bereal511 - 25-10-2007 at 11:50
Lately, I've been looking up CO2 infrared absorption charts in the context of global warming (whether or not it exists was not my prime concern, I was
just curious about what was fact and what was fiction about greenhouse gases). I don't have a strong background in physical chemistry, so please bare
with me. The graphs show 100% absorptions at approximately 15 microns, 4.3 microns, and 2.7 microns, if I am looking at them correctly, while about
25% absorption for 2.1 microns.
http://brneurosci.org/spectra.png
Reading up a little on the vibrational modes of CO2, I noticed that only the bending mode and asymmetric stretch mode correspond to any of the
absorption values, 15 microns and 4.3 microns respectively. My question is why does CO2 absorb at 2.7 and 2.1 microns? I could only think of
rotational modes or Rayleigh scattering as possible reasons, but my ignorance in photochemistry leads me to believe there is a simpler reason. Thanks
in advance for any input.
12AX7 - 25-10-2007 at 19:49
Rotational modes are much lower -- microwave region. Scattering is either wideband (e.g., white clouds) or a low pass filter (e.g., transmitting red
and green and scattering the blue that we see as the sky).
How much is absorbed depends jointly on the concentration, distance and interaction. Those peaks which seem to be 100% are actually 99-something, the
remainder depending on how "black" the gas actually is at that wavelength. Weak absorbances (like the ~2 micron lines noted) could be weaker
interactions, impurities, or even -- God forbid -- measurement errors (e.g., media, optics, etc. the light has to travel through between the source
and detector).
Tim