As indicated in the brief synopsis of the matrix isolation technique that can be found elsewhere on this site, sample preparation in our previous studies of PAHs has relied on simple thermal vaporization of a solid PAH sample - i.e. warming it to its sublimation point. This technique is illustrated in the diagram below.
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Unfortunately, PAH samples become increasingly refractory as molecular size increases and, consequently, this technique is rapidly approaching the limits of its practicality. For example, pyrene (C16H10) can be deposited at a temperature of 50 oC, while coronene (C24H12) must be heated to 180 oC, and Dicoronylene (C48H20) to nearly 500 oC. At such sizes, the energy it takes to overcome intermolecular attraction (i.e. vaporization) approaches that necessary to initiate intermolecular reaction (i.e. polymerization or graphitization). Above this limit, the sample degrades rather than volatilizes. Thus, to extend our studies to much larger PAHs, a new vaporization technique is required. |
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The technique that we are currently working to incorporate into our experiments is pulsed laser vaporization (PLV). In this technique, the pulsed beam of a moderately-sized Nd/YAG laser is focused onto the surface of a solid sample, flash vaporizing the material at that point as illustrated in the following diagram. The vaporized material will travel a short distance the cold window, where it is frozen, isolated in solid argon as before.
The PLV technique is both efficient and relatively gentle - both critical concerns in the study of large PAHs. In some cases, a sample of 5-10 mg may literally represent the world’s supply of a compound and the fruit of hundreds of hours of synthetic work. The PLV configuration illustrated above will efficiently deliver gas-phase PAHs to the sample window with an absolute minimum of waste, and it will do so while inflicting minimal damage upon the vaporized molecules and the residual solid PAH sample. Specifics about our studies of large molecular PAHs are posted elsewhere on the site. |
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