Clocking Intramolecular Charge Transfer in the Energy Domain

Philip Moriarty
School of Physics & Astronomy
University of Nottingham, Nottingham, UK

Pioneering advances in ultrafast spectroscopy -- recognised, for one, by the award of the 2023 Nobel Prize in Physics -- have revolutionised our understanding of electron dynamics on the femtosecond and attosecond timescales. Time-resolved measurements of this type, however, necessitate the sculpting of exceptionally short-lived optical pulses – an extremely challenging task. An alternative, complementary method of achieving ultrahigh temporal resolution is to work in the energy, rather than the time, domain. A particularly powerful approach is the “core hole clock” technique [1,2] first introduced by Björneholm et al. [3]. This involves exploiting the natural lifetime of a core hole created by X-ray absorption – of order femtoseconds to hundreds of attoseconds [4] - as an internal clock to probe temporal evolution, gaining key insights into the dynamics of electron transfer and delocalisation in atoms and molecules. Moreover, the technique is inherently chemically specific due to the elemental “fingerprinting” provided by photoemission and Auger spectroscopies.

I will discuss the application of core-hole clock methods to a molecular system that is especially intriguing from the perspective of electron dynamics: endofullerenes, in which an atom or molecule is encapsulated inside a buckyball. We have focussed on Ar@C60, using Auger spectroscopy in the resonant Raman regime [5] to determine the time it takes for an electron excited on the encapsulated argon to tunnel off its parent atom, for molecules in both a bulk film (see Fig. 1) and adsorbed as a monolayer on a Ag(111) surface. For the latter, the position of the Ar atom inside the C60 cage has been accurately determined using X-ray standing wave measurements [6], enabling a direct correlation of atomic position with excited state decay. Despite a lack of hybridisation of the frontier orbitals of the encapsulated atom and fullerene cage, the molecular environment dramatically influences the intramolecular tunnel rate.

 

[1] PA Brühwiler, O Karis, and N Mårtensson, Rev. Mod. Phys. 74 703 (2002)
[2] D Menzel, Chem. Soc. Rev. 37 2212 (2008)
[3] O. Björneholm, et al., Phys. Rev. Lett. 68, 1892 (1992)
[4] A. Föhlisch, et al., Nature 436, 373 (2005)
[5] See R Haverkamp, Stefan Neppl, and A Föhlisch, J. Phys. Chem. Lett. 14, 39 (2023) and references therein
[6] DP Woodruff, Rep. Prog. Phys. 68, 743 (2005)