Laser Mass Spectrometry with Circularly Polarized Light: Circular Dichroism of Molecular Ions

In recent experiments of resonance‐enhanced laser ionization, large differences between circular dichroism measured for molecular and fragment ions were found by several research groups for different molecular systems. In the case of 3‐methylcyclopentanone (3‐MCP) we attributed this effect to a large circular dichroism of the molecular ion. In the work presented here, this effect in 3‐MCP is studied by ion spectroscopy, by varying the neutral intermediate excited state involved in resonance‐enhanced multiphoton ionization (REMPI) and by performing REMPI‐induced measurements of circular dichroism at different laser pulse energies. It turns out that the dynamics of structural changes in the ionic ground state strongly influences the observed ionic circular dichroism. Excitation with circularly polarized laser light allows measurement of circular dichroism of molecular ions. Fast structural changes of these molecular ions show up in the size of their circular dichroism (see picture). This may be useful for enantiosensitive mass analysis as well as for investigation of molecular dynamics.