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
Relaxation
(picoseconds)
Figure 14.13The 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 14.14The large widths of optical
spectra usually arise from the presence of
unresolved bands.
