The set corresponding to Δ Δ ℑ ℑ = − − 1 is called the P branch. The corresponding frequencies are given by: ℏ ℏ ω ω P = E ( v + 1 , ℑ ℑ − − 1 ) − − E ( v , ℑ ℑ ) = − − 2 B ℑ ℑ + ℏ ℏ ω ω 0 ; ℑ ℑ = 1 , 2 , 3 , . . . . . .