Optics & Laser Technology 34 (2002) 99–105 www.elsevier.com/locate/optlastec Experimentalinvestigationsonenergy-transfercharacteristicsand performanceofsomelaserdyemixtures R.Ghazy a ; * ,S.A.Zim b ,M.Shaheen a ,F.El-Mekawey a a Laser Laboratory, Physics Department, Faculty of Sciences, Tanta University, Tanta, Egypt b Chemistry Department, Faculty of Sciences, Tanta University, Tanta, Egypt Received 14 August 2001; accepted 11 October 2001 Abstract Energy transfer from both Coumarin 120 (C120) and p-Bis(o-methylstyryl)-benzene (Bis-MSB), energy donors, to coumarin 7(C7) (acceptor) individually has been studied by steady-state emission measurements in methanol. The dye laser characteristics of the above-mentioned dyes have also been studied with respect to the energy transfer mechanism in the same solvent using a pulsed nitro- gen laser as a pumping source. The large values of the critical transfer distances, R0, indicate that the dominant mechanism responsible for energy transfer is due to long-range dipole–dipole interaction between the excited donor and ground state acceptor molecule. Both photophysical and photochemical stability of the donors has been examined under the eect of N2 laser pumping. ? 2002 Published by Elsevier Science Ltd. Keywords: Energy-transfer characteristics; Coumarin 120; p-Bis(o-methylstyryl)-benzene 1. Introduction While laser-pumped dye lasers often convert more than 50%ofthepumpenergyintodyelaseroutputenergy,ash lamp-pumped dye lasers rarely reach even 1%. The reason for this becomes apparent when one looks at the emission spectrumofatypicalxenonashlampandcomparesitwith the absorption spectrum of the typical laser dye rhodamine 6G.Becauseofthenarrowspectralwidthofthedyeabsorp- tion band, only a small fraction of the ash lamp output is absorbed. This fact was recognized quite early and a rem- edy was sought in dye mixtures where a short-wavelength absorbingdyewhoseuorescencebandoverlappedwiththe absorption band of the laser dye was intended to transfer its absorbed energy to the laser dye by absorption of its uorescence energy. This scheme worked in only a very few exceptional cases; normally it was counterproductive uptothecompletequenchingofthedyelaseremission.The causeofthisunexpectedbehaviorwassoonfoundtobethe triplet–tripletabsorptionoftheabsorberdyethatveryoften overlappedwiththeuorescencebandofthelaserdye,thus producing great losses at the dye laser wavelength [1,2]. ∗ Corresponding author. Fax: +20-40-350804. E-mail address: griyad@decl.tanta.enu.eg (R. Ghazy). The best way to avoid this diculty would be to make the energy transfer from the rst excited singlet state of the absorber dye to the laser dye much faster than the triplet-producing intersystem crossing in the absorber (donor) dye. This can be achieved by making the average distance between the donor and the acceptor (laser dye) so small, ¡ 30 A, that radiationless energy transfer of the F orster-type [3] occurs eciently, while still separating the two -electron clouds, thus keeping the spectral signatures of the two dyes unchanged. Even though the strict valid- ityofF orster theory is questionable in the case where the distance between donor and acceptor is smaller than the molecular dimensions, one can nevertheless estimate that thetimeforenergytransferismuchshorterthanapicosec- ond, thus making the energy transfer much faster than all radiationless transitions, leaving the donor molecule no timetomakeatransitiontothetripletstate. The subject of energy transfer between unlike molecules in solutions has received considerable attention [3–8]. The mainmechanismsthathavebeenproposedforenergytrans- fer[9]are: (i) Radiative energy transfer, that is, the absorption of donor emission by an acceptor molecule. (ii) Diusion-controlled energy transfer, this process is a nonradiative process and is viscosity dependent. This 0030-3992/02/$-see front matter ? 2002 Published by Elsevier Science Ltd. PII:S0030-3992(01)00095-0