Dissection of rovibronic band structure by polarization-resolved degenerate four-wave mixing spectroscopy

Abstract

Judicious selection of polarization characteristics in degenerate four-wave mixing (DFWM) spectroscopy is shown to provide a facile and robust means for discriminating rovibronic features according to their changes in rotational angular momentum, ΔJ. Building upon a perturbative (weak-field) treatment of the resonant DFWM response, theoretical analyses are presented for a collinear arrangement of linearly polarized electromagnetic waves that interact with an isotropic ensemble of gas-phase target molecules. The polarization unit vectors for two input fields (E1 and E3) are presumed to be fixed along the Y-axis (φ1=φ3=π/2), while the remaining incident field (E2) has its orientation within the transverse X–Y plane specified by angular coordinate φ2. For φ2=π/4, complete elimination of high-J Q-branch (ΔJ=0) and P-/R-branch (ΔJ=±1) structure is found to occur when the detected direction of linear polarization is switched between the limiting values of φ4,Q=−18.43° and φ4,P/R=+26.57°, respectively. These predictions are corroborated experimentally by probing the tunneling-split origin region of the tropolone à 1B2–X̃ 1A1 (π*←π) absorption system under ambient, bulk-gas conditions. Other polarization-based schemes for rovibronic branch suppression are discussed as are the effects incurred by (strong-field) optical saturation phenomena. The implementations of DFWM suggested by this work should prove useful for dissecting and unraveling the congested spectra often exhibited by massive polyatomic species.