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/cm², the strongest peak indicating early arrival of C<INF>3</INF>H<INF>3</INF> (m/e = 39) was observed at post-ionization delay times of 12−22 μs. As the delay time increased (16−46 μs), some prominent peaks (m/e = 140, 164, 188, 216, 234, 262, 264, 280, 288, 312, 333, and 336) and many weak peaks with m/e 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 × 10⁵ cm/s for C<INF>15</INF>H<INF>12</INF>S<INF>2</INF>O<INF>5</INF> (m/e = 336) to 4.1 × 10⁵ cm/s for C<INF>3</INF>H<INF>3</INF>. These products continued to arrive for a period of 6−15 μs, 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/cm² suggest an enhanced decomposition of PES to small fragments in the upper part of a heated surface layer. The arrival profiles of C<INF>3</INF>H<INF>3</INF> 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.