Chapter 1: Spin Precession and Rate Equations

Time dependence involves changes in the occupation of the states of a physical system. For example, the electron in a hydrogen atom may be excited from a 1s state into a 2p state due to a perturbation such as an electromagnetic field. (The states of the hydrogen atom are derived in Quantum Mechanics texts such as Konishi and Paffuti (2009, Chap. 6).) Generally, we say a transition has occurred when the physical system changes its state. It is simplest to view this as a change from the system being in state i to the system being in state j. This chapter starts with basic ideas on time dependence in Quantum Mechanics and the time evolution operator. These are applied to spin precession of a particle. Then the Einstein coefficients and rate equations are introduced. The following chapters are concerned with calculating the probability p_i(t) for the occupation of state i of a system as a function of the time t. These predictions are compared with experiments whenever possible…