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

Summary form only given. 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].In the present work [3], we made a major experimental and theoretical progress in the elucidation of the nonadiabatic relaxation mechanism by energetically and angularly resolving the dynamics of the excited electron. For that purpose, we employed a Velocity Map Imaging (VMI) spectrometer to measure the kinetic energy and the angular dependence of photoelectrons that are produced by the IR pulse. This signal, can clearly be extracted because it corresponds to low energy electrons (below 1.5 eV), and, in that range, it is the only time-dependant photoelectron contribution as shown in Fig 1.