Laser ablation of polyethersulfone films: The decomposition of the chain structure and the expansion of neutral species studied by laser ionization mass spectrometry

Laser ionization time-of-flight (TOF) mass spectrometry has been employed to probe the dynamics Of ablation of polyethersulfone (PES) at 266 nm. Neutral products arriving at an ion extraction position, which was 65 mm from a PES film surface, were detected by delaying a post-ionization laser pulse with respect to an ablation laser pulse. At a low fluence of 30 mJ/cm(2), the strongest peak indicating early arrival of C3H3 (mle 39) was observed at post-ionization delay times of 12-22 mu s. As the delay time increased (16-46 mu s), some prominent peaks (m/e = 140, 164, 188, 216, 234, 262, 264, 280, 288, 312, 333, and 336) and many weak peaks with mle up to about 690, which were assigned to direct fragments from PES or secondary products, were observed. Analysis of the products indicates that the decomposition of PES occurs due to both the scission of the polymer chain itself and the cleavage of some phenylene rings in the chain. The average flight velocities of major products ranged from 1.8 X 10(5) cm/s for C15H12S2O5 (m/e = 336) to 4.1 x 10(5) cm/s for C3H3. These products continued to arrive for a period of 6-15 mu s, which was over 1200 times the ablation laser pulse length. The distribution, the velocities, and the yields of major products for several higher fluences up to 130 mJ/cm(2) suggest an enhanced decomposition of PES to small fragments in the upper part of a heated surface layer. The arrival profiles of C3H3 at various fluences were adequately described by a shifted Maxwell-Boltzmann distribution, indicating collisions among the fragments during an unsteady adiabatic expansion process. We propose a laser-penetration-depth dependent photothermal ablation model.