Comparative study of degenerate four-wave mixing and cavity ringdown signal intensities of formaldehyde in a molecular beam

A comparison of degenerate four‐wave mixing (DFWM) and cavity ring‐down (CRD) spectroscopy is made on jet‐cooled formaldehyde under identical expansion conditions. We show that both methods are applicable to the low‐density environment of a molecular beam. However, a slightly superior signal‐to‐noise ratio for the background‐free DFWM method is observed for our experimental configurations. Absorption spectra simulated by applying an asymmetric rotor model are used to deduce Boltzmann temperatures of the rotational population distribution by a nonlinear least‐squares fit to the experimental data. In addition, partial interconversion of the nuclear spin upon supersonic expansion is observed. Consequently, a second fit parameter, describing the para/ortho concentration ratio in the molecular beam, is introduced. The resulting simulated spectra are in favorable agreement with the relative CRD intensities. A simple model for the reduction of the DFWM signal intensities to population is applied yielding absorption‐like spectra in accordance with both the simulations and the CRD measurements. We conclude that DFWM spectroscopy is applicable to deduce population distributions of an asymmetric rotor molecule under free‐jet conditions. Copyright © 2006 John Wiley & Sons, Ltd.