Theoretical and experimental studies on hydrogen migration in dissociative ionization of the methanol monocation to molecular ions H3+ and H2O+

Abstract

The dissociative ionization processes of the methanol monocation CH₃OH⁺ to H₃⁺ + CHO and H₂O⁺ + CH₂ are studied by ab initio method, and hydrogen migration processes are confirmed in these two dissociation processes. Due to the positive charge assignment in dissociation processes, the fragmentation pathways of CH₃OH⁺ to H₃ + CHO⁺ and CH₃OH⁺ to H₂O + CH₂⁺ are also calculated. The calculation results show that a neutral H₂ moiety in the methanol monocation CH₃OH⁺ is the origin of the formation of H₃⁺, and the ejection of fragment ions H₃⁺ and H₂O⁺ is more difficult than CHO⁺ and CH₂⁺ respectively. Experimentally, by using a dc-slice imaging technique under an 800 nm femtosecond laser field, the velocity distributions of fragment ions H₃⁺, CHO⁺, CH₂⁺, and H₂O⁺ are calculated from their corresponding sliced images. The presence of low-velocity components of these four fragment ions confirms that the formation of these ions is not from the Coulomb explosion of the methanol dication. Hence, the four hydrogen migration pathways from the methanol monocation CH₃OH⁺ to H₃⁺ + CHO, CHO⁺ + H₃, H₂O⁺ + CH₂, and CH₂⁺ + H₂O are securely confirmed. It can be observed in the time-of-flight mass spectrum of ionization and dissociation of methanol that the ion yields of fragment ions H₃⁺ and H₂O⁺ are lower than CHO⁺ and CH₂⁺ respectively, which is consistent with the theoretical results according to which dissociation from the methanol monocation to H₃⁺ and H₂O⁺ is more difficult than CHO⁺ and CH₂⁺ respectively.