Visualizing hydrogen atoms migrating in acetylene dication by time-resolved three-body and four-body Coulomb explosion imaging

Abstract

The visualization of ultrafast isomerization of deuterated acetylene dication (C(2)D(2)(2+)) is demonstrated by time-resolved Coulomb explosion imaging with sub-10 fs intense laser pulses (9 fs, 0.13 PW cm(-2), 800 nm). The Coulomb explosion imaging monitoring the three-body explosion process, C(2)D(2)(3+)→ D(+) + C(+) + CD(+), as a function of the delay between the pump and probe pulses revealed that the migration of a deuterium atom proceeds in a recurrent manner; One of the deuterium atoms first shifts from one carbon site to the other in a short timescale (∼90 fs), and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation. Correlated motion of the two deuterium atoms associated with the hydrogen migration and structural deformation to non-planar geometry are identified by the time-resolved four-body Coulomb explosion imaging, C(2)D(2)(4+)→ D(+) + C(+) + C(+) + D(+).

Ultrafast isomerization initiated by X-ray core ionization

“…Isomerization of acetylene (HCCH) to form vinylidene (H 2 CC) has been studied in the neutral molecule, as well as singly and doubly charged acetylene ions 1,2,[24][25][26][27][28][29][30][31][32][33][34][35][36] . Acetylene cations are important not only as a model for molecular dynamics and radiation damage investigations, but also in a range of applications, from plasma devices 37 , combustion 38 , semiconductor manufacturing 39 and to understanding planetary atmospheres 40 .…”

“…The mechanism of the unexpectedly fast isomerization could not be probed by the measurements reported in refs 1,2, because they were not time resolved. This process has been studied in time-resolved optical laser experiments coupled with three-and four-particle coincident detection 30,32,33,36 .…”

Tabletop imaging of structural evolutions in chemical reactions demonstrated for the acetylene cation

“…Actually, proton migration pumped by 800 nm pulses has been observed solely in the dication of deuterated acetylene by Hishikawa and co-workers 12,13 . Very recently, optimal control experiments confirmed that excitation with 800 nm pulses leads to isomerization in the dication 25 .…”

“…Even though acetylene (C 2 H 2 ) has been serving as a model system for decades, proton migration dynamics in its cation is not yet fully elucidated, and has recently attracted attention of both experimental and theory groups [12][13][14][22][23][24] . Given its small size, it is an ideal candidate for the application of quantitative electronic structure theory, which can be brought to bear on its ground and excited states.…”

“…Unfortunately, these techniques are largely insensitive to the more subtle and irregular structural changes that can occur within a single small molecule undergoing a chemical reaction. Pump-probe Coulomb Explosion Imaging (CEI) allows observation of these changes on a femtosecond (fs) timescale with atomic resolution [10][11][12][13] . So far, however, important phenomena such as proton migration in the acetylene cation have only been observed using vacuum ultraviolet light (VUV) from a free-electron laser (FEL) 14 .…”