Excimer Lamp Irradiation

Excimer lamps were selected to study the low fluence irradiation region, where linear (no ablation) photochemistry is taking place. This is the fluence range (e.g., insert in Fig. 47 of the previous chapter), where a linear relation between reaction products and laser fluence is observed. This may correspond to the range of linear photochemistry, i.e., below the threshold of ablation (see, e.g., Figs. 25 and 26), or the so-called Arrhenius tail. The excimer lamps emit at the same wavelengths as the excimer lasers, but with incoherent radiation, and in quasi-CW mode. The peak photon fluxes of the lamps are low compared to the excimer laser, suggesting that multiphoton processes are not important. Thin films of the triazene polymer on quartz substrates were irradiated with the excimer lamps under different conditions, i.e., in Ar, air, and 02. 

The incoherent excimer radiation from a dielectric barrier discharge (silent discharge), operating in pure xenon, gas mixtures of krypton/chlorine, and xenon/chlorine, provides intense narrow-band radiation. More details about the excimer UV sources can be found in the literature [236, 237]. As irradiation sources a XeCl (308 nm), a KrCl (222 nm), and a Xe2 (172 nm) excimer lamp were selected. Excimer lamps based on fluorine-containing excimers are difficult to operate, due to etching of the quartz housing by the F2. 

Therefore the KrCl excimer lamp was used instead of the KrF 248-nm irradiation, and additionally the Xe2 excimer lamp. Different atmospheres were used for the 172-nm irradiation, because oxygen has a strong absorption band around 172 nm and ozone, oxygen-radicals, and excited molecular oxygen species are formed upon irradiation [238]. These reactive oxygen species cause photooxidation, in addition to the direct photolytic decomposition of the polymer. The irradiance of the XeCl and KrCl excimer lamps was measured by chemical actinometry. The intensity of the excimer lamps is 24 mW cm-2 while KrCl excimer lamps have an intensity of =350 mW cm-2. 

The irradiations at 308 and 222 nm were carried out in air. Effective direct etching of polymers by excimer lamps can only be observed at reduced pressures (between 0.1 to 100 mbar) [239], which was not examined in this study where only atmospheric pressure was applied. The decomposition of the polymer was analyzed at the two maxima, i.e., 196 and 330 nm. The former corresponds mainly to the absorption of the aromatic groups, the latter to the triazene groups [68]. 

With 308-nm irradiation, decomposition of the triazene group (330 nm) is observed almost exclusively. Only very minor changes are observed for the band at 196 nm. This is shown in Figs. 50 and 51, where the changes of the two absorption maxima are plotted as a function of the irradiation time for all irradiation wavelengths. A detailed analysis of the changes of the band 

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The excimer lamp irradiation experiments show clearly that photochemical decomposition of the triazene polymer takes place, even at low fluences and quasi-CW irradiation (the excimer lamps emit bursts of UV pulses with ns duration, but with repetition frequencies in the kHz range). 

Figure 22. Human embryonic kidney cells (A), rat vascular smooth muscle cells (B, C) and human osteoblast-like MG 63 cells (D) in cultures on micropattemed surfaces. A, B PTFE irradiated with UV light produced by a Xe2 -excimer lamp for 30 min in an ammonia atmosphere through a mask with holes 100 pm in diameter and center-to-center distance 300 pm C PE irradiated with Ar ions (energy 150 keV, ion dose lO ions/cm ) through a mask with holes 100 pm in diameter and center-to-center distance 200 pm fullerenes Qo deposited through a mask with rectangular holes with an average size of 128 3 pm per 98 8 pm on glass coverslips. Day 7 after seeding. A native cells in an inverted phase-contrast microscope B, C cells stained with hematoxylin and eosin, Olympus microscope IX 50 D cells stained with fluorescence-based LIVE/DEAD viability/cytotoxicity kit (Invitrogen), Olympus microscope IX 50. Bars 300 pm (A), 200 pm (B, D), Imm (C) [10,11].

In addition to the rather trivial differences mentioned above, laser irradiation can also lead to products as a result of reexcitaion of the carbenes. Thus, excitation of 30 in isooctane with a pulse of the 249-nm line from a KrF excimer laser results in the formation of 9,10-diphenylanthrancene (103), 9,10-diphenylphenanthrene (104), and fluorene, in addition to tetraphenylethylene (Scheme 9.31). Conventional lamp irradiation of 30 results in the formation of benzophenone azine as a major product. None of the products mentioned above are detected. Moreover, the yield of both 103 and fluorene increased markedly with increased laser power. While the details of the mechanism of this reaction are not certain yet, it is clear from the dependence on laser power that some of these products arise from carbene photochemistry. " ... 

Fusion Systems (Gaithersburg, MD) at one time offered 240 W/cm microwave-driven excimer lamps within its VIP (Versatile Irradiance Platform) series. They were among the most powerful sources of UV radiant power,i available... 

Extremely inhomogeneous conditions are found in oxidative degradation processes induced by vacuum ultraviolet (VUV) irradiation of aqueous reaction systems. In fact, the absorption cross section of water for an almost monochromatic excitation at 172 nm (Xe excimer lamp, vide infra) being very... 

Figure 15. Cross section along the plane perpendicular to the axis of a photochemical reactor using (a) an annular excimer lamp of coaxial (positive) irradiation geometry and (b) of a combination of cylindrical and annular excimer lamps (see also [60],...

The narrow bandwidth emission is shown for the Xe excimer lamp (172 + 12 nm, Figure 17). The monochromacy of the light source is a great advantage in many preparative applications [2, 3, 66] and facilitates radiant power and irradiance measurements and calculations [2, 3] in up-scaling projects. 

FIGURE 31. The refractive index variation in the PMPS film by UY-light irradiation using different light sources (a) excimer lamp (308 nm) and (b) mercury-arc lamp (185, 254, 303 nm). (Reprinted from Ref. 134.)... 

When the excimer lamp of 308 nm is used for the irradiation light source, the refractive index lowers from 1.70 to 1.63. With the mercury-arc lamp the refractive index lowers to 1.58. The larger reduction of the refractive index with the shorter wavelength light derives from the elimination of the side-chain phenyl group. 

This process uses light irradiation of wavelengths lower than the UV-C, i.e., lower than 190 nm. Generally, Xe excimer lamps (Xexc = 172 nm) are used. The excitation leads, in the majority of the cases, to the homolytic breakage of chemical bonds, degrading OM in condensed and gaseous phases (for example, fluorinated and chlorinated hydrocarbons) [1,3]. However, its application is limited, and the most important use of VUV radiation is in water photolysis (Eq. 8) ... 

In addition, a novel generation of lamps with promising features for photochemical applications has been developed to industrial maturity over the last decade, the so-called incoherent excimer radiation sources (Eliasson et al., 1988). Note that these lamps are not laser sources. In contrast to well-known excimer lasers, excimer lamps are operated under different physical conditions and they emit incoherent electromagnetic radiation. Whereas pulsed laser radiation can reach very high irradiances, E up to 100 MW m , the irradiance E of excimer lamps is only in the range of 1000 W m . ... 

However, the photochemical production of ozone on a small scale represents a potential application of the novel incoherent Xe2 excimer lamps. This was first demonstrated by Eliasson and Kogelschatz (1991), and on laboratory and preparative scales by Laszlo et al. (1998) and by Hashem et al. (1997). The latter research group realized the simultaneous generation of ozone in the gas phase and its transfer to the aqueous phase followed by the VUV irradiation of this water by... 

Cross-linking of thin-film PE can also be induced by excimer UV lamp irradiation, for coating purposes, to change a hydrophilic surface into a hydrophobic one. With this method a PE film can be bonded to another material without adhesive. 

The photochemical activity of the triazene group was also confirmed by irradiation at low fluences with excimer lamps, where one photon photochemistry is expected [137]. A decomposition of the triazene chromophore was observed below the ablation threshold fluence for irradiation at 308 and 222 nm. At 222 nm, an additional decomposition of the aromatic chromophores has been detected [141]. This suggests that the decomposition of the aromatic part is related to the carbonization. This selective decomposition of the triazene group by the less energetic wavelength (308 nm) clearly indicates that the triazene is the most sensitive unit in the triazene... 

Surface Modification by Irradiation with Monochromatic UV Excimer Lamps... 

VUV irradiation experiments. For the VUV irradiation experiments, an excimer lamp emitting at 172 nm (200 mm long, 35 mm diameter was used. The lamp was immersed in a photochemical reactor of annular geometry and a total volume of 400 ml. The reactor was immersed in a water-bath at constant temperature. 

For the subsequent photo-chemical modification of surface chemistry, samples were immersed in 1,5-hexadiene, diallylphthalate (DAP), or perfluoro(4-methylpent-2-ene) (PFMP), dried and irradiated. A monochromatic (excimer) lamp emitting at 222 nm was employed and the irradiation performed for 5 or 10 min in an inert atmosphere. Samples were irradiated on both faces in all cases. In general, the samples were extracted for 4 hours in water/methanol and further 4 hours in petroleum ether before the treatments and washed after the photo-chemical modification in order to remove uncrosslinked residuals. 

D. Praschak, T. Bahners and E. Schollmeyer, Excimer UV lamp irradiation induced grcdttng on synthetic polymers, Appl. Phys. A 71,577-581 (2000). 

Imai H., Awazu K., Yasumori M., Onuki H., Hirashima H. Densification of sol-gel thin films by ultraviolet and vacuum ultraviolet irradiation. J. Sol-Gel Sci. Tech. 1997 8 365-369 Imai H., Tominaga A., Hirashima H., Aizawa M., Toki M. Ultraviolet-laser-induced crystallization of sol-gel derived indium oxide films. J. Sol-Gel Sci. Tech. 1998 13 994 Imai H., Tominaga A., Hirashima H., Toki M., Asakuma N. Ultraviolet-induced reduction and crystallization in indium oxide films. J. Appl. Phys. 1999 85 203-207 Kikuta K., Hirano S. Processing of ceramic fine patterns with excimer lamp. Cream. Trans. 1998 83 307-313... 

FIGURE 3.13 UV-vis spectra of VUV-irradiated PLA membranes for 3,6,10, 30,60, and 100 min. (Reprinted from Desalination, 287, S. Sato et al.. Effects of irradiation with vacuum ultraviolet xenon excimer lamp at 172 nm on water vapor transport through poly[lactic acid] membranes, 290-300. Copyright 2012, with permission from Elsevier B.V.)... 

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