It has to do with something called "atomic excitation." From the article (emphasis added): When photons pass through a medium ...

For the last seven years, the study authors have been studying atomic excitation, a phenomenon that occurs as a result of interaction between light and matter. During atomic excitation ...

Kelvin waves have been a mystery for decades but a new method involving superfluid helium-4 can help us control them.

Recent studies have employed sophisticated computational methods to explore atomic properties, excitation energies, and transition rates, providing valuable insights into the fundamental physics ...