Using a modified atmospheric pressure photoionization (APPI) source coupled to a tunable vacuum ultraviolet source at the SOLEIL synchrotron radiation facility, we determined the appearance energy of protonated methanol monomer and dimers at different source temperatures. The experimental data indicated that protonated monomers can be formed through the direct photoionization of neutral methanol by the second emission band of a krypton discharge UV lamp at 116.49 nm (10.64 eV) explaining why methanol can be considered as a weak dopant under APPI conditions. This point is in contradiction with the previously proposed mechanism where photoionization of methanol dimer was followed by the dissociation of the protonated methanol dimer into protonated methanol monomer.

中文翻译：

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甲醇在大气压光电离条件下是掺杂剂吗？

在 SOLEIL 同步加速器辐射设施中，使用与可调谐真空紫外源耦合的改良大气压光电离 (APPI) 源，我们确定了质子化甲醇单体和二聚体在不同源温度下的出现能量。实验数据表明，通过氪放电紫外灯的第二发射带在 116.49 nm (10.64 eV) 处，中性甲醇的直接光电离可以形成质子化单体，这解释了为什么在 APPI 条件下甲醇可以被视为弱掺杂剂。这一点与先前提出的机制相矛盾，即甲醇二聚体的光电离之后是质子化的甲醇二聚体解离成质子化的甲醇单体。