Nonlinear Photophysics and Ablation of Liquid Naphthalene Derivatives: Fluence-Dependence of Luminescence Spectra upon 248 nm Laser Excitation

Abstract

The ablation of liquid naphthalene derivatives (1-methylnaphthalene and 1-chloronaphthalene) induced by 248-nm pulsed excimer laser irradiation was investigated by means of a photoacoustic technique and by nanosecond time-resolved luminescence/absorption spectroscopy. Cavitation (ablation) was observed in both of the liquids when the laser fluence was increased beyond a certain threshold. During the photoacoustic measurements, the ablation threshold was evaluated to be 20 mJ/cm2 for both of the liquids. In the time-resolved luminescence spectroscopy, two spectral components were detected both above and below the threshold: the fluorescence of naphthalene monomers and the fluorescence of excimers. The former component is conspicuous in the later temporal stage after the excitation. One characteristic feature observed in the luminescence spectroscopy below the threshold region was that the ratio of the intensity of the monomer fluorescence to the excimer fluorescence increased with increasing laser fluence. This behavior, that is, the increase in the monomer component, was also induced by raising the temperature of the liquids. No chemical intermediates such as radicals (which would suggest photochemical fragmentation) were detected by the transient absorption spectroscopy during ablation. These results indicate that a photothermal effect is dominant for laser ablation in liquid naphthalene derivatives. The temperature at the ablation threshold was estimated on the basis of the intensity ratio of the monomer and excimer fluorescence components and the Boltzman equilibrium.