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

Abstract

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 signalto-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.

Recent Advances in linear and nonlinear Raman spectroscopy I

“…The low-wavenumber spectrum of liquid water has been investigated with four-photon coherent spectroscopy to interrogate the orientational motion of the H 2 O molecule. 148 Shimada et al 149 discovered a new molecular phenomenon in a resonance HR scattering study of liquid solutions. 145 Taking advantage of the coherent Raman emission from stimulated Raman generation, Echevarria et al 146 showed that increases in the chain length and the number of CH 3 groups in alcohols enhance the Raman emission efficiency.…”

The formaldehyde cation: Rovibrational energy level structure and Coriolis interaction near the adiabatic ionization threshold

“…MgSO 4 was mixed with the paraformaldehyde to remove water from the formaldehyde vapor [25]. The formaldehyde vapor was entrained in a stream of Kr at a stagnation pressure of 1 bar.…”

New developments in non‐linear optical spectroscopy

“…However, the reduction of the DFWM signal intensities to population densities is not fully established, in particular for polyatomic molecules. A comparative study of DFWM with Cavity Ringdown (CRD) spectroscopy has been performed on jet-cooled formaldehyde by Tulej et al 21 Absorption spectra are simulated by applying an asymmetric rotor model and assuming a Boltzmann temperature for the rotational distribution in the molecular beam. A simple model for the reduction of the DFWM signal intensities to population is applied, yielding an absorption-like spectrum in accordance with both the simulations and the CRD measurements.…”