isolatedatom

An isolated atom is a single atom that experiences negligible interactions with other atoms, molecules, or solid surfaces. In practice this condition is approximated when an atom is in the gas phase at low density, inside an ultrahigh vacuum, or confined in a trap or lattice where external perturbations are minimized. The term is often used in contrast to atoms that participate in chemical bonds in molecules or solids.

The physics of an isolated atom is governed by the electronic Hamiltonian of a single nucleus and its electrons. Energy levels are discrete, yielding atomic spectra with characteristic lines. Transitions follow selection rules and depend on the electronic structure; because there are no chemical bonds, there are no vibrational or rotational modes associated with the whole molecule. The atom can be ionized to form positive ions; multiple ionization states exist.

Isolated atoms are produced and studied in a variety of ways: effusive atomic beams, ovens, laser cooling and trapping, ion traps, and storage rings. Measurements rely on spectroscopy (absorption, emission, or photoelectron spectroscopy), mass spectrometry, and quantum-transport probes. The concept underpins atomic clocks, precision spectroscopy, and quantum information experiments, where individual atoms or ions are prepared in well-defined quantum states in optical lattices or ion traps.

However, truly isolated atoms are an idealization. In practice, residual gas collisions, electromagnetic fields (Stark, Zeeman), and van der Waals interactions with surfaces perturb energy levels and broaden lines. The isolated-atom model remains a useful reference for calculating properties and for defining energy scales in experiments and theories.