Of all the transitions that are allowed, how do I figure out intensities?
good Franck-Condon overlap:
more likely
poor Franck-Condon overlap:
less likely
Spectrum observed is built from
all Franck-Condon permitted
transitions.
Intensity of each possible
transition is proportional to
overlap of vibrational
wavefunctions squared: Sfi2
Relative displacement of potential curves involved in electronic transition can change.
What might cause these surfaces to
have no displacement?
What might cause these
surfaces to be displaced?
Displacement effects the shape of electronic spectrum observed.
255nm
Normalized Absorbance
1.0
249nm
0.8
261nm
0.6
0.4
0.2
268nm
0.0
220
230
240
250
260
270
280
290
Wavelength/nm
Notice unit on x-axis and its direction
300
What is Q for the electronic spectrum
of Benzene shown on the last slide?
Some nuclear coordinate that changes
substantially between ground and
excited state…
How can you tell this looking at
spectrum?
Set of p-molecular orbitals derived from LCAO of pure p-orbitals on each
carbon coming out of plane of ring (Hückel Theory).
Configurations:
Yground: p12 p22 p32
Yexcited:
p12 p22 p31 p41 or
p12 p22 p32 p51 or
p12 p21 p32 p41 or
p12 p21 p32 p51
So again, what could be the origin of
displaced potential energy surfaces and
what is Q?
Surfaces displaced even further
Remember that higher vibrational
wavefunctions spend most of their time (higher
probability) at the classical turning points.
Are transitions allowed above the dissociation limit?
Have we seen this type of picture?
What kind of surface is this excited
state?
An interesting case
where spectroscopy
reveals an unbound
state.
One important question: where does the transition come from?
What is the role of temperature?
Remember Boltzman statistics based on our vibrational spacing in
the ground state will determine what vibrational states are populated
and therefore what transitions are allowed and seen.
Rates of non-radiative
relaxation in the excitedstate are frequently
higher than the rates of
radiative (emissive
relaxation).
What does this mean,
i.e. where does emission
come from?
Emissive transitions, like
absorptive ones, are
vertical. This leads to
Stokes shift.
Observation of the Stokes shift.
1.0
Emission
0.8
0.8
0.6
0.6
0.4
0.4
x
0.2
0.2
*
0.0
0.0
250
300
Wavelength/nm
350
Normalized Emission
Absorption
Normalized Absorbance
1.0
Origin of Fluorescence versus Phosphorescence
S0 = ground state singlet
S1 = first singlet excited state
S2 = second singlet excited state
T1 = first triplet excited state