Transient frequency modulation absorption spectroscopy as a probe of gas-liquid scattering

Abstract

This thesis reports the first investigation of gas-liquid scattering dynamics via transient frequency modulated absorption spectroscopy (TFMAS). Using this technique in the near infrared (NIR), the inelastic scattering of gaseous CN(X 2Σ +) radicals from fluorinated and aliphatic liquid surfaces was found to be predominantly impulsive. These experiments also demonstrated the first application of a glow discharge to produce supersonically cooled CN(X) radicals in situ. A large fraction of these incident radicals was lost on contact with the potentially reactive hydrocarbon, most probably to hydrogen abstraction to form HCN. Subsequently, an entirely new TFMAS probe, with the capabilities to observe this unseen channel, was built to access the mid-infrared (MIR) region. Difference frequency generation (DFG) of tuneable NIR and single-frequency 1064 nm lasers was performed in periodically poled lithium niobate (PPLN) to generate the MIR. This probe was characterised by proof-of-principle experiments, including preliminary studies of inelastic CH4 scattering, and found to be sensitive, narrowband and possess Gaussian mode-quality. Accompanying Monte-Carlo simulations of both NIR- and MIR-TFMAS experiments supported the mechanistic conclusions drawn for CN(X) scattering and indicated that modifications to the CH4 experiments, including improvements in the signal-to-noise, are required to observe scattering in individual quantum states. On their implementation, the new MIR probe may then be used to investigate both inelastic and reactive interactions of innumerable interesting and relevant gas-liquid systems via the ubiquitous fundamental vibrational modes of closed-shell species, such as C–H, N–H and O–H stretches.