Glossary of Terms Used in Photochemistry
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TRANSITION
An electronic transition described approximately as promotion of an
electron from a "bonding"
orbital to an "antibonding"
orbital designated as *.
Such transitions generally involve high transition energies, and appear
close to or mixed with Rydberg transitions.
THERMAL LENSING
A technique that determines the alteration in the refractive index
of a medium as a result of the temperature rise in the path of a laser
beam absorbed by the medium. The lens produced (usually divergent)
causes a change (usually a decrease) in the irradiance measured along
the laser beam axis.
See also photothermal
effects.
THERMALLY
ACTIVATED DELAYED FLUORESCENCE
See delayed fluorescence.
THERMOCHROMISM
A thermally induced transformation of a molecular structure or of
a system (e.g. of a solution), thermally reversible, that produces
a spectral change, typically, but not necessarily, of visible color.
THERMOLUMINESCENCE
Luminescence arising from a reaction between species trapped in a
rigid matrix and released as a result of an increase in temperature.
See luminescence.
THROUGH-BOND
ELECTRON TRANSFER
Intramolecular electron transfer for which the relevant electronic
interaction between the donor and acceptor sites is mediated by through-bond
interaction, i.e. via the covalent bonds interconnecting these sites,
as opposed to through-space interaction.
See also through-space
electron transfer
THROUGH-SPACE
ELECTRON TRANSFER
Electron transfer for which the relevant electronic interaction between
the donor and acceptor sites is mediated either by direct orbital
overlap or by superexchange interaction via intervening molecular
entities not covalently bound to the donor or acceptor sites.
See also through-bond
electron transfer
TICT EMISSION
Electronic emission emanating from a TICT state.
See also TICT state,
twisted internal charge
transfer.
TICT STATE
The acronym derives from Twisted Internal Charge Transfer State, proposed
to be responsible for strongly Stokes-shifted fluorescence from certain
aromatics, particularly in polar medium.
See Twisted
Internal Charge Transfer
TIGHT ION PAIR
See: contact ion pair
TIME-CORRELATED
SINGLE PHOTON COUNTING
A technique for the measurement of the time histogram of a sequence
of photons with respect to a periodic event, e.g. a flash from a repetitive
nanosecond lamp or a CW operated laser (mode-locked laser). The essential
part is a time-to-amplitude-converter (TAC) which transforms the arrival
time between a start and a stop pulse into a voltage. Sometimes called
single photon timing.
TIME-RESOLVED
MICROWAVE CONDUCTIVITY (TRMC)
Technique allowing the quantitative and qualitative detection of radiation-induced
charge separation by time-resolved measurement of the changes in microwave
absorption resulting from the production and decay of charged and
dipolar molecular entities.
TIME-RESOLVED
SPECTROSCOPY
The recording of spectra at a series of time intervals after the excitation
of the system with a light pulse (or other perturbation) of appropriately
short duration.
TRANSIENT SPECTROSCOPY
A technique for the spectroscopic observation of transient species
(excited-state molecular entities or reactive intermediates) generated
by a pulse of short duration.
See also flash photolysis,
time-resolved spectroscopy.
TRANSITION (DIPOLE)
MOMENT (Mnm)
An oscillating electric or magnetic moment can be induced in an atom
or molecular entity by an electromagnetic wave. Its interaction with
the electromagnetic field is resonant if the frequency of the latter
corresponds to the energy difference between the initial and final
states of a transition (DE = hn).
The amplitude of this moment is referred to as the transition moment.
It can be calculated from an integral taken over the product of the
wavefunctions of the initial (m) and final (n) states
of a spectral transition and the appropriate dipole moment operator
of the electromagnetic radiation.
where the summation is over the coordinates of all
charged particles (electrons and nuclei). Its sign is arbitrary, its
direction in the molecular framework defines the direction of transition
polarization, and its square determines the strength of the transition.
If e is omitted one obtains Rnm in the sense used in oscillator
strength. The SI unit of the transition dipole moment is C m. The
common unit is debye (D).
TRANSITION POLARIZATION
The direction of the transition moment in the molecular framework.
TRANSMITTANCE (T)
The ratio of the transmitted spectral radiant power (P
)
to that incident on the sample 
Internal transmittance refers to energy loss by absorption,
whereas the total transmittance is that due to absorption, reflection,
scatter, etc.
See absorbance, attenuance,
Beer-Lambert law.
TRIBOLUMINESCENCE
Luminescence resulting from the rubbing together of the surface of
certain solids. It can be produced, for example, when solids are crushed.
See sonoluminescence.
TRIPLET STATE
A state having a total electron spin quantum number of 1.
See multiplicity.
TRIPLET-TRIPLET
ANNIHILATION
Two atoms or molecular entities both in a triplet state often interact
(usually upon collision) to produce one atom or molecular entity in
an excited singlet state and another in its ground singlet state.
This is often, but not always, followed by delayed fluorescence.
See also annihilation,
spin conservation rule.
TRIPLET-TRIPLET
ENERGY TRANSFER
Energy transfer from an electronically excited triplet donor to produce
an electronically excited acceptor in its triplet state.
See spin conservation
rule.
TRIPLET-TRIPLET
TRANSITIONS
Electronic transitions in which both the initial and final states
are triplet states.
TRIPLET-TRIPLET
TRANSITIONS
Same as radiative energy transfer.
TUNGSTEN-HALOGEN LAMP
See quartz-iodine lamp. Other halogens
may fill the lamp.
TUNNELLING
The passage of a particle through a potential-energy barrier the height
of which is larger than the energy of that particle. This effect is
important for some processes involving the transfer of electrons and
light atoms, particularly H atoms.
TURNTABLE REACTOR
See merry-go-round reactor.
TWISTED INTERNAL
CHARGE TRANSFER (TICT)
Intramolecular, photoinduced charge transfer between chromophores
interconnected by a single bond leading to an excited state (a TICT
state) in which the chromophores interact only weakly because of a
considerable twist about the interconnecting bond.
TWO-PHOTON EXCITATION
Excitation resulting from successive or simultaneous absorption of
two photons by an atom or molecular entity. This term is used for
successive absorption only if some of the excitation energy of the
first photon remains in the atom or molecular entity before absorption
of the second photon. The simultaneous two-photon absorption can also
be called biphotonic excitation.
See two-photon process.
TWO-PHOTON PROCESS
A photophysical or photochemical event triggered by a two-photon excitation.
