Complex sentence: A single-photon transition between S states is prohibited by quantum selection rules, so to elevate ground state positrons to the 2S level, the confinement space was illuminated by a laser tuned to half the calculated transition frequencies, stimulating allowed two photon absorption.
Split into simpler sentences: A single-photon transition between S states is prohibited by quantum selection rules. So to elevate ground state positrons to the 2S level, the confinement space was illuminated by a laser tuned to half the calculated transition frequency, stimulating allowed two photon absorption.