zero–zero (0–0) absorption or emission

https://doi.org/10.1351/goldbook.Z06744
A purely electronic transition occurring between the lowest vibrational levels of two electronic states.
Source:
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2284 [Terms] [Paper]