HALF-WIDTH (of a band)
The full width of a spectral band at a height equal to half of the height at the band maximum. Also known as full width at half maximum (FWHM). The dimension of band width should be either inverse length (wavenumbers) or inverse time (frequencies) so that the values give an indication of the energies. Note the hyphen in half-width. Half bandwidth has the meaning of half-width at half maximum.

HAMMOND-HERKSTROETER PLOT
See energy transfer plot.

HARMONIC FREQUENCY GENERATION
Production of coherent radiation of frequency k (k = 2, 3,..) from coherent radiation of frequency . In general, this effect is obtained through the interaction of laser light with a suitable optical medium with nonlinear polarizability. The case k=2 is referred to as frequency doubling, k = 3 is frequency tripling, k = 4 is frequency quadrupling. Even higher integer values of k are possible.

HARPOON MECHANISM
Reaction sequence (thermal or photoinduced) between neutral molecular or atomic entities in which long-range electron transfer is followed by a considerable reduction of the distance between donor and acceptor sites as a result of the electrostatic attraction in the ion pair created.

HEAVY ATOM EFFECT
Enhancement of the rate of a spin-forbidden process by the presence of an atom of high atomic number, which is either part of, or external to, the excited molecular entity. Mechanistically, it corresponds to a spin-orbit coupling enhancement produced by a heavy atom.

HELIUM-CADMIUM LASER
A CW laser emitting mainly at 325.0 and 441.6 nm from singly ionized cadmium.

See gas lasers.

HELIUM-NEON LASER
A CW laser emitting mainly at 632.8, 1152.3, and 3391.3 nm from excited neutral Ne atoms.

See gas lasers.

HERKSTROETER PLOT
See energy transfer plot.

HETEROEXCIMER
Same as exciplex.

HIGH-PRESSURE MERCURY LAMP (Arc)
Radiation source containing mercury at a pressure of ca. 8 MPa (ca. 80 bar) or higher which emits lines over a background continuum between about 200 and 1400 nm.

See lamp.

HOLE BURNING
The photobleaching of a feature, normally a narrow range, within an inhomogeneous broader absorption or emission band. The holes are produced by the disappearance of resonantly excited molecules as a result of photophysical or photochemical processes. The resulting spectroscopic technique is site-selection spectroscopy.

HOLE TRANSFER
Charge migration process in which the majority carriers are positively charged.

HOT GROUND STATE REACTION
A hot state reaction of the ground electronic state.

HOT QUARTZ LAMP
A term sometimes used to describe a high-pressure mercury lamp. The use of this term is not recommended.

HOT STATE REACTION
A reaction proceeding from an ensemble of molecular entities possessing a higher average vibrational, rotational or translational energy than they would at thermal equilibrium with the surrounding medium.

HUND RULES
(1) Of the different multiplets resulting from different configurations of electrons in degenerate orbitals of an atom those with greatest multiplicity have the lowest energy (multiplicity rule).
(2) Among multiplets having the same multiplicity, the lowest-energy one is that with the largest total orbital angular momentum (angular momentum rule) (valid if the total orbital angular momentum is a constant of motion).
(3) In configurations containing shells less than half full of electrons, the term having the lowest total angular momentum J lies lowest in energy, whereas in those with shells more than half filled, the term having the largest value of J lies lowest (fine structure rule).
Hund rules apply if the "Russell-Saunders" coupling scheme is valid. Sometimes the first rule is applied to molecules.

HUSH MODEL
See: Marcus-Hush relationship

HYPERCHROMIC EFFECT
Increase in the intensity of a spectral band due to substituents or interactions with the molecular environment.