Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about interpreting absorption cross sections

  1. Jun 18, 2011 #1
    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!
     
  2. jcsd
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted



Similar Discussions: Question about interpreting absorption cross sections
Loading...