(30) Again, the first two terms J φ l±1 and the others J φ l±2 cor- respond to excitation by ω and 2ω, respectively, whereas their cross-terms of them represent the two-color effect

˜Jl′ (η′ 5,η ′ 6,η ′ 7,η ′ 8), (23) where ˜Jl′ is defined as, ˜Jl′ (η′ 5,η ′ 6,η ′ 7,η ′ 8) = ∑ l6,l7,l8 i−l6−2l7−3l8Jl′−2l6−3l7−4l8 (η′ 5)Il6 (η′ 6)Il7 (η′ 7)Il8 (η′ 8), (24) and