The electronic transitions of the water molecule lie in the vacuum ultraviolet region. For water vapor the bands have been assigned as follows.
*65 nm band — many different electronic transitions, photoionization, photodissociation
*discrete features between 115 and 180 nm
**set of narrow bands between 115 and 125 nm (n) → many different Rydberg states and 3a (n) → 3sa Rydberg state
**128 nm band (n) → 3sa Rydberg state and 1b (n) → 3sa Rydberg state
**166.5 nm band (n) → 4a (σ*-like orbital)
:At least some of these transitions result in photodissociation of water into H+OH. Among them the best known is that at 166.5 nm.