Chemistry Reference
In-Depth Information
(a)
′
E
ph
=
hv
ph
=
E
ph
E
ph
=
hv
ph
Rayleigh scattering
(b)
′
′
E
ph
=
hv
ph
E
ph
=
hv
ph
Stokes radiation
′
v
ph
<
v
ph
′
Δ
v
=
v
ph
- v
ph
=
v
vib
Figure 6.9
(a) Illustration of the Franck Condon principle for a vertical excitation and
re-emission at the same frequency; Rayleigh scattering. (b) The origin of Stokes lines in Raman
bands: on relaxation, the emitted photon frequency differs from that absorbed by an amount
dependent on the vibrational energy levels. In each diagram, the lower Morse curve represents
the electronic ground state with vibrational levels included as a ladder of states. The excited
state has a different Morse curve, being higher in energy and shifted to right to indicate longer
bonds in the excited state. An electronic excitation is shown vertically on the diagram giving a
vibrationally excited molecule in the higher electronic state. (Used with permission from the
Journal of Chemical Education (1967) 44:1, page 4; Copyright 1967, Division of Chemical
Education, Inc.)
