Understanding Selectivity in Product Distributions from Laser Ablation of Organic Liquids

Pulsed laser ablation in organic solvents is widely used to produce oxide-free metal and metal carbide nanoparticles, often with carbon coatings resulting from laser-induced reactions in the organic solvent. To gain insight into how the molecular structure of the solvent affects these reaction pathways, this work investigates ablation of the C₆H₁₄ isomers <i>n</i>-hexane, 2-methylpentane, and 3-methylpentane through characterization of the gas and liquid products with mass spectrometry. Ablation of each C₆H₁₄ isomer produces a distinct distribution of product molecular weights and isomers. 2-methylpentane preferentially produces C₃ and C₉, whereas 3-methylpentane produces C₂, C₄, C₈, and C₁₀ products. These preferential product distributions, along with the lack of such selectivity in <i>n</i>-hexane, arise from differences in the most favorable C-C bond scission pathways in each C₆H₁₄ isomer. Moreover, the particular isomers of C₈H₁₈, C₉H₂₀, C₁₀H₂₂, and C₁₂H₂₆ produced by ablation of each C₆H₁₄ isomer indicate that the vast majority of reaction pathways involve addition reactions between a fragment radical and parent C₆H₁₄ or between two C₆H₁₄ molecules, without molecular rearrangement. This propensity toward direct addition suggests that the chemical reactions induced by ultrashort pulsed laser ablation proceed on faster time scales than those of radical rearrangements.