ECAMP 15

Photodetachment spectroscopy is a powerful spectroscopic technique for determining the internal state distribution of a molecular anion. Previously, our group studied the threshold photodetachment spectroscopy of CN$^-$ at both 16 K and 295 K in a 22-pole ion trap and measured the electron affinity of CN with great precision (EA: 3.864(2) eV) [1]. Here, we present results from our recent study of threshold photodetachment spectroscopy of C$_2^-$ in a radiofrequency 16-pole ion trap at two different trap temperatures, 8 K and 150 K. We investigated the behavior of the cross section near the threshold for the ground-state transition, C$_2\ X^1\Sigma_g^+ \leftarrow$ C$_2^-\ X^2\Sigma_g^+$, and the excited state transition C$_2\ a^3\Pi_u \leftarrow$ C$_2^-\ X^2\Sigma_g^+$, which are expected to show p-wave detachment and s-wave detachment behavior, respectively. By fitting the near-threshold behavior, we obtain precise values for the electron affinity of C$_2$.
We also present our results from infrared predissociation (IRPD) spectroscopy of C$_2$H$^-$ tagged with H$_2$ and D$_2$, measured in the 350–3000 cm$^{-1}$ range using the FELIX free-electron laser facility. Our experimental results, together with theoretical calculations, confirm the fundamental CCH bending, CC stretching, and the overtone of the CCH bending mode of the C$_2$H$^-$ ion and D$_2$ stretching mode associated with the C$_2$H$^-$–D$_2$ complex. Additionally, two spectral features observed in the 700–900 cm$^{-1}$ region are most likely due to intermolecular interactions with the H$_2$ tag.

[1]. M. Simpson et al., J. Chem. Phys. 153, 184309 (2020).