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.