A Comparative Study of the UV Laser Ablation of Van Der Waals Films of Benzene Derivatives

Abstract

Ablation of thick (≈ 15 μm) films of C₆H₆, C₆H₅CH₃ and C₆H₅CI at 248 nm and 193 nm is studied by means of time-of-flight quadrupole mass spectrometry. The dependence of the desorbate most probable translational energies on laser fluence is determined over the ≈20–200 mJ/cm² range. In all cases, the corresponding diagrams are found to exhibit “plateaus”, in accord with the report by Braun and Hess [J. Chem. Phys. 99 (1993) 8330]. However, no specific correlation with the thermodynamic properties of the compounds is observed, thereby questioning the attribution of the “plateaus” to phase transformation of the films under ablation conditions. A high sensitivity of the distributions and intensities on the rate of deposition and the irradiation history of the films is observed, indicating the importance of the matrix “structure” for the distribution of the absorbed energy. On the other hand, the analysis of the total translational energies of the desorbates suggests that during ablation, efficient energy transfer occurs in the film. This possibility is further demonstrated by the observation of high translational energies and sputtering yields for C₆H₁₂(nonabsorbing at 248 nm) condensed in thickness of ≈ I μm on top of C₆H₅CH₃ films. These observations can be qualitatively explained in terms of the collisional sequence model. Alternatively, a photothermal model may be applicable under the provision that energy distribution in the films is limited due to imperfections introducing barriers (bottlenecks) to its ‘flow’.