Molecular Frame Photoelectron Angular Distributions as a Probe of Geometry and Auger Dissociation Dynamics

Compex Kohn variational calculations of the molecular frame photoelectron distributions (MFPADs) for 1s core ionization of CH$_4$, NH$_3$, and H$_2$O are presented for ejected electron energies below 25 eV. Surprisingly, in these three cases there are energy ranges in which the photoelectron MFPADs effectively form ``images'' of the molecular geometry. Comparison with recent momentum imaging experiments on methane at the Advanced Light Source verify this effect. Simultaneous double Auger decay in these molecules can produce dissociation into three charged fragments, e.g., CH$_2^+$ + 2 H$^+$, allowing the complete orientation of the molecule and therefore the measurement of 3D MFPADs that test these predictions. In other Auger decay channels the measurement of 3D MFPADs verifies axial recoil (prompt dissociation) or probes its absence in the Auger dissociation dynamics of small molecules.