The response of atomic systems to ionising radiation, the fundamental process of photoionisation, is a dominant process in the universe. It involves neutral atoms, molecules and clusters as well as their ions (positive or negative) and takes place in many physical systems including a variety of astrophysical objects, the upper atmosphere, fission and fusion plasmas as well as laser-produced plasma. In many cases, the role of the photoionisation process is central for determining the overall properties of a system and in addition optical transitions are often used as a diagnostic tool.

Numerical simulations of a photoionised plasma are vital in the analysis and interpretation of spectroscopic observations. Models can describe the source of ionising radiation as well as the radiation’s influence on the surrounding gas. The success of such modeling depends on a number of factors including, importantly, the quality of the input atomic and molecular data. Absolute photoionisation cross-sections are essential to model such systems and to interpret observations made by satellite-based X-ray telescopes.

In addition, the construction of undulator beamlines at synchrotron-radiation facilities has made it possible to measure absolute photoionisation cross-sections by overlapping beams of photons and ions, the so-called merged-beam technique. Therefore it has become possible to perform detailed tests of theoretical calculations in many cases and also to guide experiments done to measure photoionisation cross-sections.
Some applications where photoionisation cross-sections knowledge is essential:

1. Fission and fusion plasmas:

Photoionisation cross-sections are required to model fission and fusion plasmas. It has been the motivation for many atomic-physics studies. Not many codes can be applied to calculate photoionisation cross-sections due to the large number and the overlap of all allowed transitions in such heavy atomic systems. These produce Unresolved Transition Arrays and make calculations inefficient.

2. Laser-produced plasma:

The interaction of intense laser beams with materials is leading to rapid phase transitions and the production of hot plumes of material. The investigation of such plumes is of intrinsical interest as examples of systems for the study of matter at high temperatures; in addition, they have several practical purposes, e.g., materials deposition, deep-ultraviolet to X-ray photon sources in analytical spectroscopy, lithography, microscopy, etc.

3. The upper atmosphere:

Photoionisation processes are a major source of charged particles (ions and electrons) and shield us against short-wavelength radiation. Charged particles represent only a minute fraction of the total mass of the Earth’s atmosphere but they play a crucial role in many geophysical phenomena such as the variations in the geomagnetic field, lightning and auroras. Also, communication by radiowaves is an essential feature of modern society. It is strongly influenced by the electrons in the F-layer of the atmosphere (150 – 400 km above the Earth), and much of the motivation for atmospheric research on charged species has derived from the need to understand how to provide secure and reliable communication.

4. Astrophysics:

Photoionised environments are common in astrophysics. The reasons are a large number of ionising-radiation sources present and the lack of shielding provided by the interstellar space. Photoionised plasma is found in/nearby the atmosphere of hot stars, planetary nebulae, H II regions, cataclysmic variables and X-Ray binaries as well as in the more distant active galactic nuclei, starburst nuclei and intergalactic medium. A broad range of physical processes determine the nature of the emission and absorption spectra of these objects, and thus such spectra contain a wealth of information concerning the physical state of the gas, its chemical composition and the ionising radiation field at unobservable wavelengths.

A recent R-matrix and Quantemol-N development is an adaptation to allow calculations of photoionisation cross-sections.
Some experimental comparisons have been performed in order to validate the code.
The calculations are done using a very simple static exchange model and the use of more sophisticated models (included in Quantemol-N suite) will necessarily improve the precision of the results. The data are compared with experimental data (N. Wainpan, Phys Review v.99, p.542).

Photoionisation cross-sections, Hydrogen

Photoionisation cross-sections, Nitrogen

Photoionisation cross-sections, Carbon Dioxide