Investigation of variation in fluorescence intensity from rhodamine 6G dye in a trapped and levitated liquid micro-drop

Abstract

Variation in fluorescence intensity from rhodamine 6 G dye was investigated. A small volume of dye solution, in the form of a trapped and levitated micro-drop, was optically excited with a 400 μW, 532 nm wavelength cw-laser light. The dye was dissolved in methanol and glycerol for a concentration of 10 mg/ml or 21 mMole per liter. With the optical excitation, initially the fluorescence intensity was observed to rise with time, and then it decayed, along with a steady shift of fluorescence peak from 562 nm to 543 nm. The observation of initial enhancement in fluorescence from start to 7 min of excitation, can partly be due to the low excitation power, less dimer formation and gradual rise in molecular population to excited state therefore slower rate of change (increase) of fluorescence intensity was observed with time. Simulation studies indicate that the photo bleaching was taking place from all the energy states of the dye molecules, which is an extension of the concept that the photo bleaching takes place at the excited triplet state whereas the fluorescence takes place due to transition between excited singlet states and ground states. A steady shift in fluorescence peak position, from 562 nm to 543 nm, was observed during the fluorescence life of the dye, at a rate of 0.0113 nm/s during fluorescence enhancement and 0.026 nm/s during photo-bleaching. The steady blue shift of fluorescence indicate insignificant dimer formation.