Fig. 4

(a) and (b) are the Newton-frame covariance maps of the momenta of \(\hbox {C}_2 \hbox {H}_2^+\) and \(\hbox {C}_3 \hbox {H}^+\) relative to \(\hbox {SeH}_y^+\) (with \(y=\) 0 or 2), calculated without momentum constraints. In each case, the momentum corresponding to the remainder of the molecule (‘\(\hbox {C}_2 \hbox {H}_2\)/\(\hbox {C}_2\)’ or ‘\(\hbox {CH}_3\)/CH’ assuming a three-body fragmentation process) is plotted in the bottom half of the image. Momentum constraints, as described in the main text, were applied to extract the components of the overall distributions that correspond to different primary fragmentation mechanisms; (c) and (d) are the distributions obtained for mechanisms where \(\hbox {SeH}_y^+\) is a primary product (\(\hbox {SeH}_{y,\textrm{I}}^+\)), while (e) and (f) are the distributions obtained when it is a secondary product (\(\hbox {SeH}_{y,\textrm{II}}^+\)). The Newton-frame covariance maps are all normalized to their own maxima, and a Gaussian blur has been added to mitigate the low signal-to-noise ratios of these channels.