Excimer
Excimer Laser
The first report of a new type of excimer system came
from Golde and Thrush at
Cambridge who observed characteristic bound-free
transitions in ArCl produced by
reacting argon metstables with chlorine in a
flowing afterglow system. At
present the rare-gas halidel asers are
undoubtedly the most important excimer
lasers and are being actively
developed for applications in laser-induced fusion
and isotope separation. A
large amount of energy, ranging from 8 eV in Xe to 20
eV in He is required to
produce the first excited state because of the closed
shell nature of the
normal state of the rare gases. Application on Water
Pollution A typical
spectrum of polluted sea water will contain the intense
water Raman signal at
344 nm, the gelbstoff fluorescence from organic and
biological waste, which
is peaked between 400 nm and 420 nm, the fluorescence of
light and heavy oils
peaked between 400 nm and 500 nm, and possibly some
chlorophyll from
phytoplancton peaked around 685 nm. The LIF spectra of the
crude oil samples
(Fig. 2d) show that, at variance with refined oil samples
emitting mostly in
the near UV, their fluorescence emission covers most of the
visible spectral
range. Although the total emission intensity decreases
dramatically at
increasing intensity, measured spectral shapes are quite similar
throughout
this region, where three maxima can be identified, roughly peaked at
460,
490, and 540 nm. Higher resolution measurements were attempted, however
did
not reveal the presence of any sharper feature. The general trend is
a
broadening of the fluorescence spectra towards longer wavelengths
with
increasing oil density. The presence of crude oils on water surface can
be
recognized from their typical emission spectra, but the direct
identification of
the specific oil seems to be rather difficult if no
additional information is
available. Measurements of the water Raman signal
have been performed at high
resolution in the range 330 nm to 365 nm in order
to discriminate both from the
intense tail of the backscattered laser
radiation and the rise of oil
fluorescence band. Measurements in the same
wavelength range have been performed
after adding fixed amounts of different
oils on the surface above a certain
water column. The spectra of Kirkuk and
Saharan Blend oil are shown in Fig. 4
and it is noticeable that the water
Raman peak intensity is progressively
reduced by the oil absorption of 308 nm
laser radiation which thus cannot
effectively penetrate in the water column.
In addition, the first peak of the
oil fluorescence spectrum was detected in
this range ( ~360 nm) which is
especially intense in the case of the lightest
oil. the dependence of oil
fluorescence intensity and water Raman intensity
upon oil (quantity) thickness
has been checked in order to use the lidar
fluorosensor for field measurements
of oil film thickness on sea water.
However the integrated oil fluorescence in
the range 360 to 364 nm, after
proper background subtraction, vs the quantity
(drops) of oil spilled upon
the water surface followed a linear behavior only at
very small quantities
and quickly reached saturation, especially for the
heaviest oils. This
demonstartes that absolute fluorescence measurements, which
also require the
knowledge of the kind of oil detected, are not suitable to
determine the
thickness of the pollutant film. Time decays curves for the four
crude oil
samples have been measured through all the visible range and the
excimer
laser pulse profile has been measured as well. In Fig 6 (a) the typical
laser
profile showing at least two well resolved cavity modes and in (b)-(e)
crude
oils appear distinguishable according to their density, in fact lighter
oils
are characterized by longer time constants. the observed trend in lifetime
is
significant to the identification of the crude oil sample. Therefore
in
conclusion, measuring accurate time decay constants should allow for
the
unambiguous identification of pollutant oils in remote sensing
experiments
together with fluorescence. In addition, according to the result,
it comes out
that an UV laser source with shorter pulses would permit more
accurate time
resloved oil fluorescence measurements. A more complete data
base for oils
recognition can be built by increasing the number of parameters
in a
multiexponential fit. Application on Optometry The Argon-Fluoride
Excimer Laser
is a revolutionary innovation and advanced treatment modality
in an attempt to
correct myopia, hyperopia and astigmatism, as well as
superficial keratectomy to
erase corneal scars and irregular corneal
surfaces. When the Argon-Fluoride
Excimer Laser is used in corneal
reshaping to correct refractive errors, it
breaks the carbon-to-carbon
molecular bonds of the corneal tissue by the
ultraviolet 193-nm wavelenghth
of emission photochemical effect called
photoablation. This photoablation
effect is extremely superficial. Minimal
thermal damage is created by the
ultraviolet excimer laser, unlike traditional
lasers in which the produced
heat causes damaging effects to surrounding tissue.
The pulsing excimer
laser removes the tissue in microscopic layers, leaving
virtually no
underlying thermal trauma. The carbon-to-carbon bond holding most
of the
tissue together has an energy requirement of 3 electron volts. If an
excimer
laser photon is introduced, it can literally crack that bond.
The
photon-energy, or energy per photon, of the excimer photon is 6.4
electron
volts, or 10-15 mj per photon. One laser pulse contains many
photons. One
excimer laser pulse contains 2.5 x 1016 photons. Therefore, the
energy per pulse
at the eye is equal to the 10-15 millijoules (single photon
energy) times 2.5 x
1016 (number of photons in one pulse), which equals
25 millijoules (mj). (2.5 x
1016 = 25 billion million.) These excimer
photons are like "photon
scissors", breaking the carbon-to-carbon bonds of
the corneal tissue.
Hence, the excimer photon is incredibly energetic,
having 3 times as much energy
as the YAG laser photon and more than twice the
energy as the Argon laser
photon. The term that has been coined for the
effect of the excimer laser on the
tissue is photoablation. The key to the
excimer laser is the short pulse
duration (10 ns or 10 x 10-9 s) with high
energy photons (energy per pulse is 25
mj at the eye) with the possibility of
concentrating large numbers of these
photons on tissue to crack the
carbon-to-carbon bonding that holds tissue
together. For the first time, a
no-touch system, or no-touch scalpel, with the
ultimate resolution of a
fraction of a micron, is available to surgeons. (One
micron equals one
one-thousandth of a millimeter.) So, without touching the eye,
the excimer
can change and sculpt the cornea (photon scissors) incredibly
accurately with
virtually no collateral damage conducted into the edges of the
tissue
affected. There is no significant mechanical effect to the
surrounding
tissues; and no crushing of tissue.