Understanding the microscopic processes ruling light-matter interaction in light-absorbing material is a crucial task to fully exploit the potential of these systems in technological applications. First-principles methods based on density-functional theory and its time-dependent extension in real time (RT-TDDFT) offer the optimal trade-off between accuracy and computational efficiency, being able to describe the properties of systems modeled by hundreds of atoms.
In this project, we adopted RT-TDDFT to investigate transient absorption spectra (TAS) in prototypical carbon-conjugated molecules. Taking thiophene as an example (see Fig. 1) we explored the response of the system to an ultrafast laser pulse of 1000 GW/cm2. On the left panel of Fig. 1, the pulse is in resonance with the first excitation of the molecule at 5.6 eV, while on the right side it carries a frequency of 7.6 eV, corresponding to the higher-energy boundary of a broad and intense absorption peak. While in the former case weak signatures of transient absorption are noticed above 2 and 4 eV, in the latter a more complex dynamic is observed, with several transient maxima emerging at in the infra-red region of the spectrum.
The same methodology is applied to more complex systems such as hybrid inorganic-organic interfaces. In Fig. 2 we show the example of a hydrogenated silicon cluster functionalized with the strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The optical spectrum of such a system (top left panel of Fig. 2) is characterized by a maximum in the visible region (HE1) stemming from transitions between occupied states hybridized across the whole system and unoccupied states mainly localized on the molecule. By applying a laser pulse in resonance with this excitation, we can monitor the population change with respect to the ground state. The graph shown on the right panel of Fig. 2 clearly shows that within 30 fs, a non-negligible amount of charge is transferred from the delocalized occupied states to the localized unoccupied ones.