Glossary of Terms Used in Photochemistry
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Q- SWITCHED LASER
A laser in which the state of the device introducing important
losses in the resonant cavity and preventing lasing operation
is suddenly switched to a state where the device introduces very low
losses. This increases rapidly the Quality factor of cavity, allowing
the build-up of a short and very intense laser pulse. Typical pulse
durations are in the ns range. The Q-switching may be active (a rotating
mirror or electro-optic device) or passive (a saturable absorber).
See also free-running
laser.
QUANTUM (of radiation)
An elementary particle of electromagnetic energy in the sense of wave-particle
duality.
See photon.
QUANTUM COUNTER
A medium emitting with a quantum yield independent of the excitation
energy over a defined spectral range (e.g., concentrated rhodamine
6G solutions between 300 and 600 nm). Also used for devices producing
an electrical signal proportional to the photon flux absorbed
in a medium.
QUANTUM EFFICIENCY
See efficiency.
For a primary photochemical process, quantum efficiency is
identical to quantum yield.
QUANTUM YIELD ()
The number of defined events which occur per photon
absorbed by the system. The integral quantum yield is
For a photochemical reaction,
The differential quantum yield is
where d[x]/dt is the rate of change of a measurable
quantity, and n the amount of photons (mol or its equivalent
einstein) absorbed per unit time.
can be used for photophysical processes or photochemical
reactions.
See also efficiency.
QUARTET STATE
A state having a total spin quantum number equal to 3/2.
See multiplicity.
QUARTZ-IODINE LAMP
A tungsten filament high-intensity incandescent lamp which contains
iodine a quartz envelope. Used primarily as a source of visible light.
QUENCHER
A molecular entity that deactivates (quenches) an excited state
of another molecular entity. either by energy transfer, electron
transfer, or by a chemical mechanism.
See quenching, Stern-Volmer
kinetic relationships.
QUENCHING
The deactivation of an excited molecular entity intermolecularly
by an external environmental influence (such as a quencher)
or intramolecularly by a substituent through a nonradiative
process. When the external environmental influence (quencher) interferes
with the behavior of the excited state after its formation,
the process is referred to as dynamic quenching. Common mechanisms
include energy transfer, charge transfer, etc. When the environmental
influence inhibits the excited state formation the process is referred
to as static quenching.
See Stern-Volmer
kinetic relationships.
QUENCHING CONSTANT
See quencher, quenching,
Stern-Volmer kinetic
relationships.