ECI Webinar: "Ultrafast photoelectron/ion spectroscopy of molecules with high-order harmonic based sources"

Wednesday the 6th of April 2022 @ 15h00 CET

Speaker: Rocio Borrego Varillas, staff researcher

Institute of Photonics and Nanotechnology, CNR, ITALY

Title of the talk : "Ultrafast photoelectron/ion spectroscopy of molecules with high-order harmonic based sources"

Abstract: The discovery of high-order harmonic generation (HHG) [1-3] set the basis to obtain attosecond pulses [4,5] and marked the birth of a new branch of ultrafast science, able to resolve the electron motion on its natural time scale. The advent of attosecond spectroscopy has paved the way for the direct observation and control of electron motion in atoms, molecules and solids [6]. Tracking these ultrafast dynamics is highly challenging both experimentally and theoretically. In particular, the relaxation pathways following excitation in the vacuum-ultraviolet (VUV) remain largely unexplored. This is due on the one hand to the large internal energy acquired by the molecule, which opens several dissociation and isomerization pathways that compete with ultrafast relaxation of the electronic excitation. On the other hand, the broad bandwidth of few-femtosecond VUV pulses, necessary to track these dynamics, excite in many cases a large number of states thus complicating the interpretation of the data. In this talk I will review some of our recent work in ultrafast spectroscopy of molecules with HHG based sources. In particular, I will present our results using a HHG source in combination with a time-delay compensated monochromator [7], which allows for a selective excitation while keeping a temporal resolution of few femtoseconds. As an example, I will show its use for the investigation of CO2 superexcited states (i.e., a neutral excited state with internal energy higher than the ionization energy [8]). Up to date, the relaxation dynamics of these superexcited states are poorly understood due to the lack of proper experimental and theoretical tools. We have employed VUV photoelectron spectroscopy with sub-15 fs temporal resolution and semiclassical as well as full quantum mechanical simulations to study the ultrafast dynamics of the super-excited states in CO2. Our results show sub-50 fs dynamics originating from a complex mechanism, where the lower vibrational levels of the super-excited state couple to a repulsive state. Finally, I will present our recent progress towards the establishment of attosecond spectroscopy for the investigation of the excited-state dynamics in bio-relevant molecules using a UV pump-VUV probe approach.

[1] G. Farkas and C. Toth, Phys. Lett. A, 1992.

[2] P. Antoine, A. L'huillier, and M. Lewenstein, Phys. Rev. Lett., 1996.

[3] M. Lewenstein, P. Balcou, M. Y. Ivanov, A. L'Huillier, and P. B. Corkum, Phys. Rev. A, 1994.

[4] P. M. Paul et al., Science , 2001.

[5] M. Hentschel et al., Nature, 2001.

[6] R. Borrego-Varillas, M. Lucchini and M. Nisoli, Rep. Prog. Phys., 2022.

[7] N. Kouchi, in Fundamentals of Mass Spectrometry, Springer, 2013.

[8] M. Lucchini, et al., Opt. Expr, 2018.