PHOTONS IN

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PHOTONS IN CHEMISTRY OUT
WHY BOTHER?
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E h ?
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E h ?
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?? c 3 108 m/s
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Visible light from the sun
450-750 nm
Take 500 nm
electronic state
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Boltzman
T oK n2/n1
300 3 x 10-42
400 7 x 10-32
1000 3.4 x 10-13
2,000 6 x 10-7
5,000 3 x 10-3
6,400 1
10,000 5.7
20,000 24
50,000 56
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Grotthuss-Draper law Only the light absorbed in
a molecule can produce photochemical Change in
the molecule (1871 and 1841)
Stark - Einstein If a species absorbs
radiation, then one particle is excited for each
quantum of radiation absorbed
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Stark - Einstein If a species absorbs
radiation, then one particle is excited for each
quantum of radiation absorbed
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Photochemical kinetics
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Transmittance
Absorbance
Beers Law
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NB 1 Beer fails when
photochemistry happens
NB 2 The photophysics
Is hidden in s
(So we havent done much yet)
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Absorption of a mixture
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Photochemical kinetics
STEADY STATE HYPOTESIS
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NB 2 The photophysics
Is hidden in s
(So we havent done much yet)
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EINSTEIN COEFFICIENT
of transitions / second
Radiation density of photons/unit freq.
of molecules
degeneracy
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Stimulated emission
Spontaneous emission
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Stimulated emission
Spontaneous emission
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Boltzman
Planck
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Oscillator strength
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Lifetimes
Einstein coefficients are rate constants
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Heisenberg may have been here
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Contributions to excited state lifetime

• Natural lifetime
• Pressure broadening
• Saturaiton broadening
• Doppler broadening

NB f(v) in a gas is Gaussian ?Doppler line
shape is Gausian
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electrons
Nuclei
(depends on coordinates of electrons and
nuclei And on time)
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NB Resonant frequency
NB 1 I f ? e0 and µ becomes permanent
dipole NB 2 ?if as beat frequency NB
3 compare to nuclear vibrations
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Compare 1015 s-1 to IR
Nuclear motion is 2 orders of Magnitude slower
than Electronic motion
Born-Oppenheimer approximation
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Orthogonal (no overlap)
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Mx is odd
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One more parameter.
a ß up down ? ? ½ -½
SPIN
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If we can separate space and spin (no spin-orbit
coupling)
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Conical Intersections
E
R2
R1