Chapter 3. Phenomena Involving Absorption of Energy Followed by Emission of Light

C. Y. She, Jonathan S. Friedman

In Chapter 3, we move to a semiclassical treatment (quantum theory) of light absorption and scattering, specifically from atoms. We start with a description of how lidar measures Doppler shift, and the fundamental difference between the measurement when the laser is in resonance with an atomic transition (resonant) and when it is not (nonresonant). We follow with a treatment of quantum polarizability and the resulting absorption cross section, leading to the differential resonance scattering cross section and its contrast with the classical result. After quantum polarizability, we demonstrate the radiation pattern of coherently excited atoms. This takes us to an interpretation of the Hanle Effect. Following these descriptions of the phenomena that impact resonance lidar, we extend our understanding by closing the chapter with an overview of the rudimentary physics of sodium laser guide stars.

Travis S. Taylor

This chapter discusses the details of what stimulated emission of radiation is, why it is the major physical phenomenon at the heart of the laser, and how it is exploited in ways to create laser light beams. It describes the Bohr model of the atom and how electrons can only make quantized leaps in energy levels from one atomic orbital or shell to the next. The chapter considers Pauli’s Exclusion Principle and singlet, doublet, and triplet states of the atom. It also describes the interaction of light with atoms through absorption, spontaneous emission, and stimulated emission. The chapter shows that the quantum mechanical harmonic oscillator and the ground state of the universe is a nonzero electromagnetic field and therefore is the trigger for spontaneous emission. It looks at the models for absorption, spontaneous and stimulated emission, and vibrational states.

Hans Joachim Eichler, Jürgen Eichler, Oliver Lux