Photons, excimer lasers

Multiphoton processes are also undoubtedly involved in the photodegradation of polymers in intense laser fields, eg, using excimer lasers (13). Moreover, multiphoton excitation during pumping can become a significant loss factor in operation of dye lasers (26,27). The photochemically reactive species may or may not be capable of absorption of the individual photons which cooperate to produce multiphoton excitation, but must be capable of utilising a quantum of energy equal to that of the combined photons. Multiphoton excitation thus may be viewed as an exception to the Bunsen-Roscoe law. 

In photo CVD, the chemical reaction is activated by the action of photons, specifically ultraviolet (UV) radiation, which have sufficient energy to break the chemical bonds in the reactant molecules. In many cases, these molecules have a broad electronic absorption band and they are readily excited by UV radiation. Although UV lamps have been used, more energy can be obtained from UV lasers, such as the excimer lasers, which have photon energy ranging from 3.4 eV (XeF laser) to 6.4 eV (ArF laser). A typical photo-laser CVD system is shown schematically in Fig. 5.14.117]... 

A number of points are clear. First, in all cases the major expense of laser photons is the hardware, not the energy (even at Austin prices). Secondly, the intrinsically greater efficiency of the lower-energy lasers, especially the economic attractiveness of the CO2 laser, is evident. One can easily understand why laser chemistry schemes based upon multiphoton infrared absorption attract so much effort. Thirdly, on a per-unit-time basis the ion laser is more than twice as expensive to operate than even the rather exotic excimer laser. This is because of the inherent energetic inefficiency of the rare-gas plasma as a gain medium and because of the extrinsic, and hideous, expense of ion laser plasma tubes (and their poor reliability). 

As shown in Figure 6, the resist film strongly absorbs KrF excimer laser light. In the KrF excimer laser exposure of the resist, photon energy absorption is highest at the top of the resist film and lowest at the interface between the resist and substrate. This is due to the attenuation of the irradiation in the resist layer. Decreases in solubility followed by such photochemical reaction occur to a much greater extent in the vicinity of the resist film surface. Moreover, the thermally decomposed azide decreases solubility of the unexposed and exposed resist film (Figure 7). 

For EPy-doped PMMA film, a 308 nm excimer laser (Lumonics TE 430T-2, 6ns) was used as as exposure source. We used a tine-correlated single photon counting systen (18) for measuring fluorescence spectra and rise as well as decay curves of a snail ablated area. The excitation was a frequency-doubled laser pulse (295 nm, lOps) generated from a synchronously punped cavity-dumped dye laser (Spectra Physics 375B) and a CW mode-locked YAG laser (Spectra Physics 3000). Decay curves under a fluorescence microscope were measured by the same systen as used before (19). 

Excimer lamps are quasi-monochromatic light sources available in UV wavelengths. The light is produced by silent electrical discharge through gas in the gap between two concentric quartz tubes. Electronically activated molecules are produced in the gas phase and decompose within nanoseconds to produce photons of high selectivity. This process is similar to the process in excimer lasers. 

Two-photon absorption chemistry of 2AP, specifically photoionization processes, can be induced by intense nanosecond 308-nm XeCl excimer laser pulses [10]. Typical transient absorption spectra of 2AP in deoxygenated neutral aqueous solutions are shown in Fig. 1. The stronger (385 nm) and weaker (510 nm) absorption bands were assigned to 2AP radicals derived from the ionization of 2AP (bleaching near 310 nm) [10], whereas a structureless absorption band from -500 to 750 nm corresponds to the well-known spectrum of the hydrated electron (eh ) [41]. 

Previous extensive studies have shown that the energy of 193-nm photons from ArF excimer lasers, E=6.42 eV, is sufficient to induce single-photon ionization of nucleic acid bases [44-47]. The energy delivered by a consecutive two-photon excitation of 2AP is E=1.11 eV this is the sum of the energy of the singlet excited state of 2AP (Eoo=3.74 eV) and the energy of a 308-nm... 

There have been several experimental developments in the last few years that have resulted in more definitive experiments on photodissociation dynamics. First and foremost is the greater availability of intense laser sources, particular in the region below 300 nm (8,9). Excimer laser sources provide large numbers of photons at 157, 193, 222, 249, and 308... 

After irradiation, a silver-colored thin polymer layer totally covers the liquid phase. During the irradiation process, the color of the liquid phase has changed from yellow to a deep red one. The red color is due to a charge transfer complex between iodine and the thiophene ring dissolved in the liquid. The polymerization process takes place only at the surface of the irradiated sample. However, it was observed that when the UV-irradiation by excimer laser is over, the polymerization process continues, but only at the surface of the irradiated sample a postpolymerization without incident UV-photons takes place. It was also observed that the presence of oxygen during... 

Laser-induced decomposition of mixtures of polychlorinated biphenyls in the liquid phase has been investigated, employing radiation from three different excimer lasers (XeCl at 308 nm, KrF at 248 nm and ArF at 193 nm)475. The mixtures can be quantitatively and efficiently destroyed by means of UV radiation at 248 nm. A single-photon dissociation process, which leads to both HC1 elimination and biphenyl bond rupture, is induced by the KrF laser radiation. 

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