Chapter 14 - Molecular Spectroscopy 2: Electronic Transitions (copy)

Selection rules concerned with changes in angular momentum

Laporte selection rule for centrosymmetric molecules and atoms

The only allowed transitions are transitions that are accompanied by a change of parity

Centrosymmetric

Those with a center of inversion

Vibronic transition

A transition that derives its intensity from an asymmetrical vibration of a molecule

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

Vertical transition

Used to denote an electronic transition that occurs without a change of nuclear geometry

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

Chromophores

Groups with characteristic optical absorptions

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

Polarized light

Electromagnetic radiation with electric and magnetic fields that oscillate only in certain directions

Plane polarized light

When the electric and magnetic fields each oscillate in a single plane

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

Radiative decay process

A process in which a molecule discards its excitation energy as a photon

Nonradiative decay

The excess energy is transferred into the vibration, rotation, and translation of the surrounding molecules

Fluorescence

Spontaneous emission of radiation occurs within a few nanoseconds after the exciting radiation is extinguished

Phosphorescence

The spontaneous emission may persist for long periods

Intersystem crossing

A nonradiative transition between states of different multiplicity, and become a triplet state

Jablonski diagram

Where the various types of nonradiative and radiative transitions that can occur in molecules are often represented

Dissociation

The breaking of bonds

Internal conversion

A radiationless conversion to another state of the same multiplicity

Metastable excited state

An excited state with a long enough lifetime for it to participate in stimulated emission

Pumping

Stimulation with an intense flash of light

Spatial coherence

The waves are in step across the cross-section of the beam emerging from the cavity

Temporal coherence

The waves remain in step along the beam

Q-switching

The modification of the resonance characteristics of the laser cavity

Pockels cell

An electro-optical device based on the ability of some crystals

Saturable absorber

A solution of a dye that loses its ability to absorb when many of its molecules have been excited by intense radiation

Multiphoton spectroscopy

States inaccessible by conventional one-photon spectroscopy become observable because the overall transition occurs with no change of parity

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

Photoionization

The ejection of an electron by the absorption of electromagnetic radiation

Photodissociation

The fragmentation of a molecule following absorption of electromagnetic radiation

Photoisomerization

The conversion of a species to one of its isomers on absorption of electromagnetic radiation

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

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

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

Nearfield optical microscopy (NSOM)

A very thin metal-coated optical fiber is used to deliver light to a small area

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

Wide-field epifluorescence method

Where a two-dimensional array detector detects fluorescence excited by a laser and scattered back from the sample