UV radiation and water

[the guy]

does UV radiation have the ability to break up water molecules? if so please specify the wavelength

[swansont]

Electrolysis takes about 1.25 eV

 

wavelength = hc/E

[the guy]

ok, what is hc and what is E?

[swansont]

h is Planck's constant, c is the speed of light and E is energy

 

hc=1240 eV nm

 

So that means the wavelength needs to be 1000 nm or shorter, which covers all the visible spectrum and gets you into the near-IR. This doesn't seem right — the electrolysis energy includes breaking off the Oxygen but also forming an O2 molecule, and happens in the presence of other molecules.

 

This calculation

http://answers.yahoo.com/question/index?qid=20081107045616AAfd4jT

 

says that you need about 5.25 eV to split off a hydrogen, which happens at 237 nm or shorter

[John Cuthber]

Water is transparent to UV light sown to at least 190nM so that calculation is wrong.

[swansont]

Water is transparent to UV light sown to at least 190nM so that calculation is wrong.

 

Vapor or liquid? AFAIK the liquid spectrum shifts the resonances to longer wavelengths

[John Cuthber]

I found some data. In the gas phase the absorbtion maximum is at about 167nM

I can't find any data for the liquid but it's certainly below the UV cutoff of most instruments because air absorbs from about 190 nM.

[the guy]

so anything under 190nm will do?

[John Cuthber]

In principle yes. the yeild will get better as you go towards 167nm.

Have fun trying to set this up; air is opaque to those wavelengths.

[the guy]

why is air opaque to those wavelengths?