Temperature and abundance profiles of CO and N₂ at the substellar point of WASP-76 b. Chemical equilibrium models with a cool, isothermal upper atmosphere (dotted) show no molecular dissociation, while those with a hot thermosphere (dashed) display thermal dissociation at temperatures above ~3000 K. Taking into account photochemistry (full) shifts the atomic-molecular transition to higher pressures. The horizontal grey line denotes the region where we adopted a parametrized upper atmosphere.