spectrum and that even the boundaries between the instrumentally based divisions are diffuse.
Atomic and Molecular Energy
The total energy of an atom or molecule includes contributions from several sources. These originate
from within the nucleus (nuclear energy), from interactions between electrons and nuclei (electronic
energy), from electron and nuclear spin, from the vibrational and rotational motion of molecules and
from translation of atoms or molecules through space. Except for translational motion, all these forms
of energy are considered to be discontinuous or quantized. According to quantum theory, for each form
of energy, an atom or molecule can exist in certain discrete energy states or quantized levels defined by
a set of quantum numbers and in accordance with a set of mathematical rules. The magnitudes of the
various forms of energy and the differences between adjacent levels vary considerably. Electronic
energy is comparatively large, and it will be seen from Figure 7.3 that the levels are much more widely
spaced than vibrational levels, which in turn are more widely spaced than rotational levels. Electron and
nuclear spin energies are small and the levels lie very close together. For simplicity, the latter and
nuclear energy levels, which are even more widely spaced than electronic levels, are not shown on the
diagram. Exactly which set of energy levels an atom or molecule occupies under ambient conditions is
determined by the rules of quantum theory and by the Maxwell-Boltzmann equation (vide infra). They
are designated ground state levels, higher ones being termed excited state levels.
Figure 7.3
Electronic, vibrational and rotational energy levels,
not to scale (only the first two electronic levels
are shown).