Time-of-flight study of the ionic and neutral particles produced by pulsed-laser ablation of frozen glycerol

The emitted particles from pulsed-laser ablation (PLA), λ = 193 nm and fluence = 88-400 mJ/cm², of frozen glycerol was examined using time-of-flight mass spectrometry. The data are analyzed using supersonic molecular-beam theory and the result is interpreted using a thermal/fluid-dynamic model. Both intact and fragmented glycerol are emitted in the PLA process at all fluences and their concentration ratio is fluence dependent. Fragmentation occurs predominantly at one of the C-C bonds forming CH₂-OH (31 amu) and HO-CH₂-CH-OH (61 amu). CH₃ is produced at the target which requires the protonation of a CH₂ fragment. At fluences higher than 250 mJ/cm², ions are detected. These ions have very high velocity, >2000 m/s, and their intensity increases with fluences. PLA is thus not suitable for glycerol transfer under these conditions due to fragmentation. The data show that particle emission proceeds as a simple thermal vaporization process at fluences <200 mJ/cm². Higher fluences will yield a Knudsen layer (KL), which is formed in front of the target surface. For fluences >300 mJ/cm², particles from the KL go through unsteady adiabatic expansion prior to free flight. Models of particle and ion formation and interaction are proposed and discussed.