Optoacoustic Relaxation of Periodically Irradiated Solutions

Abstract

Some time ago, 1964–1967, we developed a relaxation method which permits to determine radiationless transition rates in a direct way¹. Intensity — modulated light is employed to produce excited singlet and triplet states of the absorbing molecules; radiationless transitions between excited states as well as between excited states and the ground — state produce pressure waves which are measured by means of a micro — phone. With eosin in methanol at room temperature, the resultant energy dispersion curve as a function of the modulation frequency allowed to obtain a rate — constant of 1.25 · 10⁸ sec⁻¹ for the radiationless inter — system crossing from the lowest excited singlet state to the triplet state and a rate — constant of 900 sec⁻¹ for the radiationless transition from the triplet state to the ground — state. Internal conversion, i.e. radiationless transition from the lowest excited singlet state to the ground-state, appears to be negligible in comparison with intersystem crossing and fluorescence, the competing radiative process that extinguishes the low — est excited singlet state.

For the full version see: E. Hey and K. Gollnick, J. Photoacoustics, in press.