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The Horsehead Nebula is a well-known photodissociation region, or PDR.

They examine loss by mechanisms like Jeans escape, hydrodynamic escape, stellar winds, impact erosion, photoionization and photodissociation.

Theoretically, there could have been some production of oxygen by photodissociation of water vapor in ultraviolet light, but not much.

While better known as the atmospheric trace gas that protects us from much of the solar UV radiation, ozone is also an important source of hydroxyl radicals in the troposphere owing to photodissociation.

‘The predicted frequency shifts and enlargements of the spectral bands reveal that the photodissociation of ozone is strongly favoured by its interaction with water.’