Electronic Spectroscopy
Electronic spectroscopy measures transitions of electrons between molecular orbitals, mostly in the ultraviolet and visible, accounting for colour, photochemistry, and the rich phenomena of fluorescence and phosphorescence.
Definition
Electronic spectroscopy is the study of transitions between the electronic energy levels of molecules through absorption or emission of ultraviolet, visible, and related radiation, encompassing absorption spectra, luminescence, and photoelectron spectra.
Scope
This topic covers transitions between electronic states of molecules: the ultraviolet-visible absorption that arises when electrons are promoted between orbitals, the Franck-Condon principle governing accompanying vibrational structure, and the Beer-Lambert law relating absorbance to concentration. It includes chromophores and conjugation, the singlet and triplet states underlying fluorescence, phosphorescence, and intersystem crossing as summarized in the Jablonski diagram, and photoelectron spectroscopy. Time-resolved and laser methods that follow excited-state dynamics are treated in a sibling topic.
Core questions
- How do electronic transitions between molecular orbitals produce ultraviolet-visible spectra?
- How does the Franck-Condon principle explain the vibrational structure of electronic bands?
- How does the Beer-Lambert law relate absorbance to concentration?
- How do fluorescence, phosphorescence, and intersystem crossing arise from excited states?
Key concepts
- Electronic transitions and chromophores
- Franck-Condon principle
- Beer-Lambert law
- Singlet and triplet excited states
- Fluorescence, phosphorescence, and the Jablonski diagram
Key theories
- Franck-Condon principle
- Electronic transitions are so fast that the nuclei are effectively stationary during them, so the most intense vibronic bands are those whose vibrational wavefunctions overlap best between the ground and excited electronic states.
- Excited-state decay pathways
- An excited molecule can relax by emitting a photon as fluorescence from a singlet state or phosphorescence from a triplet state, or non-radiatively through internal conversion and intersystem crossing, as organized in the Jablonski diagram.
Clinical relevance
Electronic spectroscopy underlies ultraviolet-visible quantitative analysis through the Beer-Lambert law, the design of dyes, pigments, and photovoltaic and light-emitting materials, fluorescence-based assays and microscopy in the life sciences, and the understanding of vision and photochemistry.
History
The Franck-Condon principle, formulated by Franck in 1926 and given quantum form by Condon, explained the intensity patterns of electronic band spectra; Jablonski's 1930s diagram of excited-state processes systematized luminescence, and modern photoelectron and laser methods extended the field to ionization and ultrafast dynamics.
Key figures
- James Franck
- Edward Condon
- Aleksander Jablonski
Related topics
Seminal works
- atkins2018
- hollas2004
Frequently asked questions
- Why are some substances coloured?
- A substance appears coloured when its electronic transitions absorb light in the visible range, often because of extended conjugation or metal d-orbital transitions that lower the energy gap; the observed colour is complementary to the wavelengths absorbed.
- What is the difference between fluorescence and phosphorescence?
- Fluorescence is rapid emission from an excited singlet state with the same spin as the ground state, whereas phosphorescence is slower emission from a triplet state reached by intersystem crossing; the spin change makes phosphorescence forbidden and therefore long-lived.