Velocity map imaging of dissociative ionization of ICl in femtosecond laser field

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

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.

Hydrogen migration processes of methanol monocation CH₃OH⁺ to H₃⁺, COH⁺, H₂O⁺ and CH₂⁺ were studied theoretically and experimentally.

BrCl+ elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields

By using a dc-slice imaging technique, photodissociation of 1,2-C₂H₄BrCl was investigated at 800 nm looking for heteronuclear unimolecular ion elimination of BrCl⁺ in an 80 fs laser field. The occurrence of fragment ion BrCl⁺ in the mass spectrum verified the existence of a unimolecular decomposition channel of BrCl⁺ in this experiment. The relative quantum yield of the BrCl⁺ channel was measured to be 0.8%. By processing and analyzing the velocity and angular distributions obtained from the corresponding sliced images of BrCl⁺ and its partner ion C₂H₄⁺, we concluded that BrCl⁺ came from Coulomb explosion of the 1,2-bromochloroethane dication 1,2-C₂H₄BrCl²⁺. With the aid of quantum chemical calculations at the M06-2X/def2-TZVP level, the potential energy surface for BrCl⁺ detachment from 1,2-C₂H₄BrCl²⁺ has been examined in detail. According to the ab initio calculations, two transition state structures tended to correlate with the reactant 1,2-C₂H₄BrCl²⁺ and the products BrCl⁺ + C₂H₄⁺. In this entire dissociation process, the C–Br and C–Cl bond lengths were observed to elongate asymmetrically, that is, the C–Br chemical bond broke firstly, and subsequently a new Br–Cl chemical bond started to emerge while the C–Cl bond continued to exist for a while. Hence, an asynchronous concerted elimination mechanism was favored for BrCl⁺ detachment.

Concerted elimination of the molecular ion BrCl⁺ from Coulomb explosion of 1,2-bromochloroethane was studied theoretically and experimentally.

Coulomb explosion and dissociative ionization of 1,2-dibromoethane under an intense femtosecond laser field

Coulomb explosion and dissociative ionization of 1,2-dibromoethane are experimentally investigated in a near-infrared (800 nm) femtosecond laser field by dc-slice imaging technology.