Thermal confinement and temperature-dependent absorption in resonant infrared ablation of frozen liquid targets

The mechanism of matrix-assisted resonant infrared laser ablation of frozen aqueous and methanol solutions of polyethylene glycol was investigated by time-resolved plume shadowgraphy and ablation yield measurements. A picosecond free-electron laser was tuned to two wavelengths resonant with the target matrixes, one (2940 nm) that is resonant with the-OH stretch in both liquid water and methanol and the other (3450 nm) that is resonant with the-CH stretch in methanol. The plume images showed gross similarities, differing only in the time required for the shockwave to appear and in the velocity of the shock front. Primary material ejection typically commences 15-25 μs after the ablation laser pulse arrives and lasts for hundreds of μs. In all three cases studied here, the ablation plume appears to consist entirely of vapor with no droplets or solid particles. The ablation yield is found to be linear in fluence for frozen methanol but quadratic in fluence for frozen water. This dependence can be understood by considering thermal diffusion in the targets and the temperature dependence of the absorption coefficient.