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Molecular abundances are sensitive to UV-photon flux and cosmic-rayionization rate. In starburst environments, the effects of high-energy photonsand particles are expected to be stronger. We examine these astrochemicalsignatures through multiple transitions of HCO$^+$ and its metastable isomerHOC$^+$ in the center of the starburst galaxy NGC 253 using data from the ALMAlarge program ALCHEMI. The distribution of the HOC$^+$(1-0) integratedintensity shows its association with "superbubbles", cavities created either bysupernovae or expanding HII regions. The observed HCO$^+$/HOC$^+$ abundanceratios are $\sim 10-150$, and the fractional abundance of HOC$^+$ relative toH$_2$ is $\sim 1.5\times 10^{-11} - 6\times 10^{-10}$, which implies that theHOC$^+$ abundance in the center of NGC 253 is significantly higher than inquiescent spiral-arm dark clouds in the Galaxy and the Galactic center clouds.Comparison with chemical models implies either an interstellar radiation fieldof $G_0\gtrsim 10^3$ if the maximum visual extinction is $\gtrsim 5$, or acosmic-ray ionization rate of $\zeta \gtrsim 10^{-14}$ s$^{-1}$ (3-4 orders ofmagnitude higher than that within clouds in the Galactic spiral-arms) toreproduce the observed results. From the difference in formation routes ofHOC$^+$, we propose that a low-excitation line of HOC$^+$ traces cosmic-raydominated regions, while high-excitation lines trace photodissociation regions.Our results suggest that the interstellar medium in the center of NGC 253 issignificantly affected by energy input from UV-photons and cosmic rays, sourcesof energy feedback.

Starburst Energy Feedback Seen Through HCO$^+$/HOC$^+$ Emission in NGC 253 from ALCHEMI