From mackay@mrao.cam.ac.uk Sun May 17 15:24:46 2009 Date: Sun, 17 May 2009 15:24:45 +0100 To: mi259@cam.ac.uk, rmsp2@cam.ac.uk, ajr82@cam.ac.uk, lw342@cam.ac.uk, ma415@cam.ac.uk, rab200@cam.ac.uk, ajc254@cam.ac.uk, sms61@cam.ac.uk, ctst2@cam.ac.uk Cc: mackay@mrao.cam.ac.uk Subject: Re: thermal and statistical supervisions From: "David J.C. MacKay" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=us-ascii Status: RO Dear All, I looked up some facts about oxygen, because I was feeling confused about that A-question about oxygen's active and inactive modes, and how that relates to the idea that CO2 is a greenhouse gas and thus able to absorb infrared radiation. [The confusion being: if oxygen is NOT vibrationally active, how can CO2 absorb IR from a room-temperature object? Surely O2 vibrations and CO2 vibrations have similar frequencies?] This last assertion is correct: O-O stretch corresponds to a wavelength of 4852 nm. CO2 asymmetric stretch is at 4200 nm. That's a wavelength that is indeed in the infrared but it is not a wavelength found significantly in room-temperature radiation. To get the peak of a black body to 4200nm requires the temperature to be 690K. One difference between O2 and CO2 is that CO2 has got bending modes as well. Bending involves lower frequencies. The bending wavelength is 18,315nm, which is active for any temperature from 158K upwards. Bottom line: yes, oxygen is hardly vibrating at all at room temp; and maybe the way CO2 is cooking the planet is through its bending modes. All the best David -- -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - David J.C. MacKay mackay@mrao.cam.ac.uk http://www.inference.phy.cam.ac.uk/mackay/ Cavendish Laboratory, 19 J J Thomson Ave, Cambridge CB3 0HE. U.K. (01223) 339852 | fax: 337356 | home: 740511 international: +44 1223 NEW book: "Sustainable Energy - without the hot air" - withouthotair.com