Excimer-based lasers

An lasers described so far work in the nanosecond regime with laser pulse widths varying between 3-5 ns (Nd YAG-based lasers) and 10-15 ns (excimer lasers). With the more recently introduced femtosecond lasers, the energy is supphed to the solid sample within a much shorter time interval (< 1 ps) [54—56]. This reduces the amount of heat generated and results in a cleaner, more explo-sion-hke ablation ]57], especially for metaOic samples. In the latter case, avoiding heat transfer and thus melting also leads to an enhanced spatial resolution. 

The potential of LA-based techniques for depth profiling of coated and multilayer samples have been exemplified in recent publications. The depth profiling of the zinc-coated steels by LIBS has been demonstrated [4.242]. An XeCl excimer laser with 28 ns pulse duration and variable pulse energy was used for ablation. The emission of the laser plume was monitored by use of a Czerny-Turner grating spectrometer with a CCD two-dimensional detector. The dependence of the intensities of the Zn and Fe lines on the number of laser shots applied to the same spot was measured and the depth profile of Zn coating was constructed by using the estimated ablation rate per laser shot. To obtain the true Zn-Fe profile the measured intensities of both analytes were normalized to the sum of the line intensities. The LIBS profile thus obtained correlated very well with the GD-OES profile of the same sample. Both profiles are shown in Fig. 4.40. The ablation rate of approximately 8 nm shot ... 

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). 

To summarize the state of technology for the chemist wishing to practice laser chemistry the laser devices exist with the capability one would like, but they are expensive. We may expect that cheaper pulsed laser systems based upon excimer, Nd YAG, N2, alexandrite, etc. may be in the offing in the near future. This has already begun to happen with a new generation of N2 pumped dye lasers from two manufacturers. No such prospects presently exist for c.w. lasers in the visible and ultraviolet, but one may hope that the ion laser will be radically improved or supplanted soon. For chemical applications which can use infrared excitation, satisfactory devices presently exist and the price is right. 

Gas lasers, 14 681-696 carbon dioxide, 14 693-696 excimer lasers, 14 691-693 helium-neon, 14 681-683 ion lasers, 14 683-688 molecular nitrogen, 14 688-691 Gas lift electrolyte circulation, 9 621 Gas-liquid base stocks, 15 217 Gas-liquid chromatography (glc), 6 374 analysis of sugars via, 23 476 silylation for, 22 692, 697 Gas-liquid contactor, reciprocating jet,... 

Onium salts have been widely used as an acid generator for photo-, EB, and x-ray resist. In addition, aromatic polymers such as novolak and polyhydroxystyrene have been often used as a base polymer for EB and x-ray resist. The reaction mechanisms in a typical resist system have been investigated by pulse radiolysis [43,52,77-88], SR exposure [79,80,83-85], and product analysis [88]. Figure 6 shows the acid-generation mechanisms induced by ionizing radiation in triphenylsulfonium triflate solution in acetonitrile. The yields of products from electron beam and KrF excimer laser irradiation of 10 mM triphenylsulfonium triflate solution in acetonitrile are shown in Fig. 7 to clarify the... 

Here we will focus in detail on a UV pump-IR probe spectrometer described by Emsting and co-workers the system is based on an excimer laser and a dye laser operating with a pulse repetition rate ranging from 5 to 10 Hz. Pump pulses at 308 nm excite the sample and are followed at a selected time by probe IR pulses that range from 1950 to 4300 cm Absorbance changes can be recorded with a time resolution of 1.8 ps and with an accuracy in absorbance (A) of 0.001. 

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... 

Deep-UV source brightness is another issue, because the power output of a 1-kW mercury-xenon lamp in the 200-250-nm range is only 30-40 mW. For this reason, excimer lasers (such as KrCl and KrF), which can deliver several watts of power at the required wavelengths, are being considered as alternatives (7). In fact, a deep-UV step-and-repeat projection system with an all-quartz lens and a KrF excimer laser with an output at 248 nm has been reported (8). Even the laser-based systems require resists with a sensitivity of 30-70 mj/cm2. 

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