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PHOTONS IN CHEMISTRY OUT WHY BOTHER?

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E = h ν

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λν = c [3 10 8 m/s]

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~ 450-750 nm Take 500 nm

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Boltzman T [ o K]n 2 /n 1 3003 x 10 -42 4007 x 10 -32 10003.4 x 10 -13 2,0006 x 10 -7 5,0003 x 10 -3 6,4001 % 10,0005.7 % 20,00024 % 50,00056 %

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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 QUANTUM YIELD: Φ = The number of molecules of reactant consumed for each quantum of radiation absorbed Primary Φ ≤ 1 Sum of all primary Φ’s = 1

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Photochemical kinetics

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Transmittance Absorbance Beer’s Law

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Molar extinction Coefficient ~250 L.mol -1 cm -1 Cross section ~10 -18 cm 2

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NB 1: Beer fails when photochemistry happens NB 2: The photophysics Is hidden in σ (So we haven’t 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 σ (So we haven’t done much yet)

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EINSTEIN COEFFICIENT # of transitions / second: Amplitude of TRANSITION MOMENT # of molecules degeneracy Radiation density =# of photons/unit freq.

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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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(depends on coordinates of electrons and nuclei And on time) electrons Nuclei

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NB: Resonant frequency NB 1: I = f e 0 and µ becomes permanent dipole NB 2: ν if as beat frequency NB 3:compare to nuclear vibrations

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Compare 10 15 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) tells if allowed or forbidden: must be symmetric selection rules Franck-condon factors

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M x is odd:

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One more parameter………. SPIN αβ updown ↑↓ +½-½

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If we can separate space and spin (no spin-orbit coupling):

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Conical Intersections R1 R2 E

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