Fluence, photodecomposition

Photodecomposition. A greyish-white film of solid material slowly formed along the bottom of the sample cell in PuF6 cells irradiated at 1064 nm. A similar film formed considerably faster in sample cells irradiated at 532 nm with the film forming on the entrance face of the cells as well as along the walls of the cell. Film formation was evident after less than a minute of irradiation at a laser fluence of 5 J/cm2 at 532 nm. The exact chemical composition of the film has not yet been determined. It is known that it contains plutonium and... 

PhotoDecomposition (APD) that occurs above a well-defined ablation threshold usually leads to removal of polymer material leaving a new fresh "clean" surface with the same composition as the untreated material. However, in the case of PET (JL) both composition and structural modifications occur even for fluences below the ablation threshold. 

Little has been published concerning the photodynamics of PuFe gas, although this compound was first synthesized in 1942. We recently reported the first observation of fluorescence from electronically excited PuFe and found its behavior to differ significantly from that of UFe or NpFe The photophysics of PuFe excited at 532 nm and at 1064 nm has now been observed in detail using laser-induced fluorescence techniques. The fluorescence emission spectra recorded are the same at both excitation wavelengths with the fluorescence intensity peaking at about 2300 nm. The fluorescence decay of PuFe gas excited at 532 nm was found to be laser-fluence dependent and a mechanism is proposed which accounts for this observation. Net photodecomposition of PuFe was rapid at a fluence of 5 J/cm at 532 nm (7 ns pulse width). 

It was demonstrated with surface analysis techniques that the polymer surface is modified selectively with different laser irradiation wavelengths. The two laser energy regimes, above and below the threshold for laser ablation, reveal pronounced differences. For both irradiation wavelengths (248 and 308 nm) the polymer surface modification is solely chemical after treatment with fluences below the threshold. Each irradiation wavelength leads to a surface oxidation, as shown with the contact angle and XPS measurements. The oxidation is a result of the radical pathway of photodecomposition of the triazene chromophore. 

Thus it is not clear how much of the apparent difference in emission threshold for N2 and phenyl radicals in Fig. 44 is due to different emission intensities versus different detection probabilities. Since two N2 molecules must be emitted to release one phenyl radical, some difference in fluence dependence is expected. For both species, the intensity rises above the background in approximately power law fashion. Finally, at still higher fluences, the intensity rolls over, growing more slowly with increasing fluence. The slopes of the power-law portions of the two plots are 8 for N2 and 12 for the phenyl radical, respectively. Although we could suggest that this corresponds to an eight- and 12-photon process (either multiphoton or multiple photon) this seems totally unacceptable and unreasonable. Furthermore, it is inconsistent with a simple photodecomposition mechanism, where we... 

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