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

► Two pulse correlation in ultrafast transient absorption spectroscopy. ► New photo-isomerization path of Indocyanine Green including multiphoton process was found. ► Control of reaction efficiency was realized by changing interval between two pump pulses. ► Multiphoton isomerization was found by two pulse correlation of transient absorption. ► Optical order in photoreaction path was revealed by asymmetric two pulse correlation.