boundfree

Boundfree is a term used in atomic physics, spectroscopy, and astrophysics to describe transitions in which a bound electron is liberated from an atom or ion by absorbing a photon. This process is also known as photoionization and is the inverse of a recombination or free-bound transition. In a bound-free transition, the initial state is a discrete bound level, and the final state is a continuum (free electron) state. The photon energy must exceed the ionization energy of the initial bound state, defining a threshold known as the ionization edge.

The bound-free cross section characterizes the probability of photoionization and typically rises at threshold and then generally decreases with increasing photon energy, often with resonant features due to intermediate autoionizing states. The details depend on the atomic or ionic species and the specific bound state involved. Bound-free processes compete with bound-bound transitions and contribute to the overall opacity and energy balance in a system.

In practical contexts, bound-free absorption contributes to continuum opacity in stellar atmospheres and other plasmas. Notable examples include the hydrogen Lyman edge and Balmer edge, which mark ionization thresholds from different bound levels. Bound-free transitions also play a role in spectroscopy, producing absorption edges rather than discrete lines, and in modeling energy transport and ionization balance in plasmas.

Calculations and data for bound-free processes use quantum mechanical methods to determine cross sections for many atoms and ions. These data underpin opacity models, spectral synthesis, and diagnostics across astrophysics, laboratory plasmas, and atmospheric physics.