Laser ablation dynamics

Here, we briefly describe the laser ablation dynamics of phthalocyanine film where the primary species was confirmed, by femtosecond transient absorption spectroscopy [25], to be its electronically excited state. The exdton absorption band was replaced in 20 ps by a hot band of the ground electronic state. This rapid decay was ascribed to mutual interactions between densely formed excitons leading to sudden temperature elevation. The elevated temperature was estimated by comparing transient absorption spectra with the temperature difference ones. It is worth noting that quite normal dynamics of the excited states are detected, even under ablation conditions, which is the reason why we do not accept the plasma mechanism. Then we considered how to correlate these electronic processes of phthalocyanine films with their fragmentation. 

Hosokawa, Y., Yashiro, M., Asahi, T., Fukumura, H. and Masuhara, H. (2000) Femtosecond laser ablation dynamics of amorphous film of a substituted Cu-phthalocyanine. Appl. Surf. Sci., 154-155, 192-195. 

Laser ablation of TiC targets was studied by De Maria et al. (1997). TiC films were realized on oriented [111] silicon. During ablation the chamber was kept under a dynamic vacuum (1.5 10 4 Pa) the laser fluence was varied in the range 0-15 J/cm2. Different ablation mechanisms, corresponding to different film characteristics, were observed related to the laser fluence values. At low fluence values (0-3 J/cm2), a film of composition TiC2 was obtained. Films obtained at 3-8 J/cm2 showed a composition close to TiC they had the best characteristics of composition, crystallinity and compactness. 

Analyzer Q = quadrupole, CC = collision cell, DRC = dynamic reaction cell, MC = multicollector, SF = sector field. Analytical details CV = cold vapor, ETV = electro-thermal vaporization, FI = flow injection, HG = hydride generation, ID = isotope dilution, LA = laser ablation, UN = ultrasonic nebulization. Sample introduction in liquid or slurry (si) form. 

Although the sensitivity obtained with RIS is impressive, analytical calibration in specialized reservoirs presents some practical problems. In the laser ablation/RIS scheme, these limitations are particularly pronounced because of the nature of the sampling process. Relative calibration techniques, commonly used by analytical chemists, imply the availability of accurate standards at the appropriate concentration 9), The current state of knowledge about the dynamics of the ablation process did not permit absolute calibration with any confidence. The coupling of highly sensitive and selective RIS with mass spectrometry should result in more generally useful applications to analytical chemistry10). 

Fig. 37 Schematic diagram of laser-induced decomposition and ablation dynamics of the photosensitive triazene polymer film (a) before excimer laser irradiation, (b) slight expansion of the film and darkening of the irradiated surface at the beginning of the excimer laser pulse, (c) initiation of etching of the film and ejection of gaseous fragments decomposed from the polymer, (d) completion of the etching and expansion of ejected plume, and (e) after the ablation. REPRINTED WITH PERMISSION OF [Ref. 125], COPYRIGHT (1997) American Chemical Society...

A slight initial expansion of the film and darkening of the irradiated film surface are observed prior to ablation for both fluences (250 and 60 mj cm 2, shown in Fig. 37b). Ablation of the film starts, depending on the fluence, at a later time. The ending of the ablation process is also affected by the applied laser fluence. Ablation stops at +80 ns, which coincides almost with the end of the excitation laser pulse for 250 mj cm-2, while it continues until +140 ns for 60 mj cm 2 (Fig. 37d). The fluence-dependent ablation dynamics indicates that not only photochemical reactions but also pho-toinitiated thermal reactions are involved in the ablation process of the triazene polymer film. 

Direct etching of polymers in vacuum with SR which was mainly x-rays between 0.2 and 3 nm has been studied. This is a process involving only photons, like laser ablation, but it is different from laser ablation both in desorption dynamics and dissociation mechanisms. 

In previous years studies on the photoablation of polymers for etching purposes have been few. Over the last twelve months however, this area of work has expanded enormously and may be an indication of its commercial fruition. A dynamic model has been proposed for the laser ablation of polymer systems which implicates the successive absorption of two or more photons causing population to higher electronic excited states . In this work the excitation time of laser pulses was stepped and indicated that short-lived radicals play an important role in ablation. The wavelength of the... 

For conventional analysis by ICP or DCP, liquid samples are used, which are either easily prepared or commercially available. Interference problems are reduced to a minimum if the cahbration solutions are matched to the samples with respect to their content of acids and easily ionisable elements (see above). Calibration curves obtained with sparks, arcs, and laser ablation systems are usually curved so that 8—15 calibration samples or more are needed to define a suitable calibration. In the case of liquid analysis by DCP and ICP, fewer cahbration samples can be used due to the better linearity and dynamic range and absence of selfabsorption effects. With the introduction of hquids, the spray chamber is the major source of flicker noise due to aerosol formation and transport. While shot noise can easily be compensated by longer integration times, the flicker noise is of multiplicative nature so that any element can be used as an internal standard provided that a true simultaneous measurement of the analyte and internal standard line intensity is possible. 

R. Sen, Y. Ohtsuka, T. Ishigaki, D. Kasuya, S. Suzuki, H. Kataura, and Y. Achiba, Time period for the growth of single-wall carbon nanotubes in the laser ablation process evidence from gas dynamic studies and time resolved imaging Chem. Phys. Lett. 332,467-473 (2000). 

An interesting method was presented for the analysis of perfluorinated polyethers (PFPE) by Cromwell et al. The fluorinated polymers are commonly used as lubricants because of their low vapor pressures, chemical inertness, and thermal stability. Samples were examined in a cubic trap FTMS-2000 system using a 4.3 T magnetic field. Both a Kr/F excimer laser and a Nd/YAG laser were used for infernal LD. The fwo-step procedure involved using the 248-nm excimer laser for desorption of the polymer and, immediately following, a second more tighfly focused 532 nm laser ablation pulse to ablate metal cations from fhe surface beneath the polymer sample. Thus, cationization could be controlled, and the dynamics of the process are discussed. Mass spectra extended up to 10,000 Da are shown. 

There are differences in the surface of the laser-ablated polymer depending on the mode of operation, static and dynamic laser ablation mode. Rossier et al. investigated differences in surface states of polymer PET as a result of these two modes of ablation [18], Their studies revealed that the static ablation mode produced a homogeneous and hydrophobic surface with poor wettability, whereas the dynamic ablation mode produced an inhomogeneous and hydrophilic surface with high wettability. These differences were attributed to the redeposition of fragments. 

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