ECAMP 15

Molecules that are internally highly excited play an important role in a range of fields from atmospheric to plasma physics. Modelling such environments requires a detailed understanding of the molecules' behaviour at very strong excitations. However, this is a non-trivial task due to the high density of excited states as well as the variety of competing decay mechanisms available. The diatomic carbon anion C$_2^-$ presents an excellent benchmark to understand the interplay of different decay channels at high internal excitation. The system is arguably the most extensively studied molecular anion in history. Yet, its decay behaviour at high internal excitation has long remained a riddle. When produced in a hot ion source, a subset of the resulting anions spontaneously eject their excess electron with a very narrow lifetime span of about 3\,ms. While this autodetachment phenomenon has been known since the 1990s, the responsible anionic excited states and their decay mechanism have long remained elusive. Based on our measurements of autodecay of highly excited C$_2^-$ at the Cryogenic Storage Ring (CSR) facility in Heidelberg, we carried out detailed calculations of the excited states and their decay behaviour [1,2]. Here, we were able to uncover the profound effect rotational excitation has on the system's electronic landscape, causing a reshuffling of the electronic states. This in turn alters the available decay channels at high excitations. The most severe example of this effect is visible in the lowest-lying electronic quartet state, a$^4\Sigma_u^+$. Here, a newly discovered autodetachment mechanism, rotationally assisted autodetachment, explains the feature measured at different ion storage facilities throughout the world over the last three decades.

[1] V. C. Schmidt et al., Phys. Rev. Lett. 133, 183001, (2024)
[2] V. C. Schmidt et al., Phys. Rev. A 110, 042828, (2024)