Overview of Degenerate Four-Wave Mixing and its Application to Detection of OH Radical in Flame: For last decade, the degenerate four-wave mixing (DFWM) spectroscopy has been proven to be a useful tool for combustion and chemical physics studies. In the DFWM process, three input beams of the same frequency interact with a medium, producing a coherent signal beam of the same frequency. This process relies on the resonant interaction of the light frequency with an atomic or molecular transition frequency. Thus, its sensitivity is enhanced whenever the frequency is resonant with an atomic or molecular transition and absorption-like spectrum can be obtained by the intensity in the signal beam as a function of the light frequency.
In this chapter, brief theoretical description of the DFWM process and its application to a diagnostic study of OH in flame are presented. Rotational temperatures of the overlap volume of the beams in flame have been determined from the DFWM spectra.
Photodissociation Dynamics of $H_2S$ at 266 nm via the DFWM Spectroscopy: Photodissociation dynamics of $H_2S$ have been studied using the DFWM technique at 266 nm and at room temperature. The DFWM spectra of SH A$^2Σ$ - X$^2Π_i$ (0,0) transitions have been observed and the nascent rotational distributions, spin-orbit state ratio, and Λ-doublet population ratio of the SH fragments have been extracted. The rotational distributions are cold and characterized by a Boltzmann temperature of 348±20 K for the SH($^2Π_3/2$) state. The Λ-doublet population ratio and the SH($^2Π_3/2$)/SH($^2Π_1/2$) ratio have been observed to be close to unity and 3.50±0.18, respectively.
Photodissociation Dynamics of tert-Butyl Hydroperoxide at 266 nm: Degenerate Four-Wave Mixing Observation of OH State Distribution: The DFWM spectroscopy has been used to study the photodissociation dynamics of tert-butyl hydroperoxide (t-BuOOH) at 266 nm, long-wavelength tail of the UV absorption continuum. The nascent rotational dist...