Laser Flash Photolytic Studies of a Well-Characterized Soil Humic Substance

Laser flash photolysis studies of the well characterised soil humic substance, Armadale Fulvic Acid (P.E.I., Canada), have been carried out with excitation at 355 nm on picosecond and nanosecond time scales. Three principal transient absorption signals have been observed in aqueous solution: a component with a maximum absorption at 675 nm and a lifetime of about 1 microsecond (at pH = 7.0), a second component with a maximum absorption at 475 nm and a lifetime of 1-10 microseconds, and a third component with a broad, featureless transient absorption spectrum and a lifetime in excess of 100 microseconds. The 675 nm signal is believed to be a solvated electron on the basis of lifetime and quenching data, and is observed 20 ps after excitation. The 475 nm signal is believed to be a radical cation on the basis of its concurrent appearance with the electron at 20 ps. The third featureless component emerges nanoseconds after excitation and is believed to correspond to the triplet states of the humic material.