Attosecond X-ray Molecular Dynamics and Strong-Field Control

The development in recent years of ultrashort pulsed light sources in the attosecond/sub-femtosecond timescale has opened new avenues for the investigation in real-time of electronic and nuclear dynamics characterizing photochemical reactions. Such reactions, including biological reactions, such as e. g. those related to DNA damage and mutations, are triggered when light energy is absorbed by a molecule leading to a temporarily excited state that alters the physical and chemical properties of the molecules. The overall objective of this project is to understand and ultimately control photochemical reactions in excited states of polyatomic molecules. Two top-notch experimental methods are employed to achieve this goal: attosecond XUV transient absorption at the University of California at Berkeley (United States) and three-color femtosecond pump-probe velocity map imaging at Complutense University of Madrid (Spain). Ultrafast XUV transient absorption spectroscopy has been successfully employed to observe and characterize vibrational coherent motion on a molecular excited state as well as to unravel the electronic and nuclear dynamics e. g. at a conical intersection or through internal conversion, characterizing the photodissociation of different molecular targets.

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.

The achievements extending beyond the state of the art involves the use of short UV pump pulses in conjunction with attosecond probe pulses to resolve in real-time the photodynamics on neutral excited states of polyatomic molecules. Such experimental schemes are still pretty challenging. The coupled electronic and nuclear dynamics at a conical intersection have been visualized in real-time using this experimental set-up. In addition, the experiments performed on molecular iodine open the door for the experimental determination of potential energy curves of core-excited states of polyatomic molecules, which are often unknown and can be computationally demanding. The results of this project will further contribute to future exciting research concerning the understanding and control of photochemical reactions and ultrafast photodynamics. This can be especially relevant for fundamental biological reactions such as DNA damage and reactions related to human vision as well as for atmospheric chemistry and efficient materials.

Periodic Reporting for period 1 - ATTO-CONTROL (Attosecond X-ray Molecular Dynamics and Strong-Field Control)

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