Photothermal decomposition

Photothermal decomposition of palladium acetate by scanned cw Ar+ laser irradiation produces metal features that exhibit pronounced periodic structure as a function of laser power, scan speed, substrate and beam diameter, as shown in Figures 3 and 4. The periodic structure is a function of the rate at which the film is heated by absorption of the incident laser radiation coupled with the rate at which the heat of the decomposition reaction is liberated. This coupling generates a reaction front that outruns the scanning laser until quenched by thermal losses, the process to be repeated when the laser catches up and reaches unreacted material. Clearly, such a thermal process is also affected by the thermal conductivity of the substrate, the optical absorption of the substrate in those cases where the overlying film is not fully absorbing,... 

Photothermal decomposition, in which the electronically excited state undergoes internal conversion to a vibrationally ( hot ) excited ground states. Subsequent thermal decomposition is therefore a pyrolysis of the polymer, which is not very different from the processes observed with laser radiation at visible or infrared wavelengths [137,162,304,578,1240,1601]. 

Reactions of Halo Compounds. - Calculations have been carried out to investigate the decomposition paths for methyl fluoride and methyl chloride. Methyl chloride undergoes photodissociation on irradiation at 157.6 nm. Photodissociation of methyl iodide at 266 nm has been studied. The methyl radical recombination has been followed by time-resolved photothermal spectroscopy. Methyl iodide also undergoes photochemical decomposition on a GaAs(llO) surface. " Photolysis of methyl iodide at 236 nm in the gas phase brings about liberation of iodine atoms with a quantum yield of 0.69. ... 

Another important feature of ablation, which is never discussed in the photothermal models was repeatedly emphasized by Srinivasan [92] the products of pyrolysis or ablation with a CO2 laser are very different to the products of excimer laser ablation in the UV. This suggests that different processes take place between pyrolysis (thermal decomposition) and UV laser ablation. 

The thickness of the deposit increases with pulse number while the area increases with fluence. A transmission electron microscopy (TEM) picture (Fig. 75) shows that the carbon is loosely packed and that the thickness decreases from the edge of the crater (left in the TEM pictures). PI was chosen as reference polymer mainly for two reasons. The above described absorption properties, and the fact that the newest photothermal model could until now describe all experimental data (see above). Therefore, we thought that PI can be used as a typical example for a polymer which follows a photo-thermal model [89], while the triazene polymers reveal several features which might be considered photochemical (e.g., wavelength dependence, products and decomposition at low fluences, and etching during the pulse). 

All data obtained for TP strongly suggest that photochemical reactions play an important role during UV laser ablation, but also that photothermal processes are important. This is confirmed by the presence of the thermal N2 products in the TOF curves. Photothermal processes will also always be present if the polymer decomposes exothermically during a photochemical decomposition and if the quantum yields of the photochemical reaction is not equal to one (which is most of time the case). The ablation of polymers will therefore always be a photophysical process (a mixture of photochemical and photothermal processes), where the ratio between the two mechanisms is a function of the irradiation wavelength and the polymer. In addition, photomechanical processes, such as pressure produced by trapped gaseous ablation products or shock and acoustic waves in the polymer, take place and can lead to a damage of the polymer and are most important for picosecond pulses. 

In contrast to UV lasers, IR lasers ablate the substrates photothermally. When the focused laser beam hits the substrate surface, the temperature of the irradiated spot will rise rapidly that the material will first melt and then decompose, leaving a void in the substrate. The actual decomposition mechanism depends on the strength of the chemical bonds of the monomers that make up the pol)mier and the structure of the polymer itself. 

Under photothermal degradation conditions (160-170°C) the primarily formed methacrylate radicals can unzip to form monomer, but in copolymers, unzipping is inhibited by acrylate units and from a butyl acrylate copolymer a number of decomposition products such as n-butyl methacrylate, n-butyl-aldehyde and n-butanol are formed [828]. 

Sonawane [77] showed that adding Pt or Au to Ti02 favors the photothermal and photocatalytic decomposition of benzene in the gaseous phase, although an excess of dopant works against the activity. 

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