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 emplo yed to probe the dynamics Of ablation of polyethersulfone (PES) at 266 nm. Neutral products arriving at an ion extraction position, which wa s 65 mm from a PES film surface, were detected by delaying a post-ioni zation laser pulse with respect to an ablation laser pulse. At a low f luence 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 dire ct fragments from PES or secondary products, were observed. Analysis o f the products indicates that the decomposition of PES occurs due to b oth the scission of the polymer chain itself and the cleavage of some phenylene rings in the chain. The average flight velocities of major p roducts 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 perio d of 6-15 mu s, which was over 1200 times the ablation laser pulse len gth. The distribution, the velocities, and the yields of major product s for several higher fluences up to 130 mJ/cm(2) suggest an enhanced d ecomposition of PES to small fragments in the upper part of a heated s urface layer. The arrival profiles of C3H3 at various fluences were ad equately described by a shifted Maxwell-Boltzmann distribution, indica ting collisions among the fragments during an unsteady adiabatic expan sion process. We propose a laser-penetration-depth dependent photother mal ablation model.