Untitled - IRS

264 GLOSSAR
wavelength (’red end’) and shortest wavelength (’blue end’). The light might either form a continuous
spectra or consist of several lines enveloped under an amplitude function with a finite
spectral width. In the case of a laser this might be the spectral function of the medium which
envelopes different resonator modes which are simultaneously present. (Eq. (2.67)) [p. 36]
Stabiler Resonator Stable Resonator. Laser resonator which refocuses the beam priodically by means
of concave mirrors and exhibits relatively low diffraction losses. (Eq. (2.59)) [p. 32]
Stern-Volmer Gleichung The Stern-Volmer equation describes the decrease of the fluorescence intensity
as a function of the concentration of quenchers (Eq. (4.24)) [p. 103]
Stimulierte Emission → Induzierte Emission (Eq. (2.0)) [p. 14]
Strahlungsdruck Radiation pressure. A photon of energy hν carries a momentum p = hν/c =
hk/(2π). When this photon is absorbed, the momentum is transfered to the absorber, which
can be directly measured in the case of an absorbing atom and plays an important role during
laser cooling. A light wave which consists of a large number of photons transfers their
total momentum towards a macroscopic absober and this momentum transfer is denominated
’Radiation pressure’. (Eq. (3.1)) [p. 43]
Superradianz superradiance. When a pupulation-inverted ensemble deexcites by spontaneuos emssion
(fluorescence) instead of stimulated emission, the resulting light is neither coherent, nor
monochromatic. Superradiance occurs when the lasing threshold is exceeded to far (depending
on lifetime of the excited state) or when the seeding, i.e. the weak radiation that initiates
stimulated emission, is not properly aligned. (Eq. (2.30)) [p. 21]
Thomas-Reiche-Kuhn Regel also f-sum rule. If we consider the optical absorption spectrum of
a molecule, the integral of absorption cross-section over photon energy must not exceed
1.098 eV˚A 2 /electron. More precisely: One electron of a molecule accounts for a certain
maximum osillator strength of electric dipole transitions. From the moments of first order
in a randomn phase approximation calculation follows � σ(E)dE = 2π 2 q2 �c
4πɛ0·mc 2 Z =
1.098 eV˚A 2 Z. (Eq. (4.18)) [p. 96]
Threshold → breakdown threshold (Eq. (7.14)) [p. 202]
Vier Niveau Laser Four level laser. Lasertype where the lasing medium is excited to a high lying
state from which the upper lasing level is populated. Lasing action takes place to a third level
which decays towards an even lower state (or the ground state). The latter depopulation
process decreases reabsorption losses. (Eq. (3.23)) [p. 54]
Wirkungsquerschnitt (Reaction) Cross-section. This quantity, typically denominated σ or µn is used
for describing the propability of a reaction and has the unit length 2 , e.g. cm 2 . The product
of the cross-section and the flux density of reactants is the reaction rate: (Cross-section)
x (number of photons/unit area per second) = rate of absorptions. For atomic photon absorption
off resonance typical cross sections are 10 −20 − 10 16 cm 2 . Older unit: 10 16 cm 2 =
1˚A 2 (Eq. (3.3)) [p. 44]
Young-Experiment T. Young, 1801. In this experiment, Young identified the phenomenon called
interference. Observing that when light from a single source is split into two beams, and the
two beams are then recombined, the combined beam shows a pattern of light and dark fringes,