Ultrafast Extreme Ultraviolet Induced Isomerization of Acetylene Cations

Abstract

Ultrafast isomerization of acetylene cations (½HC ¼ CH þ ) in the low-lying excited A 2 AE g þ state, populated by the absorption of extreme ultraviolet (XUV) photons (38 eV), has been observed at the Free Electron Laser in Hamburg, (FLASH). Recording coincident fragments C þ þ CH 2 þ as a function of time between XUV-pump and -probe pulses, generated by a split-mirror device, we find an isomerization time of 52 AE 15 fs in a kinetic energy release (KER) window of 5:8 < KER < 8 eV, providing clear evidence for the existence of a fast, nonradiative decay channel.

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 .…”

“…In the singly charged ion the ultrafast isomerization is not launched with 800 nm radiation, but has been demonstrated to proceed from the excited A 2 AE þ g state 26,35 . Core ionization, which was employed in the current work for initiating isomerization, occurs in the weak-field regime, unlike the strong-field 800 nm experiments.…”

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

“…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 .…”

“…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.…”

“…To efficiently launch dynamics in excited states of molecules with a large energy gap between ground and excited states, it is common to use high photon energy sources such as FELs (acetylene) 14 or HHG (ethylene) 19 . If sub-fs resolution is required, one can use an XUV pump/XUV probe arrangement based on HHG for this kind of experiments 20,21 , however, at the price of a highly demanding experimental setup.…”