Laser absorption spectroscopy (LAS) plays an important role in diagnosing a wide range of combustion systems. Among various laser diagnostic techniques, LAS is relatively simple, quantitative, and versatile; therefore, it has been widely used to measure gas composition, concentration, temperature, etc. in both laboratory and industrial systems. Recently, convenient access to the entire spectrum of the species is possible because a wide range of light sources are available. Especially, mid-infrared lasers, such as quantum-cascade lasers or interband cascade lasers, have become advanced, and provide highly sensitive measuring capabilities for gas species in combustion. In this article, applications of LAS to chemical kinetics studies on the low-temperature oxidation of hydrocarbons and studies on the measurements of trace gas emitted from combustion systems (e.g. internal combustion engines) are introduced. First, fundamentals of commonly used highly sensitive measurement techniques, such as frequency modulation spectroscopy (FMS) and cavity ring-down spectroscopy (CRDS), are introduced. Next, some recent chemical kinetic studies using highly sensitive LAS techniques and emission measurements are introduced. Finally, our latest studies on HCHO measurements in the low-temperature oxidation of isooctane using a rapid compression machine and a mid-infrared LAS, and trace gas measurements of ethylene in the gasoline engine emissions have been described.
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