FERMI LEVEL (E_F)
The chemical potential of electrons in a solid (metals, semiconductors or insulators) or in an electrolyte solution.

See bandgap energy, conduction band, valence band.

FILTER (optical)
A device which reduces the spectral range (bandpass, cut-off, and interference filter) or radiant power of incident radiation (neutral density or attenuance filter) upon transmission of radiation.

FLASH PHOTOLYSIS
A technique of transient spectroscopy and transient kinetic studies in which a light pulse is used to produce transient species. Commonly, an intense pulse of short duration is used to produce a sufficient concentration of a transient species suitable for spectroscopic observation.

FLUENCE (H₀)
When applied to energy, it is the total radiant energy traversing a small transparent imaginary spherical target containing the point under consideration, divided by the cross section of this target. The product of the fluence rate and the duration of the irradiation , simplified expression: H₀ = E₀ t when the fluence rate is constant over the time considered). The SI unit is J m^-2. Energy fluence is identical to spherical radiant exposure and reduces to radiant exposure (H) for a parallel and normally incident beam, not scattered or reflected by the target or its surroundings.

See also dose, photon fluence.

FLUENCE RATE (E₀)
The rate of fluence, H₀. Four times the ratio of the radiant power, P, incident on a small transparent imaginary spherical volume element containing the point under consideration, divided by the surface area of that sphere, S_K simplified expression: E₀ = 4 P/S_K when the radiant power is constant over the solid angle considered). For energy fluence rate the SI unit is Wm ^-2. It reduces to irradiance, E, for a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings.

See intensity, radiance.

See also photon fluence rate.

FLUORESCENCE
Spontaneous emission of radiation (luminescence) from an excited molecular entity with the formation of a molecular entity of the same spin multiplicity .

FLUX (energy flux)
See radiant energy flux, radiant power.

f NUMBER
See oscillator strength.

FÖRSTER EXCITATION TRANSFER (Dipole-Dipole Excitation Transfer)
A mechanism of excitation transfer which can occur between molecular entities separated by distances considerably exceeding the sum of their van der Waals radii. It is described in terms of an interaction between the transition dipole moments, (a dipolar mechanism). The transfer rate constant is given by

where K is an orientation factor, n the refractive index of the medium, ₀ the radiative lifetime of the donor, r the distance (cm) between donor (D) and acceptor (A), and J the spectral overlap (in coherent units cm⁶ mol^-1) between the absorption spectrum of the acceptor and the fluorescence spectrum of the donor. The critical quenching radius, r₀, is that distance at which is equal to the inverse of the radiative lifetime.

See also Dexter excitation transfer, energy transfer, radiative energy transfer.

FÖRSTER CYCLE
Indirect method of determination of excited state equilibria, such as pK*^,a values, based on ground state thermodynamics and electronic transition energies. This cycle considers only the difference in molar enthalpy change (H ) of reaction of ground and excited states, neglecting the difference in molar entropy change of reaction of those states (S).

FOURIER TRANSFORM SPECTROMETER
A scanning interferometer, containing no principal dispersive element, which first splits a beam into two or more components, then recombines these with a phase difference. The spectrum is obtained by a Fourier transformation of the output of the interferometer.

FRANCK-CONDON PRINCIPLE
Classically, the Franck-Condon principle is the approximation that an electronic transition is most likely to occur without changes in the positions of the nuclei in the molecular entity and its environment. The resulting state is called a Franck-Condon state, and the transition involved, a vertical transition.
The quantum mechanical formulation of this principle is that the intensity of a vibronic transition is proportional to the square of the overlap integral between the vibrational wavefunctions of the two states that are involved in the transition.

FRANCK-CONDON STATE
See Franck-Condon principle.

FREE ELECTRON LASER
Source of coherent radiation in which the active medium is an electron beam moving at speeds close to the speed of light in the spatially periodic magnetic field produced by an array of magnets (the wiggler). The emitted wavelength, _L, is approximately given by /(4E² ), with being the wiggler period and E the kinetic energy of the electrons in MeV.

See laser.

FREE-RUNNING LASER
It applies to a pulsed laser and means that the laser emission lasts as long as the pumping process is sufficient to sustain lasing conditions. Typical pulse durations are in the s-ms range, depending on the pumping source. When the operation mode of a pulsed laser is not specified as Q-switched, mode-locked, or anything else, it must be considered as free-running.

FREQUENCY ( or )
The number of waveperiods per unit time. The linear frequency, n, is the number of cycles per unit time. The SI unit is
Hz s^-1. For the angular frequency, the symbol (= 2) is used, with rad s^-1 as the SI unit.

FREQUENCY DOUBLING
See harmonic frequency generation, nonlinear optical effects.

FWHM (Full Width at Half Maximum)
See half-(band)width.