ABSTRACT
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Any object with a net positive charge on one end and a net negative charge on the other end possesses an electric dipole moment. In ordinary classical electromagnetism this dipole moment is a vector quantity that can point in any direction, and is subject to electrical forces that are fairly straightforward to formulate mathematically. However, for a quantummechanical object like an atomormolecule, the strength and orientation of the object’s dipolemoment candepend strongly on the object’s quantummechanical state. This is a subject that becomes relevant in low-temperature molecular samples, where an ensemble ofmolecules can be prepared in a single internal state, as described in Chapters 5, 9, 13, 14, and 15. In such a case, the mathematical description becomes more elaborate, and indeed the dipole-dipole interaction need not take the classical
form given in textbooks. The description of this interaction is the subject of this chapter.
