Gas-Phase Circular Dichroism Spectroscopy Using Randomized Circularly Polarized Laser Pulses

Circular dichroism (CD) spectroscopy of gaseous molecules remains challenging because of intrinsically weak CD signals and low molecular number densities, which necessitate the use of pulsed molecular beams and high-power pulsed lasers. Pulse-to-pulse intensity fluctuations in both systems introduce variations in ionsignals, leading to fluctuation-induced artifacts in the measured CD values. To suppress these effects, we developed a CD measurement technique that employs randomly alternating left- and right-handed circularly polarized laser pulses (random-CP). Unlike conventional regularly alternating circularly polarized pulses (regular-CP), random-CP pulses decouple periodic experimental fluctuations from the CD signal, thereby minimizing systematic artifacts and improving precision. The advantages of this approach are demonstrated through resonant two-photon ionization CD and fluorescence-detected CD spectra of jet-cooled (R)-(+)-styrene oxide recorded near the origin band of the S₀ - S₁ transition. Compared with regular-CP pulses, random-CP pulses lead to more rapid convergence of CD values, more consistent CD band profiles, and effective suppression of offset biases. This random-CP pulse strategy provides a robust and broadly applicable method for enhancing the accuracy and reliability of gas-phase CD measurements.