We report the results of ab initio calculations on valence photoionization of the acetylene monocation in its ground ($X$) and electronically excited ($A$) states using the complex Kohn variational method. In contrast to the $K$-shell molecular frame photoelectron angular distributions (MFPADs), which are only sensitive to the molecular geometry, these results show that the valence MFPADs are also sensitive to the electronic state of the target. Hence, the isomerization path from acetylene to vinylidene, which is mediated by a conical intersection responsible for a change in electronic state of the monocation, could, in principle, be traced through valence photoionization.

• Received 21 June 2012

DOI:https://doi.org/10.1103/PhysRevA.86.013425