Angular correlations between the three heavy-ion fragments of ethylselenol (C<sub>2</sub>H<sub>5</sub>SeH) showing the angle between selenium and singly charged carbon ion momentum vectors (<em>x</em> axis) versus the angle between the momentum vectors of the two carbon ions (<em>y</em> axis) in a triple coincidence

<p><strong>Figure 7.</strong> Angular correlations between the three heavy-ion fragments of ethylselenol (C<sub>2</sub>H<sub>5</sub>SeH) showing the angle between selenium and singly charged carbon ion momentum vectors (<em>x</em> axis) versus the angle between the momentum vectors of the two carbon ions (<em>y</em> axis) in a triple coincidence. The respective angles produced by the Coulomb explosion simulation are shown as black dots. The two black dots correspond to two possible pathways that cannot be distinguished in the measurement. In the case of Se<sup>1+</sup>C<sup>1+</sup>C<sup>1+</sup> (a) and Se<sup>2+</sup>C<sup>1+</sup>C<sup>1+</sup> (b) the two dots originate from the ambiguity of the Se–C angle for two indistinguishable C<sup>1+</sup> ions. For Se<sup>1+</sup>C<sup>2+</sup>C<sup>1+</sup> (c) and Se<sup>5+</sup>C<sup>2+</sup>C<sup>1+</sup> (d), the two dots represent the outcome of two different simulations where the C<sup>1+</sup> either starts from the position close to the Se or from the one further away.</p> <p><strong>Abstract</strong></p> <p>The ionization and fragmentation of two selenium containing hydrocarbon molecules, methylselenol (CH<sub>3</sub>SeH) and ethylselenol (C<sub>2</sub>H<sub>5</sub>SeH), by intense (>10<sup>17</sup> W cm<sup>−2</sup>) 5 fs x-ray pulses with photon energies of 1.7 and 2 keV has been studied by means of coincident ion momentum spectroscopy. Measuring charge states and ion kinetic energies, we find signatures of charge redistribution within the molecular environment. Furthermore, by analyzing fragment ion angular correlations, we can determine the laboratory-frame orientation of individual molecules and thus investigate the fragmentation dynamics in the molecular frame. This allows distinguishing protons originating from different molecular sites along with identifying the reaction channels that lead to their emission.</p>