Time-resolved XPS for molecular electronic movies

Light excitation couples to molecular electrons. The conversion of photon energy into other molecular forms of energy is then determined by a complex interplay of electrons and nuclei, which is out of the scope of the Born-Oppenheimer approximation. We use time-resolved photoelectron spectroscopy of core electrons to observe the electron density at particular, crucial sites, thereby creating a molecular electronic movie of photoenergy conversion. I give an example of the technique and elucidate to new possibilities at the FLASH free-electron laser.

The molecule in the center of this talk is a thionated nucleobases. All nucleobases posses high absorption cross sections in the ultraviolet spectral range. Canonical nucleobases (without sulfur substitution) transfer electronic energy mostly into harmless vibrational energy. Thiolated nucleobases however show an efficient and ultrafast relaxation into long-lived triplet states, contrasting with the ultrafast relaxation to the ground states observed in canonical nucleobases. The triplet channel gives rise to applications as photoinduced-cross linkers but also to problems related to its current use of thionucleobases as medication.

We investigate the UV-induced dynamics of 2-thiouracil via time-resolved x-ray photoelectron spectroscopy (XPS) at the sulfur L-edge. We observe a shift of the photoelectron energy towards lower energies after UV excitation to the photoactive ππ* state. Subsequently, we observe a 250 fs oscillation in the electron binding energy.

We interpret the UV induced shift as a consequence of valence charge shifting from the sulfur atom to the ring of the molecule. The oscillation in the binding energy can be interpreted as a consequence of nonadiabatic dynamics over a conical intersection that drives the molecular wavepacket in- and out of the nπ* state [1].

Simulations of the XPS spectrum find a direct connection of the binding energy and the dynamically changing charge, similar to the potential model in static ground state XPS [2].

Our newer XPS studies on the carbon atoms of the same molecule probe the site that receives the electron density that is abstracted from the sulfur site. Based on the direct connection between the XPS shift and the local charge, molecular electronic movies can be constructed from the XPS data.

The FLASH free-electron laser facility is currently undergoing an upgrade, that will be highly benefitial for XPS but also for many other x-ray spectroscopy techniques. At the heart of this upgrade is the external seeding of FLASH up to 300 eV in the fundamental photon energy, with the option to also perform experiments with the third harmonic of the source.

I will give an overview of the benefits for the user community, including several of the major science fields at FLASH.

References

[1] Mayer, D.; Lever, F.; Picconi, D.; Metje, J.; Alisauskas, S.; Calegari, F.; Düsterer, S.; Ehlert, C.; Feifel, R.; Niebuhr, M.; Manschwetus, B.; Kuhlmann, M.; Mazza, T.; Robinson, M. S.; Squibb, R. J.; Trabattoni, A.; Wallner, M.; Saalfrank, P.; Wolf, T. J. A.; Gühr, M. Following Excited-State Chemical Shifts in Molecular Ultrafast x-Ray Photoelectron Spectroscopy. Nat. Commun. 2022, 13 (1), 198

[2] Gelius, U. Binding Energies and Chemical Shifts in ESCA. Phys. Scr. 1974, 9 (3), 133–147.