We report the discovery of the first spectroscopically resolved C II /C II * 1334 , 1335 doublet in the Lyman-break galaxy J0215–0555 at z _ { \text { Ly } \alpha } = 5.754 .
The separation of the resonant and fluorescent emission channels was possible thanks to the large redshift of the source and long integration time , as well as the small velocity width of the feature , 0.6 \pm 0.2 Å .
We model this emission and find that at least two components are required to reproduce the combination of morphologies of C II * emission , C II absorption and emission , and Lyman- \alpha emission from the object .
We suggest that the close alignment between the fluorescence and Lyman- \alpha emission could indicate an ionisation escape channel within the object .
While the faintness of such a C II /C II * doublet makes it prohibitively difficult to pursue for similar systems with current facilities , we suggest it can become a valuable porosity diagnostic in the era of JWST and the upcoming generations of ELTs .