Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada.
Phys Chem Chem Phys. 2013 Jan 21;15(3):940-9. doi: 10.1039/c2cp43098c.
The fragmentation of methanol, water, dimethyl ether, and acetic acid clusters upon photoionization with a single vacuum ultraviolet (VUV) photon of 10.1 eV, 13.3 eV, or 17.5 eV energy is studied with mass spectrometry. The sodium-doping method is used as an independent approximate measure of the original cluster size distribution providing information on the degree of fragmentation upon VUV ionization. The experimental results show strong fragmentation for (CH(3))(2)O and CH(3)CO(2)H clusters but minor fragmentation for H(2)O and CH(3)OH clusters. The pronounced cluster decay for (CH(3))(2)O and CH(3)CO(2)H is explained by additional intracluster chain reactions that occur after the initial fast proton transfer in the ionic clusters, i.e. the decay of (CH(3))(2)O molecules into H(2), CO, and CH(4) catalyzed by the methoxymethyl radical, and the decay of CH(3)CO(2)H molecules into CO(2) and CH(4) catalyzed by the acetyloxy radical. The absence of equivalent reaction cycles in ionic H(2)O and CH(3)OH clusters after the fast proton transfer is consistent with the much less pronounced cluster fragmentation observed upon VUV ionization. The study shows that VUV photoionization even at threshold cannot in general be considered a soft ionization method for weakly-bound clusters, largely because of potential intracluster reaction.
用 10.1eV、13.3eV 或 17.5eV 能量的单个真空紫外 (VUV) 光子对甲醇、水、二甲醚和乙酸簇进行光致电离，并用质谱法研究其碎裂情况。采用钠掺杂方法作为原始簇大小分布的独立近似测量方法，提供 VUV 电离时碎裂程度的信息。实验结果表明，(CH(3))(2)O 和 CH(3)CO(2)H 簇发生强烈碎裂，而 H(2)O 和 CH(3)OH 簇发生轻微碎裂。(CH(3))(2)O 和 CH(3)CO(2)H 的明显簇衰减解释为初始快速质子转移后在离子簇中发生的额外簇内链反应，即甲氧基甲基自由基催化的 (CH(3))(2)O 分子分解为 H(2)、CO 和 CH(4)，以及乙酰氧基自由基催化的 CH(3)CO(2)H 分子分解为 CO(2)和 CH(4)。在快速质子转移后，离子 H(2)O 和 CH(3)OH 簇中没有等效的反应循环，这与 VUV 电离时观察到的簇碎裂程度明显降低一致。研究表明，VUV 光致电离即使在阈值处也不能普遍被认为是弱结合簇的软电离方法，主要是因为潜在的簇内反应。
