breakdown threshold fluence 

in LIBS
https://doi.org/10.1351/goldbook.08460
Minimum fluence required to achieve measurable emission signals
Note:
Usually the breakdown-threshold fluence is orders of magnitude higher than the ablation-threshold fluence.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 714 [Terms] [Paper]