Absorption of photon energy by free electrons.
Note:
Inverse Bremsstrahlung increases the kinetic energy of the electrons, which is important for heating the plasma.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 723 (https://doi.org/10.1515/pac-2019-0203)