Time-resolving the ultrafast H2 roaming chemistry and H3+ formation using extreme-ultraviolet pulses

Abstract

The time scales and formation mechanisms of tri-hydrogen cation products in organic molecule ionization processes are poorly understood, despite their cardinal role in the chemistry of the interstellar medium and in other chemical systems. Using an ultrafast extreme-ultraviolet pump and time-resolved near-IR probe, combined with high-level ab initio molecular dynamics calculations, here we report unambiguously that H 3 + formation in double-ionization of methanol occurs on a sub 100 fs time scale, settling previous conflicting findings of strong-field Coulomb explosion experiments. Our combined experimentalcomputational studies suggest that ultrafast competition, between proton-transfer and longrange electron-transfer processes, determines whether the roaming neutral H 2 dynamics on the dication result in H þ 3 or H þ 2 fragments respectively.

Absence of Triplets in Single-Photon Double Ionization of Methanol

“…While the triplet remains stable for >1.5 ps of the NA-AIMD simulation, the longest-lived singlet trajectory dissociates after less than ∼400 fs due to the ultrafast roaming H 2 dynamics. 28 This in agreement with ultrafast H 3 + formation determined by time-resolved experimental studies. 28 , 46 In contrast, all of the 100 NA-AIMD trajectory simulations performed on the triplet ground state were found to resist CE.…”

“… 28 This in agreement with ultrafast H 3 + formation determined by time-resolved experimental studies. 28 , 46 In contrast, all of the 100 NA-AIMD trajectory simulations performed on the triplet ground state were found to resist CE.…”

“… 46 − 49 Here, a neutral H 2 separates from a HCOH 2+ dication decays by competing channels of proton capture, forming H 3 + , and long-range electron transfer (“inverse harpooning”), forming H 2 + . 27 , 28 When considering the triplet state energetics along a possible roaming H 2 pathway, we need to remember that H 2 is stable only in its singlet state, hence requiring a triplet HCOH 2+. As shown in Figure 1 , the optimized HCOH 2+ triplet state lies over 5.9 eV above that of the singlet, posing an ∼3.7 eV barrier that prevents triplet methanol dications formed in the FC geometry from dissociating via the roaming H 2 mechanism.…”

Formation of H3+ from ethane dication induced by electron impact

“…As a consequence of the displacement of hydrogen atoms, significant structural rearrangements could be triggered, leading to isomerization and thus alter the chemical functions of those molecules. In particular, for the ionic organic molecules, numerous studies have been conducted to elucidate the formation and evolution of ultrafast hydrogen migration through the sophisticated momentum imaging techniques [5][6][7][8][9][10][11][12][13][14][15][16][17] . It has been found that the hydrogen-migration channel can dominate over direct fragmentation in molecular cations 5 .…”

“…In addition to the well-known single hydrogen migration, another more complicated process, that is, the double hydrogen migration has been proven to play a more important role than generally assumed in molecular fragmentation 6 . Furthermore, in some of those hydrogen-migration studies special attention has been paid to the reaction dynamics in the course of H 3 + formation [12][13][14][15][16][17] . H 3 + has been brought into research focus by its unique structural and dynamical nature and astronomical significance as the most important interstellar medium 18,19 .…”