Mechanisms of Ablation

One other important motivation of our research was the controversy over the mechanism of ablation. It has been suggested that the mechanism is either thermal, photothermal, or photochemical, or a mixture of these [54, 56, 57]. 

The different models and main features may be summarized as follows  

Photochemical models [56, 72-75] Electronic excitation results in direct bond breaking. 

Photothermal models [76-80] Electronic excitations thermalize on a ps timescale, resulting in thermally broken bonds. 

Photophysical models Thermal and nonthermal features are important. The models consider two independent channels of bond breaking [81, 82], or imply different bond breaking energies for ground state and electronically excited chromophores [83, 84], These models are mainly important for short pulse (ps and fs) lasers [85], 

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Ion beam induced ablation is one of the most important electronic excitation effects [1,2]. Ablation phenomena occur both thermally and photochemically in many kinds of materials including polymers and biological systems irradiated by both ion beams and high power laser pulses. The mechanisms of ablation of polymers induced by high density electronic excitation have not been made clear yet. 

One comment is appropriate here. The modeling of ablation depths at high fluences is not sensitive to the underlying mechanisms of ablation itself. At such fluences ablation rates of most polymers are quite similar [90] and are determined by screening of the radiation by the ablated products [80, 82] or generated plasma [91]. 

The most common parameters to characterize the ablation of polymers are the ablation rates at various fluences, the threshold fluence, and the effective absorption coefficient. These values and the quality of the achieved structures can give first indications about the mechanism of ablation. For this study various designed polymers (described in more detail below) and one reference polymer were selected. 

The mechanism of ablation and the role of photochemically active groups on the ablation process. 

The data discussed above suggest that the mechanisms of ablation are not just photothermal, but contain photothermal and photochemical features. The latter has been neglected in recent models and studies about the ablation mechanism. 

Fukumura, H. and Masuhara, H. (1994) Xhe mechanism of dopant-induced laser ablation. Possibility of cyclic multiphotonic absorption in excited states. Chem. Phys. Lett., 221, 373-378. 

Hiroshi Fukumura received his M.Sc and Ph.D. degrees from Tohoku University, Japan. He studied biocompatibility of polymers in the Government Industrial Research Institute of Osaka from 1983 to 1988. He became an assistant professor at Kyoto Institute of Technology in 1988, and then moved to the Department of Applied Physics, Osaka University in 1991, where he worked on the mechanism of laser ablation and laser molecular implantation. Since 1998, he is a professor in the Department of Chemistry at Tohoku University. He received the Award of the Japanese Photochemistry Association in 2000, and the Award for Creative Work from The Chemical Society Japan in 2005. His main research interest is the physical chemistry of organic molecules including polymeric materials studied with various kinds of time-resolved techniques and scanning probe microscopes. 

They also discussed the excitation mechanism of alkali-metal atoms as follows. The addition of a metal species from a liquid solution into cavitating bubbles is through the ablation of the bubble-liquid interface, the ablation of liquid jet or the evaporation of droplets, since the evaporation of salt is negligible. The salt molecules are released and decomposed into atoms via homolysis, analogous with the projection into a flame of metal species from salt solutions. The metal atoms are... 

Considering the low separation energy between the 3A" and A states, the hypothesis that, in the reaction of laser-ablated Ti. TiO and Ti02 with CO, the excited JA state can be generated, becomes plausible. This is consistent with the B3LYP results for the PES of the insertion mechanism of Ti into the C-0 bond of carbon dioxide, which shows that no barrier is necessary for this process, in agreement with the experimental indication [49],... 

The past decade has led to the detection of new carbon allotropes such as fullerenes26 and carbon nanotubes,27 28 in which the presence of five-mem-bered rings allows planar polycyclic aromatic hydrocarbons to fold into bent structures. One notes at the same time that these structures are not objects of controlled chemical synthesis but result from unse-lective physical processes such as laser ablation or discharge in a light arc.29 It should be noted, on the other hand, that, e.g., pyrolytic graphitization processes, incomplete combustion of hydrocarbon precursors yielding carbon black, and carbon fibers30 are all related to mechanisms of benzene formation and fusion to polycyclic aromatic hydrocarbons. 

The application of ion beams to polymers has been worthy of remark in the fields of advanced science and technology since the radiation effects of ion beams on polymers are different from those of conventional radiation such as electron beams and gamma-rays. The effects of ion beams are called LET effects but the detailed mechanisms of these effects on polymers have not been elucidated so far. So-called high density excitation effects such as carbonization, blackening, ablation and formation of nuclear tracks, which only occur at high densities, have been studied by a number of advanced spectroscopic methods. 

Washboume P, Thompson PM, Carta M, Costa ET, Mathews JR, Lopez-Bendit6 G, Molnar Z, Becher MW, Valenzuela CF, Partridge LD, Wilson MC. Genetic ablation of the t-SNARE SNAP-25 distinguishes mechanisms of neuroexocytosis. Nat. Neurosci. 2002 5 19-26. 

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