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

摘要

The dissociative ionization processes of the methanol monocation CH _(3) OH ~(+) to H _(3) ~(+) + CHO and H _(2) O ~(+) + CH _(2) 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 CH _(3) OH ~(+) to H _(3) + CHO ~(+) and CH _(3) OH ~(+) to H _(2) O + CH _(2) ~(+) are also calculated. The calculation results show that a neutral H _(2) moiety in the methanol monocation CH _(3) OH ~(+) is the origin of the formation of H _(3) ~(+) , and the ejection of fragment ions H _(3) ~(+) and H _(2) O ~(+) is more difficult than CHO ~(+) and CH _(2) ~(+) respectively. Experimentally, by using a dc-slice imaging technique under an 800 nm femtosecond laser field, the velocity distributions of fragment ions H _(3) ~(+) , CHO ~(+) , CH _(2) ~(+) , and H _(2) O ~(+) 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 CH _(3) OH ~(+) to H _(3) ~(+) + CHO, CHO ~(+) + H _(3) , H _(2) O ~(+) + CH _(2) , and CH _(2) ~(+) + H _(2) O 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 H _(3) ~(+) and H _(2) O ~(+) are lower than CHO ~(+) and CH _(2) ~(+) respectively, which is consistent with the theoretical results according to which dissociation from the methanol monocation to H _(3) ~(+) and H _(2) O ~(+) is more difficult than CHO ~(+) and CH _(2) ~(+) respectively.