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Moleküler çok-fotonlu iyonlaşma etkin kesitinin hesaplanması

Decleva, Piero
Toffoli, Daniele
The theoretical treatment of multiphoton (MP) ionization processes, in the framework of lowest order perturbation theory (LOPT), has been available for more than three decades for atomic systems. In particular the possibility of obtaining extremely short pulses, now in the attosecond domain, opens the possibility of exploring nuclear as well as electron dynamics in atoms and molecules, with fascinating perspectives . The recent development of the free electron laser (FEL), pioneered at FLASH , with numerous new facilities just having entered operation or under construction has revived the multiphoton regime. In fact, despite the enormous powers available, because of the much larger photon energies, the radiation-matter interaction is well inside the MP domain. Also, except for the shortest pulses, pulse duration generally exceeds a few tens of optical cycles, making the LOPT approach completely justified. It is then interesting to expand the molecular approach which has been very successful in the study of one photon molecular ionization processes, to the evaluation of MP ionization cross sections. The use of a multicenter B-spline basis offers the capability of attaining convergent results, within the chosen Hamiltonian model, for photoionization observables also in pretty large molecules, like C60. The implementation achieved in this project is completely general, making full use of the molecular point group symmetry. Moreover the formalism has been implemented so that any field orientation with respect to the molecular frame can be treated, as well as rotational averages for randomly oriented molecules, and linear or circular light polarizations are already available. As in the case of TDSE our first implementation employs a fixed density functional (DFT) hamiltonian, therefore in the same spirit of a static exchange, or single channel approach. Already at this level a quite good description of photoionization in the linear regime is generally obtained, and it can be reasonably expected that a similar agreement, at least at a semiquantitative level, can be obtained for MP ionization processes. This was indeed observed in the comparison of single channel versus RPA calculations in water. The basic structure, however, can be in principle extended to a more refined treatment of the many-body problem, time-dependent DFT being an obvious possibility.