Ultraviolet Measurements of Interstellar C2

TL;DR

This study used Hubble Space Telescope spectra to analyze interstellar C2, revealing perturbations in certain electronic transitions caused by complex interactions, and provided new measurements for multiple sight lines.

Contribution

First detailed analysis of interstellar C2 electronic transitions highlighting perturbations and providing new observational data for 10 sight lines.

Findings

Broadened F-X band lines due to perturbation.

Anomalies in line strength and position for F-X (0,0) band.

Reliable data from F-X (1,0) band for interstellar C2 studies.

Abstract

We analyzed archival spectra acquired with the Hubble Space Telescope for a study of interstellar C2. Absorption from the electronic transitions, D ^1Sigma^+_u -- X ^1Sigma^+_g (0,0) as well as F ^1Pi_u -- X ^1Sigma^+_g (0,0) and (1,0), was the focus of the study. Our profile syntheses revealed that the lines of the F-X bands were broadened as a result of a perturbation involving the upper levels. Further evidence for the perturbation came from anomalies in line strength and position for the F-X (0,0) band. The perturbation likely arises from a combination of triplet-singlet interactions involving spin-orbit mixing between ^3Pi_u states and F ^1Pi_u and an avoided crossing between the ^3Pi_u states. Tunneling through a potential barrier caused by the 3 and 4 ^1Pi_u states and spin-orbit mixing with other close-lying triplet states of ungerade symmetry are less likely. Except for the broadening, lines in the F-X (1,0) band appear free from anomalies and can be used to study interstellar C2; new results for 10 sight lines are presented.