Dual-Beam Circular Dichroism Spectroscopy of Jet-Cooled Chiral Molecules.

Circular dichroism (CD) spectroscopy of jet-cooled molecules provides conformation-specific CD spectra. However, its widespread utilization has been limited by the weak CD effects and the low density of gas-phase molecules. Here, we developed a dual-beam method to improve the sensitivity and accuracy of gas-phase CD measurements. Circularly and linearly polarized pulses were generated from a single laser pulse and used to irradiate a single molecular-beam pulse to produce two ion peaks. The ion peaks induced by linearly polarized pulses were subtracted from those induced by circularly polarized pulses to correct the CD values for the pulse-to-pulse fluctuations in laser power and gas density. The resonant two-photon ionization CD spectrum of (1,2)-(-)-pseudoephedrine revealed that the standard deviations of CD values measured using the dual-beam method were three times lower than those measured using a single-beam method. The dual-beam method provides an effective, accurate, and easy-to-use tool to obtain gas-phase CD spectra.