Circular dichroism in molecular frame photoemission

Abstract

The paper reports on a comparative experimental and theoretical study of circular dichroism in electron angular distribution (CDAD) in the molecular frame (MF) of linear molecules photoionized by circularly or elliptically polarized light. The CDAD is derived from the analysis of the complete molecular frame angular distribution (MFPAD) I(χ,θ_e,φ_e ), where χ is the orientation of the molecule with respect to the light propagation axis and (θ_e,φ_e ) the electron emission direction in the MF, using the vector correlation method. The CDAD is quantified by the θ_e dependence of the left–right emission asymmetry maximum in the plane perpendicular to the light propagation axis k, for a space fixed molecule orthogonal to k. The experimental results for selected valence shell photoionization (PI) reactions in NO, O₂, N₂O compare very well with the multichannel Schwinger configuration interaction (MCSCI) ab initio calculations. Combined with a simple model of the ionization process, including the partial-wave composition of the initial state and phase shifts estimated from quantum defects for the various scattering partial-waves, these results provide the basis for a general discussion of the circular dichroism effect. This study enables one to disentangle the influence of the (spσ, pπ…) initial valence shell ionized orbital and that of the scattering dynamics on some fingerprint properties of the CDAD. On the other hand, the CDAD for sσ K-shell is purely assigned to a final state scattering effect. This analysis will be extended to PI of non-linear molecules where the circular dichroism characterizes recoil frame photoelectron angular distributions (RFPADs).

Keywords:

• Photoionization of molecules
• Circular dichroism
• Molecular frame photoemission

Acknowledgments

We are very grateful to L. Nahon, beamline scientist for SU5 at Super-ACO (future DESIRS beamline at the synchrotron center SOLEIL), for fruitful discussions on PECD. We thank H. Lefebvre-Brion for a nice discussion about the spectroscopy of NO⁺. We thank B. Pilette at LURE for technical assistance, as well the machine department staff for operating Super-ACO (LURE). The support of the National Science Foundation (USA) through Grant. No. INT-0089831 and the support of the Centre National de la Recherche Scientifique (France) are gratefully acknowledges. RRL acknowledge the support of the Robert A. Welch Foundation (Houston, Texas) under grant A-1020.