Attosecond transient absorption spectroscopy (ATAS) has been employed at UC Berkeley using an IR/visible/UV pump pulses (4-20 fs) and XUV probe pulses (200 as) in order to unravel photo-induced molecular dynamics of different targets. First, experiments on coherent nuclear and electronic dynamics on deuterium bromine following its strong-field ionization using IR pulses were carried out in a first stage. This work has led to one publication (Kobayashi et al, Phys. Rev. A, 2020, 101, 063414).
Second, ATAS experiments using a visible pump pulse to excite molecular iodine in its B bound state were performed. The created vibrational wavepacket was visualized with great detail and relevant information on the potential energy curves of core-excited states was extracted. Additional two-photon absorption and direct dissociation was resolved in real-time. This work was presented at the DAMOP conference of the American Physical Society (S. M. Poullain et al, Bulletin of the American Physical Society, 65, “Probing dynamics in molecular iodine using ultrafast XUV transient spectroscopy”) and a manuscript on this subject has been submitted (S. M. Poullain et al, submitted, 2021).
Third, the coupled nuclear and electronic dynamics at the conical intersection characterizing the photodissociation of different alkyl iodides upon excitation at ~270 nm in the first absorption band was investigated. The optical set-up for short UV pulses (279 nm, < 20 fs) was recently developed within K. Chang PhD thesis and used to study the structural effects on the photodissociation of a series of alkyl iodides, including CH3I, C2H5I, i-C3H7I and t-C4H9I. A first article has been published on this subject (K. F. Chang et al, Nat. Comm., 11, 4042) as well as a conference proceeding (K. F. Chang et al, Frontiers in Optics / Laser Science, OSA 2020). Finally, the photodissociation of allyl iodide was investigated in order to evaluate the role of a double bond and possible resonance stabilization on the photodynamics. A collaboration with Prof. Jesús González-Vázquez (UAM) has been established and ab initio calculations including on-the-fly trajectories are in progress to fully interpret the results. Two publications are in preparation on this subject (K. F. Chang et al, in preparation and S. M. Poullain, in preparation).
The researcher has also participated in two different beamtimes at the MeV-UED ultrafast electron diffraction instrument at SLAC (Stanford University). The first access which took place in October 2020 concerned the photodynamics of acetylacetone induced by excitation around 266 nm. Based on preliminary analysis, the geometrical change –ring-opening of the enol form and deplanarization is expected– associated with the internal conversion from the S1 (n-*) state into the T1 (-*) state is observed and occurs in a one picosecond timescale. Further analysis of the results and calculations are still required to fully interpret the results in terms of geometrical changes associated with the photo-induced dynamics. The second access to the MeV-UED instrument, in January 2021, was dedicated to the investigation of the photodissociation of dibromopropane and the possible formation of cyclopropane following two C-Br cleavages. The analysis is still in progress in order to interpret these experimental results.
