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Electronic spectroscopy involves transitions between electronic states, typically in the UV-Visible range (200–800 nm). The Franck-Condon principle states that electronic transitions are so fast that nuclear positions and momenta are essentially unchanged: the transition is 'vertical' on the potential energy surface diagram. The intensity of each vibrational component (vibronic band) is proportional to the square of the Franck-Condon factor ⟨v'|v⟩², the overlap integral between vibrational wavefunctions of the two electronic states. Excited states can relax via fluorescence (spin-allowed) or phosphorescence (spin-forbidden), with rates governed by the Einstein A and B coefficients.
Draw potential energy curves for two electronic states with different equilibrium geometries. Identify which v=0→v' transition has maximum intensity from the Franck-Condon overlap, and use this to explain vibrational structure in UV absorption spectra.
Electronic spectroscopy probes transitions between different electronic states of a molecule — typically from the ground state (S₀) to an excited singlet state (S₁ or S₂) — using UV-Visible light. From your study of molecular orbital theory, you know that electrons occupy bonding, nonbonding, and antibonding MOs. Absorbing a photon in the 200–800 nm range promotes an electron to a higher MO, changing the molecule's electronic configuration entirely. The energy of that photon must match the energy gap between the two electronic states, which is why different chromophores absorb at characteristic wavelengths.
The key physics governing which vibrational bands appear in the spectrum is the Franck-Condon principle. Electronic transitions happen on the order of femtoseconds (10⁻¹⁵ s), while nuclear vibrations occur on picosecond timescales — roughly a thousand times slower. Because nuclei are essentially frozen during the transition, the molecule is instantaneously placed on the excited-state potential energy surface at the same nuclear geometry it had in the ground state. On a potential energy diagram, this appears as a vertical arrow. The excited-state vibrational level that gets populated most is the one whose wavefunction has the greatest spatial overlap with the ground-state v=0 wavefunction — this overlap is quantified by the Franck-Condon factor ⟨v'|v⟩². If the equilibrium bond length is longer in the excited state (common when an antibonding MO is populated), the potential energy minimum shifts outward, and vertical transitions land partway up the excited-state well, producing a vibrational progression with maximum intensity at a higher v'.
After absorption, the excited molecule has several decay pathways. It can emit a photon in the reverse process — fluorescence — returning from S₁ to S₀. This is spin-allowed (singlet→singlet) and fast, typically occurring within nanoseconds. Alternatively, the molecule can undergo intersystem crossing to the lowest triplet state T₁, a spin-state change that is slow but becomes relevant when heavy atoms or extended conjugation enhance spin-orbit coupling. Emission from T₁→S₀ is phosphorescence. Because it requires a spin flip (spin-forbidden), it is much slower — persisting for microseconds to seconds — which is why phosphorescent materials glow in the dark long after the light is removed.
A common error is assuming the 0→0 transition is always the most intense peak. This is only true when the ground and excited states have nearly identical geometries. In practice, the equilibrium geometry often changes upon electronic excitation — bond lengths, angles, or even the overall molecular shape shifts — and the Franck-Condon maximum moves to higher v'. Recognizing this lets you read a UV-Vis spectrum to infer geometric changes: a long vibrational progression with maximum intensity far from 0→0 implies a large geometry change upon excitation.