Direct determination of transient molar absorption coefficient by double pulse excitation

Abstract

A new method has been devised to determine the molar absorption coefficient of short-lived photochemical species formed by laser irradiation. The technique consists of a first pump excitation pulse that significantly modifies the concentration of the initial species in its ground state, enough that the variation of the absorbance at maximum absorption of the formed species depends on the initial species concentration. A new absorbance variation is created by a second excitation pulse occurring with a time delay after the first pulse that is small compared to the lifetime of the formed species. The molar absorption coefficient is calculated using the ratio of these two variations in absorbance, assuming that the initial species does not absorb in the spectral range where the absorption variation of the formed species is observed.