Ionization of Atoms, Molecules and Nanostructures

Concerning strong field ionization of two- and polyatomic molecules there are many aspects that require a thorough theoretical understanding, such as the significance of many-electron effects beyond the single-electron approximation, the dependence on the symmetry or electronic properties of the molecule, the effect of dynamic polarizability and screening on the ionization, influence of the degree of  delocalization of the electron wavefunction of a given molecule.

Main projects studied by the group within this topic:

 Multiphoton ionization of atoms and molecules

We are interested here in comparison of properties between atoms and molecules and studying which caracteristics of molecular wavefunction influence ionization. We have studied the influence of the symetry of molecular wavefunction (ative orbital(s)) on the total ionization, orientation dependent ionizaiton, angular photoelectron spectra. We studied ionization of di- and polyatomic molecules up to C180 fullerene. More recent studied were devoted to the transition from the regime of the suppressed ionization of molecules to tunneling ionization.  Ionization if often studied in the group in parallel to high order harmonic generation.

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Resonance enhanced ionization for molecules

We have studied mechanisms analogous to CREI for multielectron di- and polyatomic molecules. For several molecules we have studied the ionization when the laser wavelength coincides with the difference in energy from (open shell) HOMO and HOMO-1. As a result one could follow in the calculations two dominant processes, namely ionization from the HOMO and resonantly enhanced ionization from HOMO-1.  In contrast to CREI the resonant coupling can be present for inner valence orbitals and not  only for HOMO-LUMO transition. In addition whereas CREI was studied only for dissociating molecules, present mechanism can be studied for equilibrium internuclear distances. Moreover for different wavelengths one can observe coupling of orbitals of different symmetry not only sigmau and sigmag coupling as it is in the case of CREI

 

Time delays in two photon resonant and nonresonant ionization of atoms and molecules

Our numerical simulations of time delays in two-photon resonant and nonresonant ionization of helium using the attosecond streaking technique confirm that the temporal shifts in the streaking traces consist of two contributions, namely a time delay acquired during the absorption of the two photons from the extreme ultraviolet field and a time delay accumulated by the photoelectron after photoabsorption. From our results we find that in the case of a nonresonant transition the absorption of the two photons occurs without time delay. In contrast, for a resonant transition a substantial absorption time delay is found, which scales linearly with the duration of the ionizing pulse. The two-photon absorption time delay can be related to the phase acquired during the transition of the electron from the initial ground state to the continuum and the influence of the streaking field on the resonant structure of the atom.

Measurements invoking the use of attosecond pulses can be incorrectly interpreted if the chirp of such pulses is not taken into account. We use a physically intuitive analytical model to understand the effect  a chirp in the extreme ultraviolet (XUV) attosecond pulse will have upon the delay observed in streaking experiments. It is known that both the photoionization cross-section of the system and the spectral and temporal characteristics of the attosecond pulse used will determine the relative time-dependent probability of absorbing a particular photon energy. We developed an analytical method to calculate the streaking delay as a function of the absorbed photon energy and the time delay between the XUV and streaking pulses. We have determined the expected value of the streaking delay observed when a chirped attosecond XUV pulse is used to initiate streaking experiments. We then demonstrate that depending on the chirp, the streaking delay can be changed by several attoseconds, which is on the order of the delays usually observed in such experiments.

 

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