ABSTRACT
This chapter invites the reader to explore the realm of modern absorption spectroscopy and see how—nearly 50 years after the first realization of laser absorption spectroscopy—traditional Fourier transform infrared spectroscopy (FTIR) is slowly being replaced by modalities based on supercontinuum (SC) or other “white-light” laser sources that combine broadband features with ultra-high spectral resolution and superb limits of detection. In particular, the reader will encounter new approaches based on frequency combs that begin to revolutionize absorption spectroscopy. For example, the 1S–2S hydrogen transition could be determined with a relative uncertainty of less than 5210>15, the most precise measurement of an atomic transition to date; and state-of-the-art direct frequency comb spectroscopy based on femtosecond-laser sources provides unrivaled versatility in absorption spectroscopy. Other, more “traditional” methodologies, based on tunable diode and quantum cascade laser sources, are grouped together under the umbrella of tunable diode laser absorption spectroscopy; their principles and applications are described, emphasizing specifically direct absorption and waveform modulation techniques. A further section is devoted to cavity-enhanced techniques, including its most utilized modality of cavity-ring down spectroscopy. The chapter concludes with accounts of the fascinating field of terahertz spectroscopy and the techniques of photoacoustic and photothermal spectroscopies.
