PHOTONS IN CHEMISTRY OUT WHY BOTHER?

E = h ν

λν = c [ m/s]

~ nm Take 500 nm

Boltzman T [ o K]n 2 /n x x x ,0006 x ,0003 x ,4001 % 10, % 20,00024 % 50,00056 %

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

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

Photochemical kinetics

Transmittance Absorbance Beer’s Law

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

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

Absorption of a mixture

Photochemical kinetics STEADY STATE HYPOTESIS

NB 2: The photophysics Is hidden in σ (So we haven’t done much yet)

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

Stimulated emission Spontaneous emission

Stimulated emission Spontaneous emission

Boltzman Planck

Oscillator strength

Lifetimes Einstein coefficients are rate constants

Heisenberg may have been here

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

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

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

Compare s -1 to IR: Nuclear motion is 2 orders of Magnitude slower than Electronic motion Born-Oppenheimer approximation

Orthogonal (no overlap) tells if allowed or forbidden: must be symmetric  selection rules Franck-condon factors

M x is odd:

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

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

Conical Intersections R1 R2 E