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

Intensity of each possible transition is proportional to overlap of vibrational wavefunctions squared: Sfi

2

Spectrum observed is built from all Franck-Condon permitted transitions.

What might cause these surfaces to have no displacement?

What might cause these surfaces to be displaced?

Relative displacement of potential curves involved in electronic transition can change.

Displacement effects the shape of electronic spectrum observed.

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Notice unit on x-axis and its direction

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 -molecular orbitals derived from LCAO of pure p-orbitals on each carbon coming out of plane of ring (Hückel Theory).

Configurations:

ground: 12 2

2 32

excited: 1

2 22 3

1 41 or

12 2

2 32 5

1 or

12 2

1 32 4

1 or 1

2 21 3

2 51

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 excited-state 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.

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Absorption

Emission

Observation of the Stokes shift.

Origin of Fluorescence versus Phosphorescence

S0 = ground state singletS1 = first singlet excited stateS2 = second singlet excited stateT1 = first triplet excited state