Vibrational%20Spectroscopy

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Lecture 9
Vibrational Spectroscopy
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Only photons of one specified wavelength
are absorbed
Excited state
Ground state
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Width
Velocity of a particle gt1000 m/s
Doppler
Speed of light 300,000,000 m/s
collisions
Approximately as Doppler
Both effects increase with the temperature and
decrease with the size of the molecule/atom
Width is around 0.01-0.001 nm in visible area
Multiple energy levels
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Energy levels of excited state
Energy sublevels
Energy levels of ground state
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A primitive rule
Molecular spectra - wide bands
Atomic spectra - narrow lines
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1 - ideal (harmonic) oscilator
Dissociation energy
2 - real (anharmonic) oscilator
Energy level/ vibrational quantum number
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IR
Raman
a1 1 CH2 s-str 2783 2782.5 S gas 2781.6 S
a1 2 CO str 1746 1746.1 VS gas 1742.3 W
a1 3 CH2 scis 1500   1500.1 S gas 1499.7 M
b1 4 CH2 a-str 2843 2843.1 VS gas 2866 W
b1 5 CH2 rock 1249   1249.1 S gas
b2 6 CH2 wag 1167   1167.3 S gas
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Frequency
Wavenumber
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Carbon Monoxide in gas
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Rayleigh scattering
No changes!
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Raman scattering
Escaping photon has smaller energy because the
molecule did not return to the ground state
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Raman scattering
Raman scattering
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