Energetically and angularly resolved non-adiabatic relaxation dynamics of PAH molecules following XUV excitation

The absorption of a single XUV photon by a complex molecule such as a PAH (Polycyclic Aromatic Hydrocarbons) can induce diverse mechanisms that can take place in a broad variety of timescales starting from the attosecond one [1], Nowadays the development of the high harmonic generation (HHG) technology allows synthesizing light pulses in the VUV-XUV spectral range, with duration down to a few tens of attosecond. Hence, the XUV induced ultrafast dynamics can be investigated using table-top experimental setups. The ionization of an inner electronic state of the valence band creates a cation in an electronically excited state. Because of the large number of coupled degrees of freedom in such polyatomic molecules, the initial electronic population can be transferred to lower states and the electronic energy can be converted into vibrationnal energy. This population dynamics has been investigated in the case of PAHs in a recent work [2] by measuring the dication rate as a function of the delay between the XUV and IR pulse. In that experiment the XUV pulse ionizes the molecule creating complex excited states, then the IR pulse ionizes further the excited cationic molecule (producing a dication) while its energy is not converted into the vibrational degree of freedom. We have found that this relaxation dynamics is very rapid, in the range of few tens of fs, and is governed by near localized non-adiabatic crossings through the potential hypersurfaces. This picture is supported by ADC and MCTDH calculations that explicitly take into account for the multielectronic character of the cationic wavefunction and the non-adiabatic couplings [2].