (ML). To select the equivalent hard sphere diameter d, several methods are tested. First, a criterion proposed by Rosenfeld and Blum and, second, a best fit of structural and thermodynamic simulation data; finally, an empirical parametrization for d as a function of density and temperature. For stable fluid states the predictions of the MHNC‐ML theory are successfully compared with a wide set of simulation resul...

Lattice dynamics and electron-phonon coupling in \beta-(BEDT-TTF)_2I_3 organic superconductor

The crystal structure and lattice phonons of (BEDT-TTF)_2I_3 superconducting \beta-phase are computed and analyzed by the Quasi Harmonic Lattice Dynamics (QHLD) method. Whereas the crystal structure and its temperature and pressure dependence are properly reproduced within a rigid molecule approximation, this has to be removed to account for the specific heat data. Such a mixing between lattice and low-frequency intramolecular vibrations also yields good agreement with the observed Raman and infrared frequencies. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The hopping integrals are evaluated by the extended Hueckel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function, which compares well with that experimentally obtained from point-contact spectroscopy. The corresponding dimensionless coupling constant \lambda is found to be around 0.4 .

Energy decay mechanisms and anharmonic lattice dynamics: The case of solid nitrogen

Abstract The anharmonic frequencies and linewidths of the lattice phonons in α-N 2 are calculated on the basis of three different intermolecular potentials which include atom-atom and electrostatic interactions. The distinction between stationary anharmonicity and decay anharmonicity is stressed and the mechanism of energy transfer between the optical lattice phonons and the two-phonon manifold of the crystal is discussed in detail. The temperature dependence of the phonon self-energy is also considered. The results thus obtained for α-N 2 are compared with predictions from previous lattice dynamics. SCP and molecular dynamics calculations. The calculated anharmonic effects are substantially independent of the adopted potential: the agreement with experimental data is reasonably good as far as the linewidths are concerned, while the anharmonic deformation of the potential wells (and thus the frequency shifts) is overestimated. We suggest that, while higher orders in the diagram expansion are necessary for a proper account of the stationary anharmonicity, the decay anharmonicity limits its effectiveness to two-phonon processes, thus allowing proper predictions of the phonon lifetimes by using the lowest-order diagrams. Finally, α-N 2 is compared to α-CO, and the role played by the translation-rotation coupling is discussed.

Pressure-induced phase transitions in 9,10-anthracene derivatives: anthraquinone

Abstract A pressure-induced phase transition is detected in crystals of anthraquinone at ≈ 2.3 GPa by Raman phonon spectra. Related fluorescence experiments indicate a phase transition mechanism similar to that recently observed in 9,10-dihalogenated anthracene derivatives, leading to a less relaxed excimer state after the transition. Lattice dynamics calculations under pressure reproduce the observed phase transition and confirm the transition mechanism.

Ab initio molecular dynamics study of ascorbic acid in aqueous solution

The ascorbic radical anion A*− in aqueous solution was studied using ab initio molecular dynamics based on density functional theory. Calculations of the spin density indicate that, both in vacuum and in solution, the unpaired electron is largely shared between the two oxygens, which, in the fully reduced acid AH2, constitute the acid hydroxyl groups, and the two carbon atoms connecting them. Of these two oxygens in RADAN, the one carrying in the reduced AN form the remaining proton is found to be the site with the largest unpaired electron density and also the site with (marginally) the higher affinity for hydrogen bonds. The hydrophilic character is almost completely lost upon oxidation of A*− to A. Reduction to AH− strengthens the hydrogen bonding of the deprotonated oxygen and weakens the hydrogen bonding of the protonated oxygen atom.

Effect of pressure on lattice modes and electronic excitations of 9,10-diiodoanthracene crystals

We have measured Raman, optical absorption, and fluorescence spectra of 9,10-diiodoanthracene (DIA) crystals as a function of hydrostatic pressure. Unlike analogous anthracene derivatives with similar quasi-one-dimensional stacking of molecules, the ambient pressure crystal structure remains stable up to at least 20 GPa. Lattice dynamics calculations, based on intermolecular potentials, reproduce well the structural parameters at zero pressure and the pressure dependence of the intermolecular Raman-active phonons. An explanation is given for the unusual stability of the one-dimensional molecular packing in DIA.