We demonstrate that optical dephasing in disordered semiconductors does not require quasiparticle interaction but can be solely caused by disorder. We show that localization effects strongly influence the optical-dephasing signal by calculating the nonlinear polarization, taking into account disorder nonperturbatively. Decay times of the order of fs to ps can be expected depending on the ratio of diagonal disorder and intersite coupling. It follows that dephasing experiments performed on disordered semiconductors cannot be analyzed simply in terms of quasiparticle interactions.PACS numbers: 72.15. Rn, 42.50.Md, 71.55.Jv The interaction of carriers with phonons determines most of the steady-state and transient properties of disordered semiconductors (for a recent review, see, e.g., Ref 1). This interaction also governs the energy relaxation of excited carriers in a nonequilibrium situation, e.g., under optical excitation. The underlying processes can be described successfully in terms of hopping in the band tails on a time scale much larger than picoseconds. On the other hand, the investigation of the very fast processes immediately following an excitation with a short light pulse is still the subject of considerable experimental work. In particular, in the case of amorphous semiconductors like a-Si:H pump and probe experiments (see, e.g., Ref. 2) have been widely employed to study the energy relaxation of carriers close to the mobility edge on a picosecond and subpicosecond time scale.Alternatively, the interaction of excitations with phonons can also be studied by photon echo 3,4 or transient four-wave-mixing experiments 5,6 in the stimulated photon echo configuration. In these experiments the sample is excited by two very short laser pulses with a time separation r and k vectors ki (first pulse) and k2 (second pulse). A photon echo then is observed at a time IT with respect to the first pulse in the case of an inhomogeneously broadened spectral line. In the four-wave-mixing experiment a third probe pulse is diffracted by the grating produced by the first two excitation pulses. In both experiments the resulting signal is asymmetric with respect to T=0 in the direction 2k2~-ki, in the case of an inhomogeneously broadened transition, since then the diffracting grating corresponds to a phased array. 5 The amplitude of the signal is proportional to the nonlinear polarization (P(2r)) and typically decays according to exp( -IxlTj), with Ti the dephasing time. This dephasing time is usually related to quasiparticle interactions like scattering with phonons or other excitations.The interpretation of the dephasing process in terms of quasiparticle interaction is justified if the sample under consideration can be taken as an ensemble of mutually noninteracting two-level systems (as in the classical photon echo experiments on ruby 3 ). The same is true for

Solvation dynamics in a Brownian dipolar lattice. Comparison between computer simulation and various molecular theories of solvation dynamicsThe transient response of ions upon sudden creation or removal of a local, external charge in a one-component Coulomb lattice gas is investigated by Monte Carlo simulations. Our model can be regarded as a simplified description of solvation dynamics processes in electrolyte systems. Effects of the nonlinearity of the relaxation are pointed out and are contrasted with predictions from linear ͑Debye-Hückel and Debye-Falkenhagen͒ theories.

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