304
PART 2
QUANTUM MECHANICS AND SPECTROSCOPY
Wavelength (nm)
Absorbance
spectrum
Absorbance (a.u.)
Absorbance
Fluorescence
Wavelength (nm)
Signal intensity
HOMO
LUMO
3
2
1
0
3
2
1
0
Absorbance
Fluorescence
R e la x a t io n
( p ic o s e c o n d s )
Figure 14.13 The absorbance is from the
lowest vibration level of the highest
occupied molecular orbital (HOMO) to
one of several different vibrational states
of the excited state, which is the lowest
unoccupied molecular orbital (LUMO).
Fluorescence is from the lowest
vibrational level of the LUMO to any
number of different levels of the HOMO.
This results in a red-shifted spectrum
for the fluorescence compared to the
absorption spectrum.
Figure 14.14 The large widths of optical
spectra usually arise from the presence of
unresolved bands.
9781405124362_4_014.qxd 4/29/08 13:09 Page 304
PART 2
QUANTUM MECHANICS AND SPECTROSCOPY
Wavelength (nm)
Absorbance
spectrum
Absorbance (a.u.)
Absorbance
Fluorescence
Wavelength (nm)
Signal intensity
HOMO
LUMO
3
2
1
0
3
2
1
0
Absorbance
Fluorescence
R e la x a t io n
( p ic o s e c o n d s )
Figure 14.13 The absorbance is from the
lowest vibration level of the highest
occupied molecular orbital (HOMO) to
one of several different vibrational states
of the excited state, which is the lowest
unoccupied molecular orbital (LUMO).
Fluorescence is from the lowest
vibrational level of the LUMO to any
number of different levels of the HOMO.
This results in a red-shifted spectrum
for the fluorescence compared to the
absorption spectrum.
Figure 14.14 The large widths of optical
spectra usually arise from the presence of
unresolved bands.
9781405124362_4_014.qxd 4/29/08 13:09 Page 304