(a) Ion time-of-flight spectra resulting from the ionization and fragmentation of methylselenol (CH<sub>3</sub>SeH) and ethylselenol (C<sub>2</sub>H<sub>5</sub>SeH) molecules by 5 fs-XFEL pulses with a nominal pulse energy of 0.4 mJ and a photon energy of 2.0 keV (methylselenol, red dashed line) and 1.7 keV (ethylselenol, blue solid line)

<p><strong>Figure 2.</strong> (a) Ion time-of-flight spectra resulting from the ionization and fragmentation of methylselenol (CH<sub>3</sub>SeH) and ethylselenol (C<sub>2</sub>H<sub>5</sub>SeH) molecules by 5 fs-XFEL pulses with a nominal pulse energy of 0.4 mJ and a photon energy of 2.0 keV (methylselenol, red dashed line) and 1.7 keV (ethylselenol, blue solid line). The two spectra are normalized on the Se<sup>1+</sup> peak such that differences in the other peak heights represent relative differences with respect to Se<sup>1+</sup>. The changes in the background signal (indicated by the arrows) are due to the higher residual gas background in the methylselenol spectrum. (b) Photoion–photoion coincidence (PIPICO) spectrum of methylselenol taken under the same experimental conditions as in (a). The two insets are zooms into low (green) and high charge channels (red). For strong channels, the five main isotopes of selenium can be identified in the spectrum as distinct parallel lines.</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>