Radiative and non-radiative (Forster type) energy transfer processes in a dye mixture of FDS and Rh B in methanol under cw Ar ion laser excitation were investigated for fixed donor concentration and varying acceptor concentration. It is found that most of the pump power absorbed by FDS is transferred to Rh B as a useful pump power. Transfer probability ( P DA ), transfer efficiency both radiative ( η R ) and non-radiative ( η NR ) and optical gain ( G ) of the system were studied for various pump powers. The gain characteristics of Rh B are found to alter due to the change in the effective fluorescence lifetime caused by energy transfer reaction. Theoretical calculations were also done to find the total transfer efficiency ( η T ) at various acceptor concentrations to identify the appropriate energy transfer mechanism responsible for gain enhancement in Rh B. Both radiative and non-radiative transfer processes are taken into consideration in all the calculations. Various energy transfer parameters viz. radiative rate constant ( K R ), non-radiative rate constant ( K NR ), critical concentration ( C 0 ), critical radius ( R 0 ) and half quenching concentration ([A] 1/2 ) are calculated by using the Stern–Volmer plots and concentration dependence of radiative and non-radiative transfer efficiencies. Concentration and pump power dependence of the peak gain and lasing wavelengths of the energy transfer dye lasers (ETDL) have also been studied. The experimental results show that the dominant mechanism responsible for the efficient excitation transfer in this mixture is of radiative nature, whereas the long range dipole–dipole (d–d) interaction (Forster type) is comparatively smaller.

Energy transfer and optical gain studies of FDS: Rh B dye mixture investigated under cw laser excitation

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Energy transfer mechanism between laser dyes doped in ORMOSILs

Energy transfer of Rh 6G:Rh B dye–mixtures in a poly methyl methacrylate (PMMA) matrix was studied by investigating the emission spectral characteristics under argon ion laser excitation The fluorescence behaviour of the acceptor dye (Rh B) was investigated for various concentration of the donor (Rh 6G) and excitation powers The energy transfer rate constants (kf ), transfer probability ( Pda) and the transfer efficiency ( η) have been evaluated and their concentration dependence on donor molecules was studied The observed decrement in the emission peak wavelength of the acceptor dye with donor concentration is attributed to the formation of complexes in the dye–mixture and also to the result of cross-relaxation taking place in the donor dye system The blue shift in the emission wavelength of the acceptor was also investigated for various donor concentrations A significant enhancement of fluorescence intensity of the acceptor (Rh B) of the order of 226% and a transfer efficiency of 90% were observed in the present dye–mixture system as compared with pure Rh B, both in PMMA matrices Further the experimental results revealed the non-radiative type energy transfer for the dye system in the PMMA matrix © 2002 Elsevier Science BV All rights reserved

Energy transfer in Rh 6G:Rh B system in PMMA matrix under cw laser excitation

Study of laser behaviour in C-440 and C-485 dye mixture

We have explored the photogeneration of the coumarin 314 radical cation by using nanosecond laser excitation at wavelengths longer than 400 nm in benzene, acetonitrile, dichloromethane, and aqueous media. In addition, time-resolved absorption spectroscopy measurements allowed detection of the triplet excited state of coumarin 314 (C(314)) with a maximum absorption at 550 nm in benzene. The triplet excited state has a lifetime of 90 μs in benzene. It is readily quenched by oxygen (k(q) = 5.0 × 10(9) M(-1) s(-1)). From triplet-triplet energy transfer quenching experiments, it is shown that the energy of this triplet excited state is higher than 35 kcal/mol, in accord with the relatively large singlet oxygen quantum yield (Φ(Δ) = 0.25). However, in aqueous media, the coumarin triplet was no longer observed, and instead of that, a long-lived (160 μs in air-equilibrated solutions) free radical cation with a maximum absorbance at 370 nm was detected. The free radical cation generation, which has a quantum yield of 0.2, occurs by electron photoejection. Moreover, density functional theory (DFT) calculations indicate that at least 40% of the electronic density is placed on the nitrogen atom in aqueous media, which explains its lack of reactivity toward oxygen. On the other hand, rate constant values close to the diffusion rate limit in water (>10(9) M(-1) s(-1)) were found for the quenching of the C(314) free radical cation by phenolic antioxidants. The results have been interpreted by an electron-transfer reaction between the phenolic antioxidant and the radical cation where ion pair formation could be involved.

Coumarin 314 free radical cation: Formation, properties, and reactivity toward phenolic antioxidants

The tunability of organic dye lasers has been reviewed by investigating the optical gain characteristics of a coumarin 485-rhodamine B (C 485-Rh B) dye mixture in ethanol subject to N 2 laser excitation. The energy transfer parameters (rate constants of radiative and non-radiative energy transfer processes and the critical transfer radius) have been evaluated with the help of Stern-Volmer plots and the pump power dependence of the gain. The calculated values of radiative and non-radiative energy transfer processes are 0.86 × 10 −11 and 0.45 × 10 −11 L mol −1 s −1 respectively, which shows that the two processes are contributing equally in the present mixture.

Arun Gaur

Papers

Energy transfer excitation in an N2-laser-pumped coumarin 485-rhodamine B dye mixture through optical gain characteristics