Hi everyone, I was wondering if I can get a little help interpreting absorption cross sections. I would like to figure out how long I would have to shine a particular light on a monolayer of molecules to expect an appreciable fraction of isomerization. Here is how far I've gotten so far: Looking at absorption curves for the isomers of stilbene, I know that from 200-350 nm I'd expect an molar extinction coefficient of 1.6 l mol-1 cm-1 (summed over wavelengths) which should yield a cross section of 6.1x10-21 cm-2 using this: http://en.wikipedia.org/wiki/Molar_absorptivity So I think I can calculate a reasonable incoming photon flux from a Xe lamp: 10 mW m-2 nm-1 x 150 nm (200-350 nm range of light) x (1m/10^9 nm)^2 x 1mJ/1mW x J/100mJ = 1.5x10-20 J/nm^2 I can then figure out the number of 250 nm photons/nm2 s: 1.5x10^-20 J/nm^2 s x 1 photon/7.9x10^-19 J = .17 photons/nm^2 s I can estimate the area of cis stilbene as 0.9 nm x 0.6 nm = 0.5 nm^2 which gives: 0.08 photons/stilbene s So since I know how many photons are hitting stilbene per second, how can I take the cross section to estimate how long I'd have to wait for an absorption event? (Or even the fraction of photons absorb) I know that stilbene has a 50/50 shot of isomerizing once optically excited. Thanks for your help!