2.4 PHYSICAL CONSTANTS USED AS ATOMIC UNITS

Sometimes fundamental physical constants, or other well defined physical quantities, are used as though they were units in certain specialized fields of science. For example, in astronomy it may be more convenient to express the mass of a star in terms of the mass of the sun. In atomic and molecular physics it is similarly more convenient to express masses in terms of the electron mass, $m_{\rm{e}}$, or in terms of the unified atomic mass unit, $1\ \rm{u}$, and to express charges in terms of the elementary charge $e$, and energies in terms of the electronvolt, $\rm{eV}$.