Quantum cascade lasers and their applications to spectral and kinetic investigations of reactive gaseous intermediate species

Infrared (IR) spectroscopy is considered to be among the most important analytical tools in laboratories, remote sensing, and industry. In addition to the commonly used Fourier-transform infrared (FTIR) spectrometers, spectral detection using IR laser sources plays important roles in varied applications. The quantum cascade laser (QCL) recently emerged as a novel, compact, tunable, and versatile IR laser source with narrow linewidth, high power, wide spectral coverage, and relatively straightforward operation. The scope of this chapter is limited primarily to mid-IR QCL and mainly their applications to spectroscopy and kinetics of reactive gaseous intermediate species. We introduce briefly the fundamental theories of QCL, their various types, advantages, and limitations. Implementations of several types are discussed, a detailed description of a continuous-wave external-cavity QCL coupled with a Herriott cell is presented. The high-resolution spectra of some stable molecules and reactive reaction intermediates, especially Criegee intermediates and other free radicals, are discussed. An account of kinetic investigations of Criegee intermediates and other free radicals in a flow reactor and of species in a shock tube is provided. Finally, we present some future perspective on research using the QCL.