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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.

Nonlinear four-wave light scattering (FWLS) on a three-level system is analyzed, adopting the density operator approach. A strong field Ω 1 is mixed by the material system with two fields Ω 2 and Ω 3 to yield an output field peaked at the frequency Ω 4 = Ω 1 + Ω 3 - Ω 2 . The theory is developed within the dipole and Markov approximations. The strong resonant field is treated “exactly”, while the other fields are regarded as perturbations. The generated intensity is obtained as the expectation value of the photon number operator, estimated at the sixth order in the weak-field expansion. The generation of a coherent field at the frequency Ω 4 is due to processes where either a single molecule or a couple of molecules are involved. The total intensity is interpreted as the superposition of four contributions: a four-photon scattering process (FPS), a three-photon process (TPS), and two more processes related to resonance Raman scattering (RRS) and resonance fluorescence (RF). The method allows the deduction of nonlinearities in the generated intensity which are not included in fully perturbative treatment, and which, in the strong-field limit, originate the Rabi intensity dip of the lines.

We discuss the transient evolution of the coherent and incoherent emission from a three-level system driven by a resonant laser field of arbitrary intensity. The effects due to the levels finite lifetimes and dephasings and to the laser power are considered, within the Markov and rotating wave approximations. An analytical expression for the time-dependent generated signal is derived, enabling the use of time-resolved spectroscopy to extract the parameters characterizing the level dynamics.

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.

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.

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.

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.