Excimer laser and its applications Introduction 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.