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How did ultraviolet radiation produce oxygen from water vapour?

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abundance of oxygen — evolution of the atmosphere A “best guess” reconstruction of the abundance of O₂ in Earth's atmosphere as a function of time. The O₂ abundance axis is logarithmic.

In the early stages of Earth's history, free oxygen was produced through a process known as photodissociation, where ultraviolet radiation from the Sun provided the energy required to break apart water vapour molecules in the atmosphere.12

The chemical reaction for this process is represented as:
$2 H_{2} O + h ν = O_{2} + 2 H_{2}$
In this equation, $h ν$ represents a photon of ultraviolet light.1

Once the water molecules were cleaved, the resulting hydrogen ( $H_{2}$ ) was light enough to escape Earth's gravitational field into space. Because the hydrogen escaped, the products of the reaction were prevented from recombining, allowing the residual oxygen to remain in the atmosphere. This oxygen subsequently reacted with reduced gases—such as hydrogen sulfide—present in the early atmosphere. While this photochemical production provided an initial source of oxygen, it was not until the later emergence of photosynthetic organisms that oxygen could accumulate in the atmosphere at a rate sufficient to create the modern oxygenated environment.3214