Energy transfer from UO++2 → Nd3+ in barium borate glass

Radiative and non-radiative energy transfer from UO⁺⁺₂ to Nd³⁺ has been studied in barium borate glass. Probabilities, efficiencies and the mechanism of non-radiative energy transfer were determined from UO⁺⁺₂ fluorescence decay rates. At low acceptor concentrations pair-wise energy transfer involving migration among donors leads to a non-radiative transfer rate which depends linearly on the concentration of Nd³⁺. It is decided that resonant radiationless dipole-dipole transfer of energy is the dominant mechanism active in this system. However, the observations of transfer yield for high acceptor concentrations are contrary to those at low acceptor concentrations as evidenced from the P_da vs C² plot. Hence at high acceptor concentrations, the transfer could happen between a donor ion and two acceptor ions which is consistent with the view of Fong and Diestler.     

Investigation of Spectral-Luminescent and Lasing Properties of Bifluorophoric Molecules. I. Bifluorophores of Linear Structure

Eight bifluorophoric (BF) molecules on the basis of paratherphenyl, stilbene, phenyl, biphenyl (donors) and coumarin 120 and phenylphtalimide (acceptors) connected by the bridging -CH₂- were experimentally studied. It is shown that the absorption spectra of the BF are virtually additive to donor and acceptor fragments. The acceptor band alone is present in the fluorescence spectrum, which points to a complete nonradiative transfer of the electronic excitation energy. The spectra of induced BF emisson under flash-lamp and laser excitations are also examined.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 74–80, February, 2005.     

Stark splitting and energy transfer between Dy3+-Ho3+ and between Tb3+-Er3+ in calcium-lithium borate glass

Fluorescence spectrum of Dy³⁺, Dy³⁺-Ho³⁺, Tb³⁺ and Tb³⁺-Er³⁺ doped in calibo glass have been studied using Ar⁺ and excimer lasers. Non-radiative energy transfer from trivalent dysprosium and terbium (donors) to holmium and erbium (acceptors) respectively has been observed on the basis of decrease in the life time of the levels and reduction in fluorescence intensity of Dy³⁺ and Tb³⁺ on increasing Ho and Er concentrations. The interaction mechanism of donor and acceptor ions is found to be dipole-dipole in both cases. Various parameters such as donor-acceptor distances, non-radiative energy transfer efficiency (η) and energy transfer probability (P _da) have been computed. Stark splitting have also been marked in several intense transition of the two.     

Intermolecular energy transfer in mixed laser dyes: photophysical properties of triplet states

Triplet-singlet energy transfer in laser dyes have been studied in EPA at 77K using N₂ laser as an excitation source. Phosphorescence of the donor (D) and the delayed fluorescence of the acceptor (A) and their lifetimes have been measured for coumarin 102 (D)-rhodamine B(A) and 9(10H)-acridone (D)-rhodamine 6G(A) dye systems as a function of acceptor concentration. These data yield energy transfer rate constants of ∼10³ dm³ mol⁻¹ s⁻¹ for the donor acceptor combinations, consistent with the Forster mechanism. The phosphorescence quantum efficiency and other spectral parameters are also reported.     

Fluorescence and Laser Emission from Coumarin-Acridine Orange Mixtures

The sensitized fluorescence and laser emissions of dye mixtures; (I) coumarin 102 (donor) and acridine orange (acceptor) and (II) coumarin 47 (donor) and acridine orange (acceptor) with Hg-lamp and N₂ laser, have been measured as a function of dye concentration and of the pump power (N₂ laser). Acridine orange which does not lase by itself on excitation with N₂ laser, lases efficiently in the presence of 7-amino-coumarins via singlet-singlet energy transfer. Energy transfer rate constants and critical distances have been estimated from fluorescence intensity and lifetime measurements. The performance of energy transfer dye lasers (ETDLs) are discussed in terms of spectral characteristics of the dyes and their penetration depths.     

Förster-type energy transfer mechanism in PF2/6 to MEH-PPV conjugated polymers

Energy transfer mechanism between Poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF_2/6) as a donor and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as an acceptor in different solvents has been studied using steady-state emission measurements. Four different solvents namely, tetrahydrofuran (THF), toluene, chlorobenzene (C.B) and benzene have been used in this study. The absorption and luminescence behaviors of the samples are measured at a fixed concentration of donor (0.1 μM) while the concentrations for acceptor are kept in the range of 0.1–1.0 μM. Based on these measurements, the energy transfer properties namely quenching rate constant (k_SV), energy transfer rate constant (k_ET), energy transfer probability (P_DA), transfer efficiency (η) and critical distance of energy transfer (R_o) are calculated. The use of THF resulted in the highest energy transfer. Long range dipole–dipole interaction between the excited donor and ground state acceptor molecules is the dominant mechanism responsible for the energy transfer as proven by the large values of R_o.     

Non-radiative energy transfer from Sm3+→Nd3+ in sodium borate glass

A study of energy transfer from optically excited Sm³⁺ to Nd³⁺ in borate glass has been performed. Contrary to the observations made by Cabezas and DeShazer, the Sm³⁺ → Nd³⁺ energy transfer has been observed as non-radiative. Energy transfer probabilities (P_da) and transfer efficiencies (η_T) have been calculated from our measurements of donor fluorescence intensity and decay times. The mechanism governing the transfer is electrostatic dipole-dipole in nature, contrary to the conclusions made by Nakazawa and Shionoya. At low acceptor (Nd³⁺) concentrations a linear dependence of P_da on acceptor concentration (C) has been observed which suggests the migration of excitation energy among donors. At high acceptor (Nd³⁺) concentration a plot of P_da vs (C_o + C)², where C_o is donor ion concentration, presents a linear dependence which is consistent with the Fong-Drestler theory of dipole-dipole energy transfer mechanism and interaction of one donor (Sm³⁺) with two acceptors (Nd³⁺).     

Photophysical properties, laser activity and photoreactivity of a heteroaryl chalcone: A model of solvatochromic fluorophore

The absorption and fluorescence characteristics of 3-(4′-dimethylaminophenyl)-1-(2-thienyl)prop-2-en-1-one (DMATP) have been investigated in different solvents. DMATP dye exhibits a large red shift in both absorption and emission spectra as solvent polarity increases, indicating a large change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMATP gave an excimer like emission at 570 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B-type class of Steven's classification. A dye solution ca. 10⁻³ mol dm⁻³ in CHCl₃ gave a good laser emission in the range 480–560 nm with emission maximum at 530 nm upon pumping by nitrogen laser (λ_ex=337.1 nm). The excitation energy transfer from 7-dimethylamino-4-methyl coumarine (DMC) to DMATP has been also studied in CHCl₃ and the values of energy transfer rate constant and critical transfer distance indicate a Főrster-type mechanism. The photoreactivity and net photochemical quantum yield of DMATP in chloromethane solvents are also determined. We applied semiempirical MO calculation using AMI and ZINO/S calculation to understand the geometric and electronic structure of DMATP molecule in both ground and excited states.     

Optically pumped high-pressure DF-CO2 transfer laser

Pulsed sealed off CO₂ laser operation at 19 atm gas pressure has been demonstrated by pumping DF/CO₂/He mixtures with radiation from a pulsed DF laser. DF acts as an absorber for the pumping radiation, and the CO₂ upper laser level is populated via subsequent energy transfer. CO₂ pulse energy at 19 atm was 0.5 mJ corresponding to a photon conversion efficiency of 4% measured relative to the number of absorbed pumping photons.     

Laser Charecteristics of a New Dye: 7-Diethylamino-3-cyano Benzimidazo(1,2-a)quinoline by Energy Transfer Dye Laser Mechanism

An experimental study of the laser characteristics, by energy transfer machanism of a new dye: 7-Diethylamino-3-Cyano Benzimidazo (1,2-a) Quinoline (DCBQ) using 7-Diethylamiho-4-Methyl Coumarin (Coumarin 1,DAMC) as a donor are presented. The effect of the concentration of the components on the lasing characteristics like peak power and wavelength of lasing of dye mixture is studied and discussed. The effect of the solvents on the fluorescence energy transfer has also been studied to ascertain the nature of the transfer mechanism involved. The fluorescence energy transfer rate constant(k_ET) and the critical transfer distance(R_o) are experimentally determined and also calculatee theoretically. These results indicate the dominance of resonance transfer in this energy transfer dye laser.     

Nature of the Electron Excited States and Energy Transfer in Bichromophore Coumarin Molecules

Results of experimental and theoretical research for three bichromophore molecules, trans-stilbene-CH₂-coumarin 120 (I), 4-methylumbelliferone-CH₂-UC 17, and 4-(3-fluoro)-methylumbelliferone-CH₂-UC 17 (II, III), are presented. Schemes of photophysical processes in the bichromophore molecules based on quantum chemical calculations by the INDO method and theory of radiationless transitions in polyatomic organic molecules are suggested. After optical excitation to the strong donor absorption band, the fast internal conversion processes develop there. As a result, the molecule is found in the S ₁ ^* -state localized on the acceptor moiety. It is shown that a mechanism of intramolecular transfer energy in bichromophores different from that proposed by Förster may be realized. Excitation energy, initially located on D, will be transferred from the donor moiety to the acceptor chromophore in convenience of the internal conversion process. The intramolecular electronic energy transfer from energy donor to energy acceptor may be interpreted as the internal conversion process. The rate constants of internal conversion are calculated.     

7-Hydroxy-4-methyl-8-(4′-methylpiperazine-1′-yl)methyl coumarin: An efficient probe for fluorescence resonance energy transfer to a bioactive indoloquinolizine system

Solvatochromic effects on the fluorescence behavior of 7-hydroxy-4-methyl-8-(4′-methyl-piperazine-1′ yl)methylcoumarin (HMMC) was studied in different solvents. The fluorescence of HMMC was found to be highly sensitive to both the polarity and the protic character of the solvent. Exploiting the polarity-sensitive fluorescence property of HMMC, its excited-state dipole moment has been determined. Fluorescence (Förster) resonance energy transfer (FRET) process from HMMC to a potent bioactive molecule 3-acetyl-4-oxo-6,7-dihydro-12 H indolo-[2,3-a] quinolizine (AODIQ) was studied. From the determined K_SV and R₀ values, it is argued that a long-range dipole-dipole interaction is operating for the energy transfer mechanism. The energy transfer efficiency (E) and the distance between the acceptor and the donor (r₀) have been determined.     

Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye

The optical properties including electronic absorption spectrum, emission spectrum, fluorescence quantum yield, and dipole moment of electronic transition of 7-diethylaminocoumarin (DEAC) laser dye have been measured in different solvents. Both electronic absorption and fluorescence spectra are red shifted as the polarity of the medium increases, indicating that the dipole moment of molecule increases on excitation. The fluorescence quantum yield of DEAC decreases as the polarity of solvent increases, a result of the role of solvent polarity in stabilization of the twisting of the intramolecular charge transfer (TICT) in excited state, which is a non-emissive state, as well as hydrogen bonding with the hetero-atom of dye. The emission spectrum of DEAC has also been measured in cationic (CTAC) and anionic (SDS) micelles, the intensity increases as the concentration of surfactant increases, and an abrupt change in emission intensity is observed at critical micelle concentration (CMC) of surfactant. 2×10⁻³ mol dm⁻³ of DEAC gives laser emission in the blue region on pumping with nitrogen laser (λ_ex=337.1 nm). The laser parameters such as tuning range, gain coefficient (α), emission cross section (σ_e), and half-life energy have been calculated in different solvents, namely acetone, dioxane , ethanol, and dimethyforamide (DMF). The photoreactivity of DEAC has been studied in CCl₄ at a wavelength of 366 nm. The values of photochemical yield (?_c) and rate constant (k) are determined. The interaction of organic acceptors such as picric acid (PA), tetracyanoethylene (TCNE), and 7,7,8,8-tetracynoquinonedimethane (TCNQ) with DEAC is also studied using fluorescence measurements in acetonitrile (CH₃CN); from fluorescence quenching study we assume the possible electron transfer from excited donor DEAC to organic acceptor forming non-emissive exciplex.     

Donor-acceptor recombination in type-II GaAs/AlAs superlattices

A study is reported of steady-and nonsteady-state photoluminescence of intentionally undoped and uniformly silicon-doped type-II (GaAs)₇(AlAs)₉ superlattices grown by MBE simultaneously on (311)A-and (100)-oriented GaAs substrates. It has been established that at elevated temperatures (160>T>30 K) the superlattice spectra are dominated by the line due to the donor-acceptor recombination between donors in the AlAs layers and acceptors located in the GaAs layers. The total carrier binding energy to the donor and acceptor in a pair has been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1734–1739 (September 1998)     

Laser transitions under resonant optical pumping of donor centres in Si:P

Terahertz stimulated emission of phosphorus donors in silicon optically excited by radiation from the free-electron laser FELIX has been studied. It is found that a spectral line of the Si:P laser emission depends on pump frequency. Stimulated emission arises on the 2p₀→1s(E) intra-centre transition (21.2 meV) under resonant pumping of the 2p₀ state and on the 2p₀→1s(T₂) transition (22.3 meV) under pumping of the 2p_± or higher odd-parity donor states. The line shift is attributed to the Auger redistribution of the 1s(E)- and 1s(T₂)-state populations. Received: 7 November 2002 / Revised version: 7 April 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-30/6705-55-07, E-mail: sergeij.pavlov@dlr.de RID="**" ID="**"On leave from: The Institute for Physics of Microstructures, Nizhny Novgorod, Russia     

Effect of different donors and a polymer environment on photophysical and energy transfer studies using C540 as the acceptor

The dyes (C450, C480 and C540) and their dye mixtures (C450:C540 and C480:C540) were doped in polymer matrices (solid). Their photophysical studies were recorded. These results were analysed by comparing them with the data of the dyes and the dye mixtures in monomer compositions (liquid). The absorption and fluorescence spectral profiles of the dyes in the polymer matrix were found to be identical to those in the monomer compositions. The effect of different donors on the energy transfer technique using C540 as acceptor, in polymer matrix and monomer compositions, was studied in detail. The results obtained for the energy transfer technique in two binary dye mixtures containing different donors but same acceptor, in solid and liquid media, were intercompared. The gain of the acceptor without donor and with different donors was determined experimentally. The gain coefficient in the polymer matrix (solid) was less than that in the monomer medium (liquid). Also, the gain of the acceptor C540 was found to be more when C450 was used as the donor compared to that when C480 was used as the donor. Using nitrogen laser, the photobleaching effect in the two binary dye-doped polymer rods (with different donors but same acceptor) was studied. It was observed that photobleaching of the acceptor C540 in the presence of C450 as donor is slower than that in the presence of C480 as donor.     

Nitrogen related excitation processes in a mercury bromide laser discharge

The role of nitrogen as a buffer gas constituent in an avalance discharge mercury bromide laser has been examined, and it is shown that a change over in excitation from electron impact dissociation of HgBr₂ to energy transfer dissociation by excited nitrogen occurs at 1.5%N ₂. This is explanable in terms of excitation cross-sections if considerable pumping occurs from excited states of nitrogen which are higher in energy than the metastableA state. When excitation via nitrogen is dominant, a high dischargeE/N value is needed to avoid coupling of energy to unwanted vibrational excitation.     

Gain studies on a uranine-damc dye mixture laser under nitrogen laser pumping

The radiative and Forster type of energy transfer processes in a dye mixture laser of 7-diethylamino-4-methyl coumarin (donor) and fluorescein disodium salt (acceptor) under nitrogen laser pumping were investigated. The Forster transfer rate calculated from the absorption and emission spectra of acceptor and donor is 1.3 × 10¹¹ liter mole^?1sec^?1. The gain of acceptor at 550 nm was measured for acceptor concentrations N_A from 10^?3M to 3 × 10^?3M for a fixed ratio F = 1 of donor to acceptor concentrations at different pump powers. The results agreed with the rate equation model proposed for the dye mixture laser. The radiative rate constant calculated from these results is 3.1 × 10¹⁰ liter mole^?1sec^?1. Numerical simulation of the rate equations showed that the acceptor reaches peak emission with a time lag of 3 ns with respect to the donor peak emission for F = 0.998, N_A = 10^?5M. This time lag decreases with increasing N_A and becomes zero for N_A = 10^?1M, F = 0.048.     

用受激拉曼泵浦方法高效制备v=2振动激发态氘分子

We report the preparation of D₂ molecules in v=2 level in molecular beam condition. A single longitudinal mode laser system was used for excitation of D₂ from (v=0, j=0) to (v=2, j=0) with the scheme of stimulated Raman pumping. An excitation efficiency of 25.2% has been achieved, which was determined by the scheme of resonance-enhanced multiphoton ionization. Dependence of relative excitation efficiency on laser energy has been measured. We found that the increasing rate of excitation efficiency became slower as pulse energy of Stokes laser increase, while the excitation efficiency still increases approximately linearly with pump pulse energies up to 60 mJ. The spectral line shapes of Raman transition was also measured at different laser energies and considerable dynamical Stark effect was observed. A single peak was found on the three dimension surface of relative excitation efficiency, indicating the process occurred in the present study is a process of stimulated Raman pumping instead of stimulated adiabatic Raman passage.     

Pumping of titanium sapphire laser

Two methods of Ti:Sapphire pumping for the generation of tunable laser radiation in the visible region were studied. For coherent pumping, the radiation of the second harmonic of a Nd:YAP laser was used and a maximum output energy ofE _out=4.5 mJ was reached from the Ti:Sapphire laser. For noncoherent pumping, two different lengths of flashlamp pulses were used and a maximum ofE _out=300 mJ was obtained. Preliminary estimations of the wavelength range of tunability were made.