Is methanol a dopant under atmospheric pressure photoionization conditions?

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.