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Lecture 9
Vibrational Spectroscopy
Only photons of one specified wavelength areabsorbed
hchE
Ground state
Excited state
Width
Dopplerc
v
Speed of light 300,000,000 m/s
Velocity of a particle >1000 m/s
collisions Approximately as Doppler
Both effects increase with the temperature and
decrease with the size of the molecule/atom
Multiple energy levels
Width is around 0.01-0.001 nm in visible area
Energy levels of ground state
Energy levels of excited state
Energy sublevels
A primitive rule:
Molecular spectra - wide bands
Atomic spectra - narrow lines
Dissociation energy
Energy level/vibrational quantum number
2 - real (anharmonic)oscilator
1 - ideal (harmonic) oscilator
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
RamanIR
21
21 where,2
1
mm
mmk
Frequency
21
21 where,2
1
mm
mmk
cc
Wavenumber
Carbon Monoxide in gas
cm) (in )(2 21
N
d
Rayleigh scattering
No changes!
Raman scattering
Escaping photon has smaller energy because the molecule did not return to the ground state
Raman scatteringRaman scattering
