Ionization time delay dynamics from planar molecules

When an electron in an atom (or a molecule) absorbs an energetic photon, it may gain enough energy to escape and become photoionized. During its exiting excursion, the resulting photoelectron imprints the details of the parent potential in its phase. Following the semi-classical interpretation of Wigner, these phases can be understood in terms of delays in the photoionization process.

In this presentation, I will focus on the ionization time delays from extended molecules. At first glance, one might assume that large molecules result in scattering over a greater distance and consequently, a larger ionization time delay. However, we show that this simple view does not hold for planar molecules where the outgoing electron interacts with the delocalized hole smeared across the molecular plane. The shape and the symmetry of the ionic potential lead to a strong quadrupole contribution that significantly affects the electrostatic interaction between the photoelectron and the residual ion. The quadrupole contribution creates a less attractive potential for electrons emitted perpendicularly to the molecular plane. A weaker attraction implies a repulsive behavior, leading to photoelectrons being emitted in advance, and thus, exhibiting a negative ionization time delay.

To shed light on this physical process, we applied the RABBITT (Reconstruction of Attosecond Beating By Interference of Two-photon Transitions) method, resolved in angle and photon energy to two different C₁₀H_x molecules. One has the atoms distributed in a plane (naphthalene) while the other forms a near-spherical structure (Adamantane). To confirm the generality of our findings, we also investigated pyrene, which is nearly twice as large, and fluorene, which has two hydrogen atoms protruding out of the molecular plane. In all cases, the planar molecules exhibit a negative ionization time delay relative to their 3D counterparts. The interpretation is supported by a simple analytical model and a state-of-the-art Static-Exchange Density Functional Theory (SE-DFT) calculation.