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