Theoretical description of time-resolved photoelectron momentum microscopy of ultrafast photoexcited dynamics in molecular systems

Abstract

The recent development in photoelectron momentum microscopy, covering light pulses and measurement instruments, brought orbital tomography into the time domain. Simultaneously the development of theoretical approaches is crucial for the interpretation and explanation of the data. In addition, simulations can guide the design of these expensive experiments. I extend photoelectron momentum microscopy to image excited state properties in molecular systems from a theoretical perspective. This work aims to capture and understand the dynamics of excited isolated molecules and molecules at interfaces down to attosecond time resolution and with atomic spatial resolution.

In particular, I describe excited states of isolated molecules with ab initio configuration interaction calculations. I consider the broad bandwidth of the probe pulse, where the interaction with the molecular system leads to the emission of photoelectrons. I demonstrate the opportunities of this developed framework by comparing my theoretical calculations with data from two experiments performed by our collaborators.

In the first experiment, the sample is a bilayer pentacene film adsorbed on a silver substrate. My calculations reveal electronic and structural dynamics observed in the experiment up to hundreds of femtoseconds after the excitation. The second experiment involves a thin film of copper phthalocyanine on TiSe2. In this case, my calculations on angle-integrated C1s core spectra and angle-resolved photoelectron distributions from the valence states of neutral and ionized CuPc gain insights into the electronic and structural properties of the sample in the experiment after excitation. Furthermore, charge transfer properties between the substrate and the molecules could be investigated.

In a purely theoretical part, I studied the possibility of photoelectron momentum microscopy for imaging charge oscillations on the attosecond timescale in neutral photoexcited molecules. In particular, I considered charge migration, described by a superposition of excited states, in pentacene induced by a pump pulse. The photoelectrons are emitted due to the interaction with an XUV probe pulse. I demonstrate that the excited-state dynamics of a neutral pentacene molecule in real space map onto unique features of photoelectron momentum maps.

Finally, my thesis shows the potential of photoelectron momentum microscopy for imaging processes upon photoexcitation in molecular systems. The findings of this study give a deeper understanding of the dynamics in different molecular systems.