To illustratre the field of computational chemistry and materials science, here is a description of one of the projects currently using our system. The list of all current projects contains links to more project descriptions.

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.

Figure 1: Transient absorption spectra (TAS) of laser-excited thiophene molecule (shown in the inset) with a Gaussian pulse of 8 fs duration polarized along the direction pointing from the C-C bond to the S atoms. Left: pulse frequency of 5.6 eV in resonance with the first maximum marked in the TAS by the red horizontal bar. Right: off-resonance pulse frequency of 7.6 eV marked in the TAS by the red horizontal bar. The applied pulses are sketched on the top panels.

Figure 2 - Left: Optical absorption spectrum of the hybrid structure formed by a hydrogenated silicon cluster and the acceptor molecule F4TCNQ. The spectrum of the hybrid system (red curve) is contrasted with that of the Si cluster alone (blue). The maximum in the visible region (HE1) corresponds to the transition shown in the bottom.
Right: Population change with respect to the ground state of selected occupied and unoccupied states. The resonant pulse is sketched on top.