Absolute Determination of Quantum Yields of Photosensitization by Time Resolved Thermal Lensing

Abstract

Time-resolved thermal lensing (TRTL) can be used to study the time profile of heat release after absorption of radiation. The part of the energy released as heat through radiationless processes produces local changes in temperature, and hence density and refractive index, in the solution such that along the beam axis the medium acts as a lens (in general divergent since in most media the variation of the refractive index with temperature is negative). The time dependence of the formation of the so-called thermal lens is governed by the relative rates of radiationless processes occurring in the system and is probed by an overlapping CW laser beam, of wavelength not absorbed by the sample. After passing through a pinhole, the probe beam impinges on a photodiode which detects the reduction of the probe beam as a consequence of the formation of the lens. The time resolution of the thermal lens effect is determined by the acoustic transit time in the system, τ_a, the time taken for the accompanying acoustic or shock wave to traverse the radius of the laser beam (τ_a = R/v_a, where R = radius of beam and v_a = velocity of sound in the solution). For focussed beams τ_a ≃ 100 ns. Thus, all heat deposited in solution in a time shorter than τ_a constitutes the ‘prompt’ heat component whereas any energy storing species living longer than τ_a will be responsible for the delayed heat component of a the TRTL signal.