Velocity profiles of species ejected in ultraviolet laser ablation of several polymers examined by time-of-flight mass spectroscopy

Abstract

Velocity distributions of molecular species ejected by ∼80 mJ/cm2, 266-nm laser ablation of polycarbonate, polyimide, poly(ethylene terephthalate), and poly(α-methylstyrene) are presented and discussed. Time-of-flight mass spectroscopy in conjunction with both 248- and 193-nm laser ionization was used to probe the escaping vapor. Up to three distinct waves of material pass through the ionization zone. The fastest wave (6–8×105 cm/s) appears to consist of highly degraded species such as C3; the arrival profiles are well fit by a velocity offset Maxwell–Boltzmann distribution with offsets typically 3–6×105 cm/s and transverse temperatures above 10 000 K. The second wave has a characteristic velocity of 1–2×105 cm/s, and, except with the poly(α-methylstyrene) target, the associated material is not cleanly ionized to parent ions under our typical conditions. It is hypothesized that this wave consists of hot, fairly heavy (up to a few hundred amu) radicals. The slow wave (2–5×104 cm/s) is composed of stable molecules which do not readily condense on the chamber walls. Its arrival profile is too broad to be described by a simple Maxwell–Boltzmann velocity distribution. A mechanism involving a thermal velocity distribution combined with laser-associated background vapor might explain the broad profiles. Problems related to the largely unknown and highly variable ionization cross sections of diverse organic molecules with 193- and 248-nm light are briefly discussed.