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 in this topic:

**1. Resonance enhanced ionization for open shell 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 sigma_u and sigma_g coupling as it is in the case of CREI

**2. Enhanced ionization for closed shell molecules in the vicinity of exceptional point **

**3. Time delays in two photon resonant ionization**

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.

**4. Efficient calculations of streaking time delays using cut off Coulomb potential**

Based on our numerical streaking simulations we have shown previously that the observed time delays with respect to the instant of ionization are related to the finite range in space, which the emitted electron probes along the polarization direction of the streaking pulse after its emission until the streaking pulse ceases. We have further explored the finite range aspect and used cutoff Coulomb potential. The calculations for the Wigner-Smith time delays for the cut off Coulomb potential can be performed analytically. These time delays can be used in the formula derived by us, in which the time delay measured in a streaking experiment can be represented as a sum of piecewise field-free time delays over a finite-range weighted by the instantaneous field strength. The calculations performed in this way are very efficient and agree well with ab initio numerical simulations. One of the advantages is that it can be easily extended to calculations for three dimensional case and for multielectron systems within 'single active approximation' model potentials.

**5. Time Delays in Two-Photon Ionization: effect of attochirp**

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.

Past projects studied in this topic:

**1. Suppressed ionization **

**2. Alignment dependent ionization of molecules**

**3. Sequential multiple ionizaiton of icosahedral fullerenes**

**4. Transition from regime of suppressed molecular ionization to tunneling ionization**