Imaging Molecular Isomerization Using Molecular-Frame Photoelectron Angular Distributions

Techniques such as X-ray diffraction and ultrafast electron diffraction can potentially be taken to the time domain to image chemical reactions on their natural timescale. Photoelectron diffraction from fixed-in-space molecules, where an electron is launched from an inner shell by photoabsorption, offers a similar promise. We illustrate the idea here with the results of {\it ab initio} calculations using the complex Kohn variational method of molecular-frame photoelectron angular distributions (MFPADs) on the acetylene monocation (HCCH$^+$). Photoionization of neutral acetylene, which is linear at equilibrium, in the 20-40 eV range produces ground (X) and excited (A) HCCH$^+$ in roughly equal amounts. The electronically excited A-state cation can follow a downhill path to a conical intersection with the X-state near a trans-symmetric geometry and from there to a vinylidene (H$_2$CC) isomeric structure. We will show that the MFPADs produced by C k-shell photoionization of HCCH$^+$, while relatively insensitive to the electronic configuration of the valence electrons at a given photoelectron energy, are much more sensitive to nuclear geometry and can therefore be used to track the acetylene to vinylidene isomerization.