We report a study of the 3E excited-state structure of single
nitrogen-vacancy (NV) defects in diamond, combining resonant excitation at
cryogenic temperatures and optically detected magnetic resonance. A theoretical
model of the excited-state structure is developed and shows excellent agreement
with experimental observations. Besides, we show that the two orbital branches
associated with the 3E excited-state are averaged when operating at room
temperature. This study leads to an improved physical understanding of the NV
defect electronic structure, which is invaluable for the development of
diamond-based quantum information processing.Comment: 4 pages, 4 figure