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.
The realization of this project will provide the chance to perform pump-probe experiments with a superior temporal resolution in a broad spectral region, ranging from the near-UV (260 nm) to the near-IR (1000 nm) for the study of a wide range of solution processed molecular compounds, such as nucleic acid bases [1], hemeproteins [2] and thiophene based photochromic molecular switches [3]. In principle, the GVD effects could be mitigated using low-dispersion glasses, with reduced GVD in specific spectral ranges [4]. However, employing reduced-dispersion materials to hold the sample does not match the requirement of a broad tuning tunability for both pump and probe wavelengths, which indeed is imperative for simultaneously selecting the multiple energy domains involved in physiological processes [5]. On the contrary, if this proposal will be accepted, by fully avoiding the GVD introduced by transmission cell with windows, TA and IVS techniques will benefit one order of magnitude in terms of temporal resolution. Notably, the capability of collecting and handling 10 fs laser pulses directly on the sample will provide the chance, in IVS experiments, of detecting high frequency modes (with energy on the order of 3000 cm-1), where the measurement of the C-H and N-H stretching Raman modes can provide a wealth of information on the system geometrical configuration.
Accessing sub-50 fs temporal realms will disclose the chance to address the open case of vibrational relaxation and intersystem crossing processes in DNA and RNA nucleic acid bases. The joint action of TA and IVS has the potential to access at the same time electronics dynamics and vibrational fingerprints, enabling us to map all the degrees of freedom of photoexcited systems. The quest to understand how these molecules transform the absorbed energy into heat, redistributing it as kinetic energy among all the de-excitation channels and dissipating it into the environment avoiding damage, has recently become a hot topic [6-7]. In fact, highly efficient nonradiative decay pathways can guarantee that most excited states do not lead to deleterious reactions but instead relax back to the ground electronic state. Understanding nucleic acid basis photo-physics requires to identify the concurring pathways that reduce DNA damaging (as photon emission or nonradiative transitions to the ground state) as opposed to photo-reaction processes that are the doorways to structural deformations and intersystem crossing [7-8]. Although the last decade has witnessed a large number of experimental and theoretical studies to address this subject, identifying the vibrational degrees of freedom that lead to a de-excitation from the initially populated electronic state to an hot 1n-pi* excited singlet state is a crucial and still uncharted challenge. In fact, while it has been possible to identify the presence of such 1n-pi* electronic state, the conventional temporal resolution of pump-probe experiment is still inadequate to measure and assign the different and fastest transition timescales, which are theoretically expected on sub-50 fs regimes [8]. For this reason, the present project can provide a powerful tool to capture the first stages of nucleic acid bases photochemistry. Most important, enabling IVS to detect 3000 cm-1 modes, it would enable to identify the reaction-driven excited state manifold, responsible for the promptly harvesting of the photo-deposited energy, in these molecular compounds.
References:
[1] Richter M. et al., Phys.Chem.Chem.Phys., 16, 24423 (2014).
[2] Ferrante C. et al., Nature Chem. 8, 1137 (2016).
[3] Batignani G. et al., J. Chem. Phys. Lett., 7, 2981 (2016).
[4] Horn A., Ultra-fast Material Metrology, Wiley-VCH (2009).
[5] Zewail A. H., J. Phys. Chem. A, 104, 5660 (2000).
[6] Gustavsson T. et al., J. Phys. Chem. Lett., 1, 2025 (2010).
[7] Crespo-Hernandez C. E. et al., J. Am. Chem. Soc., 137, 4368 (2015).
[8] Doorley G. W. et al., Angew. Chem., Int. Ed., 7, 123 (2009).