Saturated absorption as a result of two-stage transitions in dye solutions

A report is given of the first observation of saturated absorption caused by two-stage filling of the second excited singlet state of complex molecules in solution. The main requirements on the system, subject to which nonlinear bleaching occurs as a result of two-stage vibronic transitions (excitation wavelength, initial absorption coefficient, energy structure of the dye molecules), were determined. It was found that the degree of bleaching of a nonlinearly absorbing solution was limited by transient thermal defocusing of the pump beam even in the nanosecond range of durations.