Photochemical Cleavage of a Polymeric Solid: Details of the Ultraviolet Laser Ablation of Poly(methyl methacrylate) at 193 and 248 nm

Abstract

The products of the laser ablation of poly(methyl methacrylate) (PMMA) (Mn ~ 800 000) at 193 or 248 nm range from C2 through methyl methacrylate (MMA) and a solid that is a low molecular weight (Mn= 2500) fraction of PMMA. While the products are the same at both wavelengths, the mix is quite different. At 193 nm, 18% of the ablated polymer is MMA, whereas at 248 nm less than 1% of the polymer appeared as MMA. A semilogarithmic plot of the mass of material removed vs. the fluence shows three distinct regions. The central portion at each wavelength (80–300 mJ/cm2 at 193 nm, 600–2000 mJ/cm2 at 248 nm), which corresponds to rapid etching, is identified as “ablative photodecomposition”. At lower fluences, the etching efficiency falls off rapidly, indicating that there is a threshold fluence. At fluences above the range for ablative photodecomposition, the etching levels off, probably due to the secondary absorption of the incoming photons by the products. The formation of C2 as a product was monitored by laser-induced fluorescence. The velocity distribution of the product as a function of fluence was also measured. The velocity distribution approaches but does not exactly fit a Maxwell-Boltzmann equation. Average translational energies as high as 6 eV were recorded even at the fluence threshold for this product. It is suggested that ablative photodecomposition involves both a one-photon process, which produces MMA and low molecular weight polymeric fragments, and a multiphoton process, which gives rise to products such as C2 with high translational energy. At longer wavelengths, a greater temperature rise within the ablated volume may be necessary to increase the quantum yield for bond-breaking. The mass of material ablated per joule of energy absorbed in the ablated volume was remarkably similar at both wavelengths. Ablative photodecomposition is a novel method to cleave an organic solid. In contrast to alternative methods such as mechanical pressure or thermal decomposition, both of which occur in the ground electronic state of the bonds undergoing rupture, ablative photodecomposition involves the electronically excited state of the bond that is broken. It may therefore limit secondary effects of the cleavage to a highly localized region, a feature that can be of considerable interest in living systems. © 1986, American Chemical Society. All rights reserved.