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 C_{180} 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.

**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 sigma_{u} and sigma_{g }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.

**Publications:**

1. Effect of attochirp on attosecond streaking time delay in photoionization of atoms, C. Goldsmith, A. Jaron--Becker, A. Becker, Journal of Physics B: Atomic, Molecular and Optical Physics 51, 025601 (2018).

2. Analytical estimates of attosecond streaking time delay in photoionization of atoms, C. Goldsmith, J. Su, A. Becker, A. Jaron-Becker, Physical Review A 96, (2017).

3. Time Delays in Two-Photon Ionization, J. Su, H. Ni, A. Jaron-Becker, A. Becker, Physical Review Letters 113, 263002 (2014).

4. Attosecond-streaking time delays: Finite-range property and comparison of classical and quantum approaches, J. Su, H. Ni, A. Becker, A. Jaron-Becker, Physical Review A 89, 013404 (2014).

5. Numerical simulations of attosecond streaking time delays in photoionization, J. Su, H. Ni, A. Becker, A. Jaron-Becker, Chinese Journal of Physics 52, 404 (2014).

6. Observation and Control of Electron Dynamics in Molecules, A. Becker, F. He, A. Picón, C. Ruiz, N. Takemoto, A. Jaron-Becker, Attosecond Physics 177, 207 - 229 (2013).

7. Theoretical analysis of time delays and streaking effects in XUV photoionization, J. Su, H. Ni, A. Becker, A. Jaron-Becker, Journal of Modern Optics 60, 1484 - 1491 (2013).

8. Finite-range time delays in numerical attosecond-streaking experiments, J. Su, H. Ni, A. Becker, A. Jaron-Becker, Physical Review A 88, 023413 (2013).

9. Numerical simulation of time delays in light-induced ionization, J. Su, H. Ni, A. Becker, A. Jaron-Becker, Physical Review A 87, 033420 (2013).

10. Attosecond intramolecular electron dynamics, A. Becker, N. Takemoto, A. Picón, A. Jaron-Becker, EPJ Web of Conferences 41, 02008 (2013).

11. Wavelength Dependence of the Suppressed Ionization of Molecules in Strong Laser Fields, J. Durá, A. Grün, P.K. Bates, S.M. Teichmann, T. Ergler, A. Senftleben, T. Pflüger, C.D. Schröter, R. Moshammer, J. Ullrich, A. Jaron-Becker, A. Becker, J. Biegert, The Journal of Physical Chemistry A 116, 2662 - 2668 (2012).

12. Molecular Dynamics in Strong Laser Fields, A. Jaron-Becker, IEEE Journal of Selected Topics in Quantum Electronics 18, 105 - 112 (2012).

13. Suppressed molecular ionization due to interferences effects, A. Jaron-Becker, A. Becker, Laser Physics 19, 1705 - 1711 (2009).

14. Single-active-electron ionization of C 60 in intense laser pulses to high charge states, A Jaron-Becker, A Becker, FHM Faisal, The Journal of chemical physics 126, 124310 (2007).

15. Saturated ionization of fullerenes in intense laser fields, A Jaron-Becker, A Becker, FHM Faisal, Physical review letters 96, 143006 (2006).

16. Ionization of N2, O2, and linear carbon clusters in a strong laser pulse, A Jaroń-Becker, A Becker, FHM Faisal 2004/2/27, Physical Review A 69, 023410 (2004).

17. Signatures of molecular orientation and orbital symmetry in strong-field photoelectron angular and energy distributions of diatomic molecules and small carbon clusters A Jaron-Becker, A Becker, FHM Faisal - LASER PHYSICS 14, 179-185 (2004)

18. Dependence of strong-field photoelectron angular distributions on molecular orientation, A Jaron-Becker, A Becker, FHM Faisal, Journal of Physics B Atomic Molecular Physics 36, L375-L380 (2003)