The fluorescence lifetime of rhodamine B in a series of normal alcohols (C"H2^.1OH, -1-6) was measured as a function of temperature. The nonradiative rate constants were calculated from the fluorescence lifetimes and quantum yields. Activation energies were obtained from Arrhenius plots of the nonradiative rate constant. The variation of the nonradiative rate constant with solvent polarity and temperature was consistent with a photophysical mechanism that involves equilibrium between the planar and twisted configurations of the diethylamino groups on the xanthene ring of rhodamine B and internal conversion from the twisted configuration. The activation energy is equal to the free energy difference between the twisted and planar configurations. The solvent polarity dependence of the free energy difference and of the rate constant for internal conversion from the twisted configuration determines the variation of the nonradiative rate constant with solvent. When solvent polarity effects are taken into account by using the parameter £T(30), the nonradiative rate constant shows weak or no dependence on the solvent viscosity.

The term ‘‘incubation pulses’’ is applied to the pulses of ultraviolet laser radiation which are directed initially at a polymer surface and which etch less depth per pulse than subsequent pulses which remove identical depths of material at each pulse. This incubation effect which is particularly strong when the polymer absorbs moderately at the laser wavelength has been studied in polymethyl methacrylate at 193 and 248 nm. The transformation of the surface during incubation pulses has been followed by 1) photographing the blast wave that is produced by the products that emerge from the surface, using a fast (<1 ns) dye laser that is electronically triggered to light the ablation plume after a set delay, and 2) by treating the exposed surface with a solvent to dissolve out any photolyzed material that is left in the exposed area. The results show that ablation, as shown by the ejection of the products at high velocity, occurs even during the incubation period but the surface is not etched at all or only partially. The etch pit is filled with chemically transformed material which is removed by dipping the sample in a solvent. In the photochemical model of Sutcliffe and Srinivasan [J. Appl. Phys. 60, 3315 (1986)], if UV laser ablation is seen as a volume explosion, then incubation can be attributed to insufficient production of the gaseous products that are needed to build up sufficient pressure to eject the large fragments into which the solid is largely broken up.

Ablation and etching of the surface of polymethylmethacrylate (PMMA) by pulses of 248 nm laser radiation [∼20 ns full width at half maximum (FWHM)] have been probed by pulses of visible laser radiation (596 nm; <1 ns FWHM). The results were recorded photographically. Modification of the surface structure of the polymer is first visible at 12 ns and appears to be complete in about 60 ns. Emerging solid material, preceded by a shock wave which first becomes visible at 60 ns, reaches a maximum in intensity at 6 μs and continues until about 20 μs. The average velocity of the solid material, which is probably a low molecular weight polymer of PMMA, is 1.5×104 cm/s.

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Laser irradiation effects on the CdTe/ZnTe quantum dot structure studied by Raman and AFM spectroscopy J. Appl. Phys. 112, 063520 (2012) Formation of nanostructured TiO2 by femtosecond laser irradiation of titanium in O2 J. Appl. Phys. 112, 063108 (2012) Ultralow-power local laser control of the dimer density in alkali-metal vapors through photodesorption Appl. Phys. Lett. 101, 091107 (2012) Optical absorption of silicon nanowires J. Appl. Phys. 112, 033506 (2012) Crystallization of fused silica surfaces by ultra-violet laser irradiation