Time Resolved Frequency Comb Spectroscopy for Studying Gas Phase Free Radical Kinetics

Abstract

Gas phase free radicals are key intermediates in the chemistry of many important processes, including air pollution, combustion, stratospheric ozone depletion and planetary atmospheres. These trace species exist typically in very low concentrations in both natural settings and laboratory experiments, thus requiring detection methods with high sensitivity, microsecond time resolution, and chemical specificity. Here we present a new approach for time-resolved absorption spectroscopy that exploits advances in the development of frequency comb lasers. This technique, mid-Infrared Time Resolved Frequency Comb Spectroscopy (TRFCS), takes advantage of the large spectral bandwidths of ultrafast lasers and the high sensitivity and spectral resolution of cavity-enhanced absorption spectroscopy. In this approach, the coherent output of a stabilized mode-locked laser generates high resolution frequency combs [1] that are coupled to an external high-finesse cavity. The radiation transmitted through the cavity is dispersed with a Virtually Imaged Phase Array (VIPA) spectrometer [2], permitting simultaneous broad-band detection with high (25 us) time resolution. The cavity encloses a gas flow cell, in which radicals are produced by excimer laser photolysis of precursor gases. Free radicals, transient intermediates and primary reaction products are detected as a function of time. We demonstrate this technique by observing DOCO radicals produced from the 193 nm photolysis of acrylic acid-d1. We have detected simultaneously the depletion of precursor, the formation and loss of DOCO, as well as other primary and secondary products. Current limitations and potential applications will be discussed.