Basic Quantum Mechanics of Atomic Structures and Transitions

Abstract

To set up a problem in mechanics one needs to specify:

• (a) the system, as, for example, point particles, specified by properties which determine their inertia (mass, spin) and their interactions (charge, magnetic moment). These properties are supposed to remain constant during the motion that is being analysed and the particles are, in that context, elementary particles. But for interactions in some other energy range one may make a different choice of elementary particle. For example, atomic nuclei, of specified spin, are ‘elementary particles’ in the context of atomic spectroscopy (energy range eV), but in the energy range MeV they are composite particles.

• (b) the geometry, as, for example, position vectors relative to some origin in three-dimensional physical space — but, frequently, one makes a transformation to a co-ordinate system moving with the centre of mass of the’ system’. Four-vectors are used in relativistic problems.

• (c) the field of force, commonly specified by a potential function of the position vectors. Such functions exist for electromagnetic forces, but not for all conceivable types of force.

• (d) the law of motion, as, for example, in classical mechanics, Newton’s second law.