ABSTRACT
In this first chapter on laser-spectroscopic methods—absorption spectroscopy—the following (and related) questions are addressed: Do all particles absorb photons with similar probability? What can be learned from the shape of a spectral line? How many photons can be absorbed by a single atom/molecule? In this context, detailed accounts of the two main modalities, linear and nonlinear absorption spectroscopy, are given (depending on whether a single or several photons are involved in the induced transition, respectively). First, basic concepts are outlined, including absorption cross sections and spectral line profiles. In particular, the relation of the latter to homogeneous and inhomogeneous line broadening factors is put into a concise, formalistic framework. Following this, an overview on nonlinear spectroscopy is provided, with detailed presentations for saturation or Lamb dip spectroscopy, as well as for multiphoton absorption spectroscopy. The latter comprises the three important modalities of two-photon absorption spectroscopy, Doppler free two-photon absorption, and multiphoton ionization. Finally, infrared multiphoton dissociation is addressed, describing its molecular mechanisms and applications to state-selective chemistry and isotope separation. The chapter concludes with highlights on modern incarnations of absorption spectroscopy, predominantly those incorporating tunable diode lasers, quantum cascade lasers, frequency combs, or terahertz radiation.
