Ricerc@Sapienza

Ultrafast spectroscopy aims to investigate non-equilibrium dynamics of molecular compounds. Studying photoactive molecules in the liquid phase by means of temporally compressed pulses requires the use of a flow system to guarantee fresh sample at every laser shot. This is conventionally allowed by using a peristaltic pump to flow the sample through a transmission cell with windows. Critically, while propagating along dispersive windows, different pulse spectral components have different group velocities, and hence they lose their temporal overlap and experience a temporal broadening. Moreover, the presence of glass in the optical path can result also in nonlinear absorption and cross phase modulation effects. Therefore, in multiple-colour ultrafast studies, the presence of windows compromises the temporal resolution of the experiment. In the case of 10 fs light pulses, 1 mm of fused silica reduces the temporal resolution up to 200 fs when combining near ultraviolet and visible light pulses.
Building on an existing pump-probe setup, which exploits third harmonic generation and noncollinear optical parametric amplification to synthetize 10 fs laser pulses in a broad spectral region, ranging from the near-UV (260 nm) to the near-IR (1000 nm), we propose the realization of a wire-guided, gravity-driven jet nozzle apparatus, able to provide stable thin films of flowing liquids without window cells. Avoiding the limitations dictated by the use of transmission cells, we aim to provide a tool to perform transient absorption and Raman pump-probe experiment with 10 fs-time resolution, which otherwise would be limited to more than 100 fs.
The setup capabilities will then be tested: harnessing the possibility to access 10 fs temporal realms, we will be able to track the sub-50 fs response of DNA and RNA nucleic acid bases, in order to address the highly debated case of vibrational relaxation and intersystem crossing processes in these essential molecular building blocks.