Resonantly Enhanced Multiphoton Ionization Circular Dichroism (REMPICD)

Linear and non-linear circular dichroism of $R$-(+)-3-methylcyclopentanone is reported in the gas and liquid phase. Measurements of (2+1) resonance-enhanced multiphoton ionization circular dichroism (REMPICD) for nozzle-jet expanded molecular beams of the equatorial conformer of $R$-3MCP are presented. Monitoring either mass-selected cations or photo-electrons produced via (2+1) REMPI through the $n \to $ 3s Rydberg transition yielded a REMPICD of +1.5$\pm $ 0.5{\%} [ REMPICD $\equiv $ 2(I$_{L}$ -- I$_{R})$/( I$_{L}$ + I$_{R})$, where I$_{L/R}$ refers to the ion/electron signal for left-/right-circularly polarized light. A racemic mixture of 3-methylcyclopentanone showed no significant CD. The REMPICD is larger and of opposite sign than the 1-photon CD at the $n \to $ 3s transition. It appears that the REMPICD is dominated by the continuum transition. Measurements of 1-photon CD as a function of temperature provides information on the enthalpy difference between the equatorial and axial conformers of 3MCP. Density Function Theory calculations also support these measurements.