The role of the intermediate state in angle-resolved photoelectron studies using (2+1) resonance-enhanced multiphoton ionization of the chiral terpenes, α-pinene and 3-carene

Photoelectron angular distributions (PADs), ranging up to the maximum 6th order Legendre polynomial term set by the Yang theorem, have been recorded for the (2 + 1) resonance enhanced multiphoton ionization (REMPI) of two terpene isomers, 3-carene and alpha-pinene, employing femtosecond lasers and electron velocity map imaging detection. PAD measurements made with coincident photoion detection allow ion fragmentation effects to be assessed. Using circular polarization and enantiomerically pure samples the PAD measurements are extended to include chiral (odd) Legendre polynomial terms, and these are analysed and discussed as multiphoton photoelectron circular dichroism (MP-PECD). Comparisons are also made with single photon (synchrotron radiation) PECD of these compounds. Although for a given compound a common final cation state is reached, pronounced differences are observed between PECD and MP-PECD, and between the alternative identified REMPI intermediate states in the case of MP-PECD.