Why/how do molecules absorb light?
PSet 1 - due in class 9/25
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9/16/13
1

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F
B
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λ T
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σ   = 2/15 π 5 k 4 c
= 5.671 x 10 -8
-2 h -3
W m -2 K -4
S&P
© John Wiley and Sons. All rights reserved. This content is excluded from our Crea
Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/ .
tive
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= hc
9/16/13
© Cengage Learning. All rights reserved. This content is excluded from our Creative
Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/ .
Zumdahl, Chemistry , 8 th ed.
2

A + h v → A*
Excited state: vibratonal, rotatonal, electronic
Molecule can absorb radiaton efciently if:
1)   The photon energy matches the energy spacing
between molecule's quantum levels
2)   Optcal transiton between these quantum levels is allowed by "selecton rules" ("forbidden" transitons are weaker)
3)   Electronic: overlap between wavefunctons is high
Image courtesy of Mark Somoza on Wikimedia Commons. License: CC-BY-SA. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/ .
htp://wiki.verkata.com/en/wiki/Morse potental
[ Note: A dditional material is discussed here during lecture.
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9/16/13
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bands more common
Finlayson-Piis and Piis
© American Chemical Society. All rights reserved. This content is excluded from o
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v
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+v
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band of H
2
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(from S. Nizkorodov, VC/)
Courtesy of Sergey Nizkorodov. Used with permission.
4
J' 3
2
Quantum numbers: Λ, S , Ω
4
J" 3
2 v '=1 v "=0
Excited (upper) electronic state
For more detail see F-P&P, pp. 46-47; or
Herzberg, Molecular spectra and molecular structure , Van Nostrand 1950
4
J' 3
2
4
J" 3
2 v '=1 v "=0
[ Note: A dditional material is discussed here during lecture.
]
Ground (lower) electronic state
9/16/13
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