Attosecond correlation dynamics during electron tunnelling from molecules

Zachary B Walters (1) , Olga Smirnova (1)

1 MBI - Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie

We present analytical theory of strong field ionization of molecules, which takes into account rearrangement of multiple interacting electrons during the ionization process. We show that such rearrangement offers an alternative pathway to the ionization of orbitals more deeply bound than the Highest Occupied Molecular Orbital (HOMO). This pathway is not subject to the full exponential suppression characteristic of the direct tunnel ionization from the deeper orbitals. The departing electron produces an " attosecond correlation pulse " which controls the rearrangement during the tunneling process. The shape and duration of this pulse are determined by the electronic structure of the relevant states, molecular orientation, and the laser parameters. Attosecond correlation dynamics during electron tunneling from molecules 2 Tunnel ionization in static electric fields is exponentially sensitive to ionization potential I p: ionization from deeper bound states is exponentially suppressed. Similar suppression is commonly expected during ionization in intense low-frequency laser fields [ 1 ]. It has thus been a natural and nearly universal assumption that only the least bound electron may escape the atom or molecule, leaving the resulting ion in its lowest electronic state. We show that the conventionally assumed exponential suppression of ionization rates is strictly valid only if the state of the ion is frozen during the tunneling process. The ionization dynamics change if the state of the ion is allowed to evolve, reflecting the interaction between the departing electron and the ionic core. We show that correlated electron dynamics during ionization provides a new pathway to the creation of excited states of the ion, which is not subject to the full exponential suppression accompanying direct tunneling from a more deeply bound state. One would think that the importance of excited electronic states of the ion during ionization could be easily verified by examination of photoelectron spectra. Indeed, different ionization channels, corresponding to different I p 's should lead to photoelectron peaks with different energy. While this is indeed the case for one-photon ionization, highly non-linear nature of strong-field processes make observation of the alternative ionization channels quite challenging in the strong field regime [ 2 ]. The first experiment [ 3 ] aimed at observing ionization channels with higher I p during strong-field ionization of molecules has been performed with H 2. However, the large (about 20 eV) energy needed to excite the H + 2 ion resulted in a vanishingly small (10 −6) excited state population [ 3 ]. An important problem arising in such experiments is the need to separate the creation of excited ions during and after ionization. Recent studies of strong field ionization of hydrocarbon molecules [ 4 ] solved this problem by using covariance technique, correlating energies in the photoelectron spectra with the ionic fragments. These experiments have unambiguously demonstrated the contribution of several ionization channels corresponding to the population of different electronically excited states during ionization. Together with an avalanche of alternative experimental evidence [ 5, 6, 7, 8, 9, 10, 11 ], the body of new experimental data poses challenges to the theory of strong field ionization. While strong field ionization of atoms is well understood (see e.g. [ 13, 12 ]) in the single channel limit, a quantitative explanation of single channel ionization of molecules or an accurate treatment of multiple coupled channels represent problems at the frontier of strong field theory (see e.g. [ 23]). The importance of channel coupling due to the laser field has been emphasized theoretically in [ 9, 20, 23 ]. Ref. [ 9 ] suggested the importance of electron correlations during tunnelling and provided the first evidence of non-trivial dynamics induced by this coupling. We present a multichannel generalization of the strong field ionization theory originally proposed in the classic papers of Popov, Perelomov and Terent'ev (PPT)[13 ]. We include channel coupling induced by electronic interactions between the departing electron and the electrons left behind in the molecule. Interactions with the departing electron produce an " attosecond correlation pulse " which may drive transitions between

Mots clés: multichannel strong field ionization; attosecond multielectron dynamics; core rearrangement; molecular orbitals

Zachary B Walters, Olga Smirnova. Attosecond correlation dynamics during electron tunnelling from molecules. Journal of Physics B: Atomic, Molecular and Optical Physics, 2010, 43 (16), pp.161002. ⟨10.1088/0953-4075/43/16/161002⟩. ⟨hal-00597865⟩