Jan A. Leegwater

Photon echoes in impulsive optical spectroscopy of phonons

Multidimensional off resonant spectroscopy of crystals using a train of optical pulses can be effectively used to probe nuclear dynamics in solids. We predict a clear photon echo signal even in the absence of inhomogeneous broadening. This technique may be used for studying phonon dynamics in solids, in structurally frozen systems such as glasses, and in systems where the validity of the concept of phonons is not equally well established, such as liquids and gases. With this technique it is also possible to obtain information on the lifetime of phonons. We predict a long time tail of the nonlinear signal proportional to t−n/5, where n is the order of the response function studied.

Velocity autocorrelation function of Lennard-Jones fluids

A kinetic equation describing tagged particle motion in a fluid of particles interacting through continuous potentials is used to calculate the velocity autocorrelation function. The kinetic equation takes into account only binary collisions. It is an extension the Lorentz–Boltzmann equation to higher densities, by inserting a factor g(r) in such a way that in the hard sphere limit the Lorentz–Enskog equation is obtained. The velocity autocorrelation function of a fluid consisting of Lennard‐Jones particles calculated using this theory compares well with simulation results, even for remarkably high densities. Close to the triple point the theory fails due to the neglect of perturbed binary collisions and correlated collisions. Approximate expressions for the collision operator are given for r−n interactions and a closed expression for the self‐diffusion coefficient in terms of the pair distribution function is presented.

The optical dielectric function of polarizable liquids

We report numerical simulations of the density of states and the frequency dependent dielectric function for optical frequencies of a polarizable Lennard‐Jones atomic fluid from gas to liquid densities. Our simulations at high densities can be successfully interpreted in terms of fluctuations around a fcc solid reference model. The mean spherical approximation, which is based on a resummed density expansion, is shown to give reasonable predictions for intermediate densities but is shown to fail at liquid densities. We discuss the transition from the gas phase limit to the liquid phase theories, and show that for liquids nuclear motions do not strongly affect the line shape, whereas local field effects do.

Semiclassical Green function calculation of four wave mixing in polarizable clusters and liquids

A theory for the nonlinear optical response of an assembly of four level atoms representing an s–p transition, with dipole–dipole interactions is developed. Simulation results for two‐pulse ultrafast four wave mixing measurements in atomic clusters and in liquids demonstrate that the electronic density of states can be obtained by combining absorption and four wave mixing experiments. We show that a sensitive direct probe for interatomic interactions and correlations is provided by reversing the pulse time ordering in a photon echo sequence. The effect of nuclear motions on the linear absorption spectrum is investigated.

Exciton transport and degenerate four wave mixing in topologically disordered systems

The static and dynamical properties of excitons in a lattice with a random distribution of polarizable atoms are studied using Green’s function techniques. Exciton transport is related to the configurationally averaged particle–hole Green’s function which is calculated using the ladder diagram approximation. Degenerate four wave mixing (D4WM) using resonant pump beams and an off resonant probe is shown to provide a direct probe for exciton transport. A disorder induced extra resonance is predicted whose width is proportional to the exciton diffusion coefficient. Numerical calculations are presented for the diffusion coefficient and the D4WM signal as the excitation energy is tuned across the exciton band.

Four-wave mixing signatures of exciton Bose condensation

Abstract We propose a transient grating experiment in order to observe exciton Bose condensation. We show that for typical parameters in semiconductors the velocities of first and second sound are comparable, resulting in a characteristic behavior of the decay of the longitudinal modes.

Transient grating spectroscopy of exciton sound waves in dense exciton fluids

Abstract A dynamic description is developed for the motion of an exciton fluid at high exciton density. Coherent oscillatory modes of the exciton density are predicted provided the exciton—phonon coupling is dufficiently small. The physical mechanism is similar to that of sound waves in an ordinary gas. A method to probe these motions using transient grating with strong pump fields is proposed. The grating recurrence time is estimated to be about 100 ps for typical molecular crystals. Exciton Bose condensation could also be probed using the same technique.

Kinetic theory of Lennard‐Jones fluids

A kinetic theory that describes the time evolution of a fluid consisting of Lennard‐Jones particles at all densities is proposed. The kinetic equation assumes binary collisions, but takes into account the finite time duration of a collision. Furthermore, it is an extension of a kinetic equation for the square well fluid as well as the hard sphere Enskog theory. In the low density limit, the Boltzmann theory is obtained. It is shown that the proposed theory obeys all the conservation laws. The exchange of potential and kinetic energies is studied and it is shown that at high density this is a fast process. The dominant mechanism for energy exchange is found to be collisions at the strongly repulsive part of the potential that are disturbed by third particles. The kinetic equation is also used to calculate the Green–Kubo integrands for shear viscosity and heat conductivity. The major structures found in molecular dynamics simulations are reproduced at intermediate densities quantitatively and at high densit...

Nonlinear optical properties of confined excitons in clusters

Size effects in femtosecond photon echo spectroscopy of neat clusters are calculated using a quasiparticle representation of the nonlinear response. We extend our previous study of cooperative effects on the nonlinear response of assemblies of two level molecules [J. A. Leegwater and S. Mukamel, Phys. Rev. A46, 452 (1992)] to allow for nuclear motion and to have an s-p model of polarizable atoms. Photon echos in Benzene/Argon clusters are calculated using a semiclassical phase averaging procedure [L. E. Fried and S. Mukamel, Adv. Chem. Phys. (in Press)].