Core excitation of gaseous HCl to the dissociative s ‫ء‬ state leads to a deexcitation spectrum with two qualitatively different contributions -a broad molecular background and narrow atomic lines. The experiment demonstrates that the ratio of integrated intensities from these two contributions depends crucially on the excitation frequency; the molecular background is enhanced with increased detuning of the photon frequency. This is discussed in a time-dependent model, using the concept of an effective duration time for the resonant x-ray scattering process. [S0031-9007(97)

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The propagation of laser pulses of different lengths in nonlinear media of organic absorbers is described starting out from a recently suggested dynamical theory for two-photon absorption ͑TPA͒ of molecules in solutions ͓J. Opt. Soc. Am. B 19, 937 ͑2002͔͒. The roles of saturation effects and pulse duration on the suppression of TPA are emphasized. The numerical simulations of the pulse propagation are performed for a two-photon active charge transfer molecule using molecular parameters obtained from first principle calculations.

Optical power limiting is theoretically studied using an approach that combines quantum electronic structure calculations of multiphoton excitations and classical calculations of dynamical wave propagation. We illustrate the capability of such a combined approach by presenting results for a couple of organometallic compounds; basic metal-base porphyrins, vinylphenylamine porhyrin, and the so-called type IVc platinum compound. A comparative analysis of their electronic properties related to nonlinear absorption of electromagnetic radiation and their optical limiting capability has been performed based on dynamical simulations of the nonlinear pulse propagation taking account of resonant as well as off-resonant effects. Several key features and ratelimiting steps in the transmission have been examined in relation to various characteristics of the pulse. It is found that the resonant vs off-resonant conditions, the saturation conditions and the dephasing play critical roles for the nonlinear transmission. The saturation effects are sensitive to the pulse duration, the intersystem crossing rate and the quenching of the higher triplet state. The inter-system crossing rate has to be comparable with the inverse pulse duration in order to boost the stepwise two-photon channel associated with singlet-singlet followed by triplet-triplet transitions. It is illustrated that structure-to-property relations of the rate-limiting steps serve as important criteria for choices of compounds suitable for the application of interest.