Two-photon cross-section measurement of meso-tetra-hydroxyphenyl-chlorin using femtosecond laser pulses

Experiments were carried out using the femtosecond laser to compare the fluorescence amplitude produced by two-photon excitation at 800 nm and 400 nm (i.e. to the strong absorption band in mTHPC). Using an experimental arrangement where the fluorescence sampling geometry was kept unchanged between the two exposure wavelengths a value for the two-photon cross-section of mTHPC at 800 nm of ?1.8 × 10^?57 m⁴ s/molecule/photon was derived. This is larger than the value estimated using a simple theoretical model, probably because the calculation neglects ??resonant?? assistance provided by the real state at ??650 nm (S ₀). In terms of order of magnitude the values are in agreement with the two-photon cross-section for other organic species.