MULTIPHOTON-EXCITED VISIBLE EMISSION BY SEROTONIN SOLUTIONS

Nonlinear excitation of the neurotransmitter serotonin (5HT) in aqueou s solution is shown to generate a blue-green-emitting photoproduct in addition to UV fluorescence characteristic of native 5HT. The visible emission rate in diffusional steady-state measurements scales as the s ixth power of excitation intensity, demonstrating that absorption of s ix near-LR photons is required to generate emission of one visible pho ton. Transient measurements reveal that this process is composed of tw o sequential nonlinear steps, the first excited by four photons and th e second by two photons. These results, in combination with measuremen ts of multiphoton-excited serotonin UV fluorescence, support a model i n which 5HT is photochemically transformed as a consequence of four-ph oton absorption (E-tot approximate to 6 eV) to a photoproduct that the n emits in the visible region via two-photon excitation. A minimum bou nd of similar to 10(-51) cm(4) s photon(-1) is observed for the two-ph oton emission action cross section at 830 nm. Photoionization, rather than reaction with a dissolved oxygen species, appears to be the prima ry mechanism for generation of the blue-green-emitting photoproduct. T he peak intensities required to generate significant blue-green emissi on (similar to 5 x 10(11) W cm(-2) from 80 MHz 150 fs titanium:sapphir e laser pulses) are approximately five-fold higher than are typically used in two-photon laser scanning microscopy but are still substantial ly lower than the estimated intensity needed to induce dielectric brea kdown of water.