Fragmentation pathways for selected electronic states of the acetylene dication

Abstract

Coincident measurement of the Auger electron and fragment ion momenta emitted after carbon core-level photoionization of acetylene has yielded new understanding of how the dication fragments. Ab initio calculations and experimental data, including body-frame Auger angular distributions, are used to identify the parent electronic states and together yield a comprehensive map of the dissociation pathways which include surface crossings and barriers to direct dissociation. The Auger angular distributions for certain breakup channels show evidence of core-hole localization.

Ultrafast isomerization initiated by X-ray core ionization

“…2 used non-resonant core ionization, the same dicationic state distribution is created in both experiments. By measuring the energy of the Auger electrons simultaneously, Osipov et al 2 concluded that the CH þ /CH þ channel originates from excited electronic configurations of the dication, while the CH þ 2 =C þ channel comes from the 1p À 2 u configuration (see Fig. 1).…”

“…This relaxation begins before interruption by the Auger relaxation. The extent of the geometry change during the core excitation depends on the corehole lifetime and is typically neglected in considering core ionization 2,4,41 . This approximation is better for cases of core excitation of nuclei with fast Auger relaxation rates, as well as for heavier nuclei that experience unscreened fields.…”

“…Measurements reported by Osipov et al 1,2 indicate that, when initiated by X-ray core ionization, this isomerization can occur on a timescale shorter than 60 fs. This ultrafast rate was unexpected based on the predicted barrier heights in the electronic states populated in relaxation on core ionization 3,4 .…”

“…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 .…”

“…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 . Isomerization of the dication has been investigated in synchrotron experiments 1,2,27,28,34 . Osipov et al 1,2 performed ion-ionphotoelectron and ion-ion-Auger electron coincidence measurements to study the fragmentation of the dication formed on core ionization of acetylene.…”