Very Large Telescope observations of interstellar comet 3I/ATLAS III: High-resolution monitoring of CN and forbidden oxygen emission across the perihelion passage with ESPRESSO

3I/ATLAS is only the third known interstellar object to traverse the Solar System. We obtained high-resolution (R 140000) spectroscopy of 3I/ATLAS on 23 nights with VLT/ESPRESSO (r h=1.67-2.45 au). CN production rates were derived by fitting the B 2+-X 2+ band, using CometSpec, a publicly released Python package for flexible fluorescence modeling. The three forbidden [OI] lines were modeled to derive the green-to-red (G/R) flux ratio and a CO 2 /H 2 O proxy. We fit Q CN=a(r h/1 au) b to the production rate versus heliocentric distance relation. The pre-perihelion CN power-law index is b pre=-4.62+1.25-1.22, within the range observed for solar-system comets. The CN residuals about the smooth model show substantial scatter that we attribute to a combination of systematics and possible physical drivers that the present cadence cannot disentangle. The G/R ratio decreases from about 0.42 plus or minus 0.14 at r h 2.4 au to about 0.114 plus or minus 0.004 near perihelion. The asymmetric piecewise oxygen fit is strongly preferred over the symmetric one, consistent with thermal-inertia models, although denser post-perihelion sampling would be needed to confirm this. The CO 2 /H 2 O proxy spans about 0.06-1.06 and is comparable to the values reported by Subaru/HDS and SPHEREx, and to those measured for 2I/Borisov. We additionally identify NiI and FeI, and report post-perihelion production rates for C 2 and CH. Our high-resolution monitoring of 3I/ATLAS reveals a CO 2-rich coma that becomes progressively H 2 O-dominated near perihelion, with a G/R asymmetry consistent with delayed H 2 O sublimation predicted by thermal-inertia models. Together with the public release of CometSpec and the reduced data, these results provide both an empirical reference and a methodological framework for the next generation of interstellar-object monitoring campaigns.