Selection rules concerned with changes in angular momentum
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Laporte selection rule for centrosymmetric molecules and atoms
The only allowed transitions are transitions that are accompanied by a change of parity
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Centrosymmetric
Those with a center of inversion
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Vibronic transition
A transition that derives its intensity from an asymmetrical vibration of a molecule
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Franck-Condon principle
Because the nuclei are so much more massive than the electrons, an electronic transition takes place very much faster than the nuclei can respond
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Vertical transition
Used to denote an electronic transition that occurs without a change of nuclear geometry
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Franck-Condon factor
Because the transition intensity is proportional to the square of the magnitude of the transition dipole moment, the intensity of absorption is proportional to
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Chromophores
Groups with characteristic optical absorptions
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Charge-transfer transitions
When the electron moves through a considerable distance, which means that the transition dipole moment may be large and the absorption is correspondingly intense
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Polarized light
Electromagnetic radiation with electric and magnetic fields that oscillate only in certain directions
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Plane polarized light
When the electric and magnetic fields each oscillate in a single plane
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Circular polarization
The electric and magnetic fields rotate around the direction of propagation in either a clockwise or a counter-clockwise sense but remain perpendicular to it and each other
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Radiative decay process
A process in which a molecule discards its excitation energy as a photon
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Nonradiative decay
The excess energy is transferred into the vibration, rotation, and translation of the surrounding molecules
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Fluorescence
Spontaneous emission of radiation occurs within a few nanoseconds after the exciting radiation is extinguished
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Phosphorescence
The spontaneous emission may persist for long periods
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Intersystem crossing
A nonradiative transition between states of different multiplicity, and become a triplet state
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Jablonski diagram
Where the various types of nonradiative and radiative transitions that can occur in molecules are often represented
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Dissociation
The breaking of bonds
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Internal conversion
A radiationless conversion to another state of the same multiplicity
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Metastable excited state
An excited state with a long enough lifetime for it to participate in stimulated emission
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Pumping
Stimulation with an intense flash of light
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Spatial coherence
The waves are in step across the cross-section of the beam emerging from the cavity
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Temporal coherence
The waves remain in step along the beam
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Q-switching
The modification of the resonance characteristics of the laser cavity
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Pockels cell
An electro-optical device based on the ability of some crystals
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Saturable absorber
A solution of a dye that loses its ability to absorb when many of its molecules have been excited by intense radiation
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Multiphoton spectroscopy
States inaccessible by conventional one-photon spectroscopy become observable because the overall transition occurs with no change of parity
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State-to-state reaction dynamics
Where a specific state of a reactant molecule is excited and we monitor not only the rate at which it forms products but also the states in which they are produced
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Photoionization
The ejection of an electron by the absorption of electromagnetic radiation
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Photodissociation
The fragmentation of a molecule following absorption of electromagnetic radiation
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Photoisomerization
The conversion of a species to one of its isomers on absorption of electromagnetic radiation
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Photodeflection
Based on the recoil that occurs when a photon is absorbed by an atom, and the linear momentum of the photon is transferred to the atom
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Time-resolved spectroscopy
Here laser pulses are used to obtain the absorption, emission, or Raman spectrum of reactants, intermediates, products, and even transition states of reactions
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Continuum generation
In which focusing an ultrafast laser pulse on a vessel containing liquid results in an outgoing beam with a wide distribution of frequencies
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Nearfield optical microscopy (NSOM)
A very thin metal-coated optical fiber is used to deliver light to a small area
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Far-field confocal microscopy
A laser light focused by an objective lens is used to illuminate a very dilute sample placed beyond the near field
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Wide-field epifluorescence method
Where a two-dimensional array detector detects fluorescence excited by a laser and scattered back from the sample
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