ABSTRACT
Atoms and atomic ions have long held a place at the very heart of precision measurement and metrology. For example, the second is defined in terms of the Cs hyperfine structure interval, the Rydberg constant is measured by spectroscopy of atomic hydrogen, and the ratio of electron-to-protonmass is known from the oscillation frequencies of trapped atomic ions. The importance of atoms in this field lies partly in the detailed understanding that we have of atomic structure but also in the technical capabilities that exist for preparing and manipulating atoms and ions. In recent years, both the computational methods for understanding molecules more fully and the experimental methods for producing and controlling them have advanced enormously. This has led to a surge of interest in using molecules for precision measurements, especially where they offer new properties that are not available from atoms and atomic ions. For example, the rotational, vibrational, and electronic structures within a molecule offer a wider range of coexisting frequencies than one finds in atomic systems. Moreover, polar diatomic molecules have a built-in cylindrical symmetry, whilst more complex molecules can have a handedness-structural conformations that atoms cannot offer. In this chapter, we discuss some applications of molecules to current problems in precision measurement and we outline recent technical advances that make some of these applications possible.
