J. Baghdad for Sci. Vol.9(2)2012 243 Study the spectroscopic characteristics of Rhodamine B Dye in Ethanol and Methanol mixture and Calculation the Quantum Efficiency Rafed A. Ali* Oday Mazin Abdul-Munem* Ahmed Naji Abd* Received 13, October, 2010 Accepted 11, May, 2011 Abstract: The effect of ethanol and methanol solvent, and their mixture has been studied on the absorption and fluorescence spectra of laser dye Rhodamine B at concentration of (10 -4 ) Molar at room temperature. The molar absorption coefficient has been determined for mixture which was (3.223) at wave number (18181.8 cm -1 ), Also the Quantum Efficiency of the two solvents (ethanol and methanol) and their mixture have been calculated ,which was for mixture spectrum (38.94%) and it was larger comparing with other and solvents. The characteristics of spectrum has been determined by calculating (Δλ) of absorption spectrum for the solvents and its mixture at maximum wave number (  ) cm -1 depending on solvent polarity and the transitions between molecular energy levels in each solvent of Rhodamine B dye . Key wards: Rhodamine B, Quantum Efficiency, laser dye, ethanol and methanol. Introduction: In the past decade we have made some surface photochemical studies of several dyes, including Rhodamines, absorbed onto a scarcely used absorbent powdered microcrystalline cellulose [1-6 ]. From these studies, a large amount of information concerning room-temperature fluorescence and phosphorescence [1- 4] triplet-triplet energy transfer,3 electron and hydrogen transfer and the nature of the absorption process [5]. As studied in field in the Rhodamine B and related xanthene dyes have been extensively in solution [7-10]. Both molecular structure and solvent play a very important role in the nonradiative pathway of deactivation of the first excited state of these dyes. The cationic forms of Rhodamine B and Rhodamine 101 in acidic ethanol present different [11]. The amino groups in Rhodamine B are fully ethylated, while in Rhodamine 101 the amino groups are rigidly attached to the xanthenes ring by methylenic bridges. The precise interpretation of the mechanism of internal conversion appears not to be completely stab Rhodamine 101. it also shows a monoexponential decay in ethanol (τ F in ethanol is 4.3 ns and in water is 4.2 ns),while that absorbed on glass with distorted adsorption sites shows a marked decrease in the lifetime [10]. This fact is indicative of the formation of nonplanar conformers. The most important distinguishing characteristic of reactive dyes is that they form covalent bonds with the substrate that is to Rhodamine B xanthene dye[12]. Upon excitation, there is a change in its dipole moment associated with the internal twisting of the diethylamino group about the CN bond (see figure 1), A twisted- intramolecular-charge-transfer (TICT) state that it formed by the internal *Department of Physics, College of Science, Al-Mustansiriyah University