29 8
Reference: JPL Publication 94-26
           Chemical Kinetics and Photochemical Data for Use in Stratospheric
           Modeling
           Evaluation Number 11
           December 15, 1994
           page 117

H2O + hv -> H + OH

"Water vapor has a continuum absorption spectrum at wavelengths longer than 145
nm, with a maximum around 165 nm, the cross sections falling off rapidly toward
longer wavelengths; the photodissociation threshold occurs at 246 nm.  Below 69
nm the spectrum is also a continuum, and between 69 and 145 nm it consists of
diffuse bands.  In the atmosphere water vapor is photodissociated mainly by the
solar Lyman alpha line (121.6 nm).

The absorption cross sections and the photochemistry of water vapor have been
reviewed, for example, by Hudson [716, 717], by Hudson and Kiefer [718]; by
Calvert and Pitts [241]; and by Okabe [1204].

The recommended absorption cross sections are taken from the review by Hudson
and Kiefer [718], and are listed in Table 10 between 175 and 190 nm.  At these
wavelengths the quantum yield for production of H and OH is unity.  At shorter
wavelengths H2 and O are also formed as primary products; Stief et al. [1498]
report a quantum yield of 0.11 for this process between 105 and 145 nm."

Wavelength (nm)    Cross section (1E-20 cm^2)
175.5			262.8
177.5			185.4
180.0			78.1
182.5			23.0
185.0			5.5
186.0			3.1
187.5			1.6
189.3			0.7