Positrons are short-lived subatomic particle with the same mass as electrons and a positive charge.

They are used in medicine, e.g.

in positron emission tomography (PET), a diagnostic imaging method for metabolic disorders.

Positrons also exist as negatively charged ions, called positronium ions (Ps-), which are essentially a three-particle system consisting of two electrons bound to a positron.

Now, commercially available lasers are capable of producing photons that carry enough energy to bring the electrons of negatively charge ions, like Ps?, to doubly-excited states, referred to as D-wave resonance.

Positronium ions are, however, very difficult to observe because they are unstable and often disappear before physicists get a chance to analyse them.

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