The Spectroscopic Behaviour of Rhodamine 6G in Liquid and Solid Solutions

: Absorption, fluorescence, quantum yield and lifetime of rhodamine 6G in chloroform, methanol and dimethyl sulfoxide were measured. From a comparison of these quantities, with those for solid solutions (solid solutions are obtained by mixing constant volume proportions of dye at a concentration of 1*10 -4 M/l with different volume proportions from the concentrated solution of polymer in chloroform and dimethyl sulfoxide). The results showed that the addition of polymer to liquid concentrated solutions (1*10 -4 M/l )of rhodamine 6G dye from expecting [which leading to development active medium for laser dye at high concentration] increase the spectra shift toward high energies, and the luminescence quantum yield but decreasing radiative lifetime.


Introduction:
Along the years dye lasers have demonstrated to be useful and versatile sources of tunable coherent radiation with wide applicability in many different fields [1] .Dye oscillators and amplifiers operate under intense optical excitation which results in significant population build up in the lowest excited singlet S1.Absorption from S1 can produce a number of deleterious effects such as reduction of effective gain, interference with optical pumping, and introduction of an extra thermal load due to the energy released as heat in the subsequent relaxation back to S1, as well as photochemical reaction, Thus knowledge of the spectral properties of the spectral properties of S1 state would be necessary to design efficient dye lasers and to determine optimum pumping conditions [2][3][4][5][6][7][8][9] .
In this paper we have study the absorption, emission spectra with a different solvents, and, concentrations of Rh6G, firstly as a solution, and secondly as a solid solution when it mixed with PMMA at a different volume.
A series of solutions of Rh6G at concentrations varying from [10 -6 -10 -4 ] M/l were prepared in chloroform, methanol, and, dimethyl sulfoxide.The solid Solutions were prepared by mixing a constant volume proportion from a concentrated of Rh6G (1*10 -4 M/l) with different volume proportions for concentrated solutions of the polymer in chloroform and dimethyl sulfoxide (VRhB/VPMMA= without polymer, 1/0.25, 1/4, 1/8).
The absorption and florescence spectra were measured by using spectrofluorimeter model RF-500, Shimadzu Com.Which illustrated in Fig. (2).*Baghdad University, Collage of Science, Physics Department,Baghdad, Iraq The quantum efficiencies were measured by the comparative method [10] using as standard solutions [11] {as solution of Rh6G in methanol which has a quantum efficiency of 94%}.

Results and Discussion:
In figures (2, 3, 4) which represents the absorption, emission spectra (A,B,C,a,b,c respectively) of Rh6G in chloroform, methanol, dimethyl sulfoxide for concentrations (10 -4 , 10 -5 , 10 -6 M/l).Absorption maxima, wave numbers, and, lifetime are measured in table (1) [12] .Emission maxima, wave numbers, and, quantum efficiencies were measured in table (2).From this figures and tables we can see that there are shift to the low energy (long wave length) as the concentration increases, while the quantum yield decreases and lifetime increases.The shift in spectra toward long wave length as the concentration is increases.These changes have been interpreted in two different ways.On the one hand there are considered as due to dimerization process [13][14][15][16][17][18][19] .On the other, they are attributed to the change of RhB from its neutral form (Rh6G + ) to the cationic one Rh6GH + [20][21][22][23] Fig. (5).
This theory is based on the decreasing of PH produced when dye concentration increases.The decreasing of quantum yield as the concentration increases, have been interpreted by dimmer molecules which absorb the emission photons.This is due to the band energy of dimer molecules under the band energy of RhB molecules (monomer).These results are agreement with Lopez Arbeloa, and, his team [24] .The decreasing of quantum yield as the concentration increases, have been interpreted by dimmer molecules which absorb the emission photons.This is due to the band energy of dimer molecules under the band energy of Rh6G molecules (monomer).These results are agreement with Lopez Arbeloa, and, his team.
Fig. (6)  represent the absorption spectra (A, B) of Rh6G in chloroform, and, dimethyl sulfoxide of concentration 10 -4 M/l, where it mixed with different proportions from the concentrated solution of polymer in chloroform and dimethyl sulfoxide respectively.The absorption maxima, FWHM and lifetime, are summarized in table (3).Fig. (7)  represents emission spectra (a, b) of Rh6G in chloroform and dimethyl sulfoxide of concentration 10 -4 M/l when it mixed with different proportions from the concentrated solution of polymer in chloroform and dimethyl sulfoxide respectively.Emission maxima, quantum yield of mixed Rh6G solution, and, polymer solutions are summarized in table (4).
From these curves and tables we can get that the addition of the polymer to concentrated solutions (10 -4 M/l)of Rh6G; [from expecting that leading to development active medium for laser dye at high concentrations] where increasing spectra shift toward high energies increasing luminescence quantum yield and decreasing relative radiation lifetime, either in relation liquid solutions that increasing Concentration leading to increasing spectra shift toward low energies, decreasing luminescence quantum yield and increasing relative lifetime.These results are agreement with [25][26][27][28][29][30] .(A, B, C, a, b,c

Fig
Fig. (6B):-The absorption spectra of Rh6G in dimethyl sulfoxide of concentration 10-4M/l with different volume proportions polymer.