LASER-ABLATION OF A PYRENE-DOPED POLY(METHYL METHACRYLATE) FILM - DYNAMICS OF PYRENE TRANSIENT SPECIES BY SPECTROSCOPIC MEASUREMENTS

The 248 nm ablation of pyrene-doped poly(methyl methacrylate) (PMMA) has been investigated by various spectroscopic and etch depth measurements with much attention to the dynamics of pyrene transient species in PMMA under intense excitation. It is shown that pyrene is rather stable even in the case that it absorbs more than several 248 nm photons during the laser pulse. It suggests that the photodecomposition of a dopant has no or Little importance in laser ablation. The lowest excited singlet (S-1) and triplet (T-1) states and the cation of pyrene were detected within and after the laser pulse by time-resolved emission and absorption spectroscopy. The broadening and red shift of the fluorescence of pyrene at 440 mJ/cm(2) suggest that not only pyrene but also the matrix PMMA becomes hot, which supports the thermally induced ablation of pyrene-doped PMMA. It is suggested that the T-1 state and cation of pyrene are produced from a highly excited state of pyrene which is produced by successive photoabsorption by the S-1 state of pyrene. It is also suggested that the temperature increase due to the photoabsorption facilitates the S-1-S-1 and T-1-T-1 annihilation and the recombination of the cation and electron or anion. A detailed analysis of transient absorbance at the excitation wavelength (248 nm) at 150 mJ/cm(2), a little below the ablation threshold, suggests that there should be a further unknown or undetected species which contributes to the observed multiphoton absorption in addition to the detected ones.