Upon irradiation with laser

Polymers Designed to Cross-link Upon Irradiation with Laser Beams... 

FIGURE 6.3 TOF spectra depicting ionization and fragmentation of CSj molecules upon irradiation with laser pulses of (left) 10-fs duration and (right) 40-fs duration. Note the relative absence of atomic fragments in the former case. The peak laser intensity was adjusted to be -40 TW cm in both cases. 

Clathrate. 2. Any molecular entity that is nonplanar and often polycyclic that encloses a cavity. 3. Photosensitive precursors that generate chemically reactive species upon irradiation with a laser or flash-lamp. See Photore-... 

For our studies, we chose the potassium dimer (K2) as a molecular prototype system, since K2 can be treated experimentally [73,74] and theoretically [83,116, 117] on a highly accurate level. The relevant potential energy surfaces (PES) of K2 are shown in Figure 6.16a. Before the interaction with the laser, the molecule resides in the ground state Upon irradiation with a weak 790 nm, 25 fs... 

Figure 3. Top. Ion/molecule reaction product formed by reaction of cis-dichloro-trans-dihydroxo-bis-2-propanamine platinum (IV) with its 50 eV electron impact fragments. Bottom. Dissociation pattern produced upon irradiation with cw CO2 laser, same experimental conditions and delay times as in the top spectrum. (Reproduced from ref. 18. Copyright 1987 American Chemical Society.)...

Dissociation obtained upon irradiation with cw CO2 laser, same... 

Figure 7. Top. (M-H)- ion of sucrose formed by desorption with pulsed CO2 laser beam. Bottom. Dissociation upon irradiation with the cw CO2 laser. Experimental conditions are same as in the top spectrum.

Figure 9. Top. Negative fragment ion formed from the substituted sulfonamide by laser desorption using pulsed CC>2 laser. Bottom. Dissociation upon irradiation with a cw CO2 laser, all other conditions identical to those in the top spectrum.

Peroxide Sensitizers. Poly(methylmethacrylate), PMMA, is the first polymer reported to exhibit an increase in refractive index upon irradiation with uv light (41). Irradiation at either 325 nm (Cd laser) or 365 nm (Hg arc) was found to give an index increase of up to 0.3%, and the resolution, determined from gratings, was shown to be at least 5000 lines/mm. Applications to optical waveguides and holographic elements were suggested. This phenomenon was a serendipitous discovery. The PMMA had been intended for use as an inert matrix for reversible photodimerization when its own photosensitivity was uncovered. 

Recently, Shimizu et al. [30] reported an efficient enantioselective destruction of the L isomer of a o/L-tartaric acid mixture, using focused r-CPL from an XeF excimer laser. Initial attempts to destroy optically pure samples of each enantiomer resulted in a decrease of the L-tartaric acid sample by 14% when irradiated with 1 X 10 J of focused r-CPL at 351 nm, and photodestruction products such as CO, CO2, H2, etc., were observed. Unusually, the D isomer (1) was found to remain almost unchanged upon irradiation with the same light source. Thus, treatment of a racemic mixture with r-CPL afforded a sample enriched in 1, which had an op of 7.5%, a result which is exceptionally high for this type of process (see Scheme 1). It was observed that when unfocused r-CPL was used, there was hardly any decrease in the op of the L-tartaric acid (0.11%),... 

Ablation with femtosecond pulses is comprehensively reviewed by Kruger and Kautek in this issue. The present discussion focuses exclusively on photoproduct formation in the irradiation of Arl-doped systems with fs pulses. This examination indicates several subtle mechanistic possibilities. Upon irradiation with 500-fs pulses at 248 nm, only ArH-like product formation is observed as the fluence is raised above the ablation threshold (Fig. 17). Recombination (e.g., Nap2-type) or other by-products are not observed even for dopant concentrations as high as 4 wt% [83-84]. In contrast, at low flu-ences, a number of by-products are observed after irradiation with few laser pulses. In this case, the accumulating radicals evidently react with each other to produce ill-defined products. Most interestingly, ill-defined species are not observed above the threshold even after extensive irradiation (Fig. 17). It is clear that photochemical processes in the ablation with fs pulses must differ distinctly from those in ns ablation. 

In Fig. 14.3, the decomposition pathway of PMMA upon irradiation with UV laser light is shown [78] as it has been defined from products detected with time-of-flight mass spectroscopy (TOF-MS). The first three steps can be associated with incubation, whereas in step 4 depolymerization takes place. Incubation, as observed for PMMA, is often present in polymers that show weak absorption at the irradiation wavelength... 

Further, a large photoinduced birefringence intensity is found as well as a good reversible optical storage upon irradiation with a 532 nm Nd YAG laser [96]. 

---