PHOTOACOUSTIC DETECTION OF THE DECOMPOSITION KINETICS OF POLYMERS - INTERPRETATION OF ACOUSTIC-SIGNALS

A broadband piezoelectric PVDF foil transducer was used for time-resol ved detection of the acoustic signals generated by laser-induced ablat ion of polyimide. The fast signal observed within the time resolution of the detection system of several nanoseconds was assigned to thermoe lastic excitation. The second, strongly fluence-dependent acoustic sig nal with a delay varying between 5 and 500 ns was observed for the fir st time and interpreted as due to the recoil momentum of the ablating species. The corresponding delay times measured for polyimide and KrF (248 nm) and XeCl (308 nm) radiation correlate with the deposited ener gy density. This interpretation is not in agreement with previous assi gnments and points to much longer delays for the ejection of particles at low fluences.