# Vibrationally Resolved Inner-Shell Photoexcitation of the Molecular Anion C$_2^-$

## TL;DR

This study investigates the vibrational structure of C$_2^-$ anion during inner-shell photoexcitation using high-resolution synchrotron techniques, revealing molecular contraction and rotational effects on spectral features.

## Contribution

It provides the first detailed vibrational and rotational analysis of core-excited states in C$_2^-$, including spectroscopic parameters and molecular contraction insights.

## Key findings

- Observation of vibrational structure in core-excitation cross section
- Identification of molecular contraction of 0.2 Å upon excitation
- Rotational broadening effects on vibrational spectral features

## Abstract

Carbon $1s$ core-hole excitation of the molecular anion C$_2^-$ has been experimentally studied at high resolution by employing the photon-ion merged-beams technique at a synchrotron light source. The experimental cross section for photo--double-detachment shows a pronounced vibrational structure associated with $1\sigma_u\to3\sigma_g$ and $1\sigma_g \to 1\pi_u$ core excitations of the C$_2^-$ ground level and first excited level, respectively. A detailed Franck-Condon analysis reveals a strong contraction of the C$_2^-$ molecular anion by 0.2~\AA\ upon this core photoexcitation. The associated change of the molecule's moment of inertia leads to a noticeable rotational broadening of the observed vibrational spectral features. This broadening is accounted for in the present analysis which provides the spectroscopic parameters of the C$_2^-$ $1\sigma_u^{-1}\,3\sigma_g^2\;{^2}\Sigma_u^+$ and $1\sigma_g^{-1}\,3\sigma_g^2\;{^2}\Sigma_g^+$ core-excited levels.