Laser mass spectrometry with circularly polarized light: two-photon circular dichroism.

Extensive studies of two-photon circular dichroism (TPCD) on 3-methylcyclopentanone are presented following the first TPCD experiment of gas phase molecules by Compton et al. [J. Chem. Phys.125 (2006) 144304]. (2 + 1)-Multiphoton ionization in a specially designed time-of-flight mass analyzer has been used to perform these studies. CD of two-photon transitions from the molecular ground state to low lying Rydberg states is strongly enhanced with respect to corresponding one-photon transitions in good agreement with Compton. Differences between CD values determined via the molecular ion and via fragment ions indicate strong molecular ion CD effects. This would be the first time that circular dichroism of isolated molecular ions has been measured.