24 MolecularSpectra

05 700k 1300k 2200k 004 003 002 001 0 4900 4950 5000

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Unformatted text preview: -1 avk(cm-1 ) , cm 0.05 700K 1300K 2200K 0.04 0.03 0.02 0.01 0 4900 4950 5000 High T 5050 Frequency, cm-1 5100 5150 5200 Low T !"#$%$ Molecular Spectra -7 Copyright © 2009 by Jerry M. Seitzman. All rights reserved. Broad regions, doesn't go to zero. S o many molecules as one goes d own another goes up Small Segment of o ther part Why does it go up at 10 a tm? Raise P, number d ensity goes up. It's also g etting a lot broader. Collision broadening. School of Aerospace Engineering Example: H2O High Pressure • Lines can “merge,” especially at higher pressure 1 0 times more number d ensity but twice the a bsorption coefficient. Why? Because as you b roaden it out, the peak g oes down. Integral under the curve is what actually g oes up by a factor of 10. 0.007 Higher pressures and temperatures, where lines start merging in a lot 0.006 av k , cm-1 (cm-1) 0.005 1 atm 10 atm 0.004 0.003 0.002 0.001 0 5000 Molecular Spectra -8 Copyright © 2009 by Jerry M. Seitzman. All rights reserved. 5005 5010 5015 Frequency, cm-1 5020 !"#$%$ 4 Why does it get smoother a t 1 bar? Collision b roadening, merged subtle features (for rotation) and merged them together 300K, 0.01bar School of Aerospace Engineering Example: CO2 300K, 1bar 1000K, 1bar from M. Modest, Radiative Heat Transfer (2003) !"#$%$ Molecular Spectra -9 Copyright © 2009 by Jerry M. Seitzman. All rights reserved. At 1000 K, collision freq goes like number d ensity and number d ensity goes down. The 'new' stuff is means we're now p opulating areas that were not populated b efore. School of Aerospace Engineering Band Structure • Spectral lines from diatomics/polyatomics group in bands with selection “rules” limiting “possible” transitions – pure rotation transitions ( J=1) – rotational lines within a specific vibrational band ( v=1:fundamental; J= 1: R, P branches, R(J)> P(J); in some cases J=0, Q branch) – ro-vibronic lines within electronic bands ( v=?; J=0, 1,Q, R, P branches) Molecula...
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