Theoretical and experimental studies on hydrogen migration in dissociative ionization of the methanol monocation to molecular ions H and HO.

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