Pier Francesco Fracassi

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.

Calculated lifetime and decay mechanism of the 991 cm−1 mode in crystalline benzene at 1.6 K

Abstract The lifetime of the 991 cm −1 Ag mode in benzene crystal at 1.6 K has been calculated in good agreement with the experimental value. The main decay mechanism has been shown to consist of a three-phonon scattering involving the ν 10 860 cm −1 e 2g internal vibration and high-frequency lattice modes.

Phonon dispersion curves and phonon lifetimes in crystalline ammonia

Abstract The theory of lattice dynamics of molecular crystals using atom—atom and multiple—multipole potentials developed in previous papers is extended to take account of anharmonic effects. Calculations of the dispersion curves and the density of states for the harmonic phonons in solid ammonia are presented. The finite phonon lifetimes due to anharmonicity are calculated for the zero wave vector modes and compared with experiment.

Lattice dynamics of solid α-carbon monoxide

Abstract The lattice dynamics of α-CO is studied assuming a fully-ordered antiferro P213 structure, using a potential model which includes electrostatic, dispersion and exchange interactions. The coupling of translational and librational motions leads to mixed modes at finite wave vectors and to anomalous acoustic dispersion. Anharmonic frequency shifts and the damping due to phonon—phonon interactions are evaluated for the zero wave vector modes. The single-particle potential wells for the high-frequency librational modes are investigated and suggest that translation—rotation coupling plays a primary role in the molecular reorientations.

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.

Translation-rotation coupling and the lattice dynamics of solid carbon monoxide

Abstract Several intermolecular potentials have been used to investigate the lattice dynamics of solid carbon monoxide assuming a fully antiferro P2 1 3 structure for the T = 0 K ground state. All modesl give a reasonable account of the zero wave-vector observed in Raman and infrared measurements. At finite wave vector translation-rotation coupling leads to anomalous dispersion in certain transverse acoustic phonon branches.

Potential models and torsional stability in molecular crystals

Abstract Unlike translational stability, torsional stability in molecular crystals can be extremely sensitive to the details of the adopted potential model. The

Anharmonic interactions in molecular crystals. Two-phonon absorption in crystalline OCS☆

Abstract The use of atom—atom potentials for the evaluation of third order coupling coefficients in molecular crystals is discussed. Expressions of the two-phonon absorption coefficients for combination bands involving an internal and an external phonon are obtained. The method described is applied to the phonon side bands of crystalline OCS. A satisfactory agreement with experiments is obtained.

Theoretical analysis of resonant raman scattering: Simulations of lineshapes and excitation profiles

Abstract The steady-state response of a three-level system in the presence of a strong laser field is described with the density operator formalism, in order to derive an analytical expression for the intensity of resonance Raman scattering. The Liouville equation for the density operator is written in quantum form for both the molecular system and the radiation field, making use of the dipole and rotating-wave approximations for the matter-radiation interaction, and of the Markov approximation for the molecular decays. The equation is solved exactly for the laser field, and in perturbative way for the generated field. The results account for power broadening of the generated line. Rabi intensity-dip of the generated signal, and for the excitation profiles in the Franck-Condom approximation.

Effects of dampings and dephasings in resonance raman spectroscopy

Abstract In resonance Raman scattering experiments both coherent and incoherent processes occur, their relative intensities being determined by the magnitudes of the statistical fluctuations and laser beam intensity. In the weak-field limit coherent processes dominate, and the competition between resonance fluorescence and inverse Raman scattering is determinant. In the strong-field limit incoherent processes are enhanced, and the emitted radiation is essentially fluorescent in character. In particular we show that, under stimulated conditions, inverse resonance Raman scattering can, in principle, be observed directly.