New photo-isomerization path of indocyanine green in condensed phase investigated by two-pump excitation

In cyanine dye, a photo-isomerization path involving a multiphoton process was found in addition to a conventional reaction path with a one-photon process by two-pulse correlation of transient absorption and bleaching. That photo-isomerization was completed within 3 ns and its efficiency was dependent on the interval time between two pump pulses indicate that the new path involves a multiphoton process: S0 → Sn → Sm, where Sn is ∼3.1 eV and Sm is ∼4.7 eV higher than S0 and the lifetime of intermediate state in Sn is approximately 0.2 ps. Changing the pump power controlled the branching ratio between the two paths.