4. Hello Jablonski Diagram
Types of electronic transitions
We will use the diatomic for our illustration:
S0 is the g.s. which is a singlet state, S1 and T1 are the singlet and triplet e.s., respectively
T1 < S1 from Hund’s rules
Eq bond lengths, Re: Re (S0) < Re (S1) < Re (T1)
Hello Jablonski Diagram
Types of Decay – what goes up must come down
Radiative: occur through absorption or emission of radiation (solid lines in diagram 1, 2, & 5)
Nonradiative: still through absorption or emission of radiation (wavy & dashed lines in diagram 3, 4, & 6)

5. Hello Jablonski Diagram
Types of electronic transitions
Absorption Energy (Line 1)
Isolated systems can only undergo radiative decay back down to g.s.
When collisions can occur – energy is transferred through vibrational relations – wavy lines
A molecule will quickly relax back to the lowest vibronic state within the electronic wavepacket as shown in the diagram
Hello Jablonski Diagram

6. Hello Jablonski Diagram
Types of electronic transitions
Ways to get back to S0:
Photon emission, Line 2
Occurs between 2 states with same multiplicity
AKA fluorescence
Nonradiative decay, Line 3
AKA internal conversion
Intersystem Crossing, Line 4
nonradiative decay between two excited states with different multiplicities (S1  T1)
Requires a spin change so it is a much slower process
The systems can then relax back to lowest vibronic state through nonradiative decay.
Hello Jablonski Diagram

7. Hello Jablonski Diagram
Types of electronic transitions
Ways to go from T1  S0
Photon Emission, Line 5
Occurs between 2 states with different multiplicities
AKA phosphorescence
Much slower than fluorescence again due to spin flip
Nonradiative decay, Line 6
As there is a multiplicity change this is another example of intersystem crossing
Hello Jablonski Diagram

8. Transition Time Scale

9. Translation to Spectra
Relation of these transitions to our spectra
Let Re(S0) = Re (S1) such that the Morse potentials are aligned
We also assume that relaxation to the lowest vibronic state (10-19s) occurs before it can fluoresce (10-9s) to ground state
Translation to Spectra
Absorbance v”  v’ transitions require more energy
Fluorescence v’  v” transitions
Spacing between lines:
For absorbance: depends on vibronic spacing in e.s.
For fluorescence: depends on vibronic spacing in g.s.
If the vibronic state gaps in both e.s. and g.s. are the same then the two branches would look like a mirror image

12. Translation to Rate Eqns
Working toward those rate equations
Consider a system of Natoms which has one g.s. and one e.s.
Occupation is T dependent
Energy of light descriptors:
Or proportional to kBT (kB = Boltzmann constant)
If E2 – E1 >> kBT, the atoms do not possess enough energy to occupy the e.s. so they stay in the g.s., so NTOTAL = Natoms = N1
When h12  E2 – E1 then some atoms will have enough energy to be promoted to e.s.
Translation to Rate Eqns
Radiant energy density,  = radiant energy/unit volume (J/m3)
Spectral radiant energy density  = d/d (Js/m3)
We are most interested in (12)

13. Translation to Rate Eqns
Einstein proposed …
In words: the rate of excitation from g.s. to e.s. is proportional (12) to and to N1(t) the number of atoms in the g.s. at time t
The (-) occurs because N1(t) decreases with time
For Absorption:
Translation to Rate Eqns

14. Translation to Rate Eqns
Einstein proposed …
Spontaneous Emission: atoms emit a photon energy, h12 = E2 – E1
Translation to Rate Eqns

15. Translation to Rate Eqns
Einstein proposed …
Stimulated Emission: exposure of the system to another photon of energy, h12, will stimulate the atom to return the g.s.
Translation to Rate Eqns
NOTE: this type of emission amplifies light intensity since one atom is stimulated to emit another and thereby generates another photon, h12

16. Translation to Rate Eqns
Putting it all together:
Exposure of our sample to light will generate all three processes (absorption, spontaneous and stimulated emission)
Hence our equation is really:
Translation to Rate Eqns

17. Translation to Rate Eqns
Relating our coefficients
Translation to Rate Eqns

18. Translation to Rate Eqns
Now we add in T
Translation to Rate Eqns

20. How they work
Lasers

21. The 2-Level System
Lasers

23. Getting a Population Inversion
3-Level System
Getting a Population Inversion

24. Getting a Population Inversion
The Key is State 2
Getting a Population Inversion

25. Getting a Population Inversion
Putting it all together
Getting a Population Inversion

27. Laser Applications They are:
Resolution: how well we can distinguish between absorption/emission peaks
Spectral resolution, in particular is the limit of the spectrometer
In a lamp-based instrument this is around 0.03 cm-1
The monochromatic light generated by a laser in the range of visible has a spectral width of 3.5 x 10 cm-1
Photochemical Dynamics
A photochemical process is initiated by light absorption
Some of these process are given below (see p. 615 in text for other pretty examples):
Photodissociation
Photoisomerization
Photodimerization
Laser Applications
Finding Quantum Yield
Often used to judge the success of the reaction and given by 

29. Controlling Decay How do we do so?
The ability of a molecule to fluoresce or phosphoresce is dependent on lifetimes
Additionally it is relative to their competitors of intersystem crossing & internal conversion
Controlling Decay
We can decrease/increase these processes using reagents or through environmental controls
To facilitate phosphorescence we can add a paramagnetic heavy atom to introduce O2
Additionally, increasing one will decrease the other
Lowering T is another way to get more phosphorescence
Solvent is yet another parameter