(a) Measured distribution of angles between C and Se ion momentum vectors detected in coincidence for methylselenol (CH<sub>3</sub>SeH)

<p><strong>Figure 6.</strong> (a) Measured distribution of angles between C and Se ion momentum vectors detected in coincidence for methylselenol (CH<sub>3</sub>SeH). The distributions are peaked around cos(α<sub>Se,C</sub>) = –1 corresponding to 180° between the momentum vectors of the ions ('back-to-back' emission). α is the angle between the momentum vectors of the ionic fragments. (b) Angular distribution of protons relative to the Se–C axis as defined by the direction of the carbon ion momentum. The peaks centred around cos(α<sub>C,H</sub>) = –0.4 and cos(α<sub>C,H</sub> ) = + 0.4 correspond to the Se-H and the C-H angles of 114° and 66°, respectively. The integrals of the two peaks also show a 3:1 ratio corresponding to the numbers of protons bound to carbon and selenium. (c), (d) Kinetic energy distributions for protons with C and Se neighbours, respectively, detected in a triple coincidence with (c) Se<sup>2+</sup> and C<sup>2+</sup> or (d) Se<sup>5+</sup> and C<sup>2+</sup> .</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>