ECAMP 15

Synopsis In this work, we demonstrate how the discrete-state-in continuum model, previously used for electron-molecule collisions, can be generalized to treat the vibronic dynamics in electron photodetachment from molecular anions. The theory is tested on models motivated by diatomic molecules, exploring phenomena known from electron-molecule collisions, such as boomerang oscillations, Wigner cusps, and vibrational Feshbach resonances.

We extend the nonlocal discrete-state-in-continuum model based on the projection-operator formalism, previously successfully applied to inelastic electron-molecule collisions [1], to describe electron photodetachment from molecular anions. A crucial aspect of this process is the absorption of a photon by a molecular anion in its ground state, leading to the formation of a metastable molecular anion, which then undergoes vibronic dynamics before decaying into the electron molecule scattering continuum. By tracking the energy of the released electrons as a function of photon energy, a 2D spectrum can be obtained, similar to electron energy loss spectroscopy (EELS) [2], which serves as a suitable experimental method for studying these processes.

In the preprint [3], we present the theoretical framework and apply it to a simplified model of a diatomic molecular anion inspired by LiH$^-$. In later research, we vary the parameters of the diatomic model in an attempt to observe additional resonance phenomena known from electron molecule collisions, including boomerang oscillations, Wigner cusps, and vibrational Feshbach resonances, with preliminary results available in [4]. Furthermore, we implement two approximations: the adiabatic nuclei approximation and the local complex potential approach. We analyze the limits of these approximations and their applicability to describe different phenomena.

The photodetachment cross-section amplitude shows contributions from three competing processes: direct background detachment $(1)$, resonant autodetachment $(2)$, and temporary photodetachment followed by reattachment and subsequent autodetachment $(3)$:
\begin{align}
&\gamma + M^- \to M + e^- \tag{1}, \
&\gamma + M^- \to (M^-)^\ast \to M + e^- \tag{2}, \
&\gamma + M^- \to M + e^- \to (M^-)^\ast\to M + e^- \tag{3}.
\end{align}
The relative contribution of these mechanisms is illustrated in Figure 1 on a model exhibiting boomerang oscillations.

Figure 1 Photodetachment cross-section as a function of photon energy, illustrating contributions from different processes in a model exhibiting boomerang oscillations.

References
[1] Domcke W., Phys. Rep. 208 97–188, (1991)
[2] Anstöter C. S. et al, Phys. Rev. Lett. 124, 203401, (2020)
[3] Čížek M., arXiv arXiv:2309.05830, (2023)
[4] Zlatník J., Bc thesis, Charles University, Prague hdl:20.500.11956/191309, (2024)