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Published byBerniece Long Modified over 5 years ago
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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)
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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
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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
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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
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Transition Time Scale
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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
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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 h12 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 (Js/m3) We are most interested in (12)
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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
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Translation to Rate Eqns
Einstein proposed … Spontaneous Emission: atoms emit a photon energy, h12 = E2 – E1 Translation to Rate Eqns
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Translation to Rate Eqns
Einstein proposed … Stimulated Emission: exposure of the system to another photon of energy, h12, 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, h12
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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
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Translation to Rate Eqns
Relating our coefficients Translation to Rate Eqns
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Translation to Rate Eqns
Now we add in T Translation to Rate Eqns
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How they work Lasers
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The 2-Level System Lasers
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Getting a Population Inversion
3-Level System Getting a Population Inversion
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Getting a Population Inversion
The Key is State 2 Getting a Population Inversion
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Getting a Population Inversion
Putting it all together Getting a Population Inversion
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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
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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
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