Fluorescence Line Narrowing Technique Research Articles

Eu3+ Site Distribution and Local Distortion of Photoluminescent Ca3 WO6 :(Eu3+ , K+ ) Double Perovskites as High-Color-Purity Red Phosphors.

In this study, Eu3+ and K+ co-doped Ca3 WO6 double perovskite, a high-color-purity red phosphor, is quantitatively investigated using synchrotron X-ray diffraction Rietveld refinement, energy dispersive X-ray spectroscopy, and high-resolution PL spectroscopy. The Eu3+ fluorescence line-narrowing (FLN) results, used to estimate the Eu3+ occupation at given sites (so-called A and B sites) in the host crystal (A2 BMO6 ; A, B=Ca; M=W), reveal that the Eu3+ ions have a twin distribution in both the A and B sites with high asymmetry ratios of ΛA =9.7 and ΛB =10.7, respectively. More interestingly, at lower Eu3+ doping levels, the ions are predominantly located at the B sites (≈75%), indicating that the high color purity of Ca3 WO6 :(Eu3+ , K+ ) is mainly caused by the high asymmetry ratios of the Eu3+ sites. Rietveld refinement combined with the FLN technique provides more reliable results for increasing the Eu3+ substitution at the A site with Eu3+ and K+ doping concentrations, which lower the lattice energy of Ca3 WO6 :(Eu3+ , K+ ). The structural distortions owing to K+ co-doping in terms of the quadratic elongation and bond-angle variance exhibit good correlations with the Ca(Eu)O12 A-site cuboctahedra and Ca(Eu)O6 B-site octahedra, partially accounting for the higher Λvalues.

A Comparative Study of the Band-Edge Exciton Fine Structure in Zinc Blende and Wurtzite CdSe Nanocrystals

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright-dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

Fluorescence studies of porphycene in various cryogenic environments

Low temperature fluorescence of porphycene, a structural isomer of porphyrin, has been studied using polymer samples, matrix isolation, and fluorescence line narrowing (FLN) techniques. Contrary to the case of the chromophore embedded in a nitrogen matrix, the emission from polymer samples at temperatures above 10 K exhibits strong dependence on the wavelength of excitation: increasing the excitation energy leads to gradual broadening and, finally, loss of vibrational structure. A rather unusual observation is the similarity of the structured fluorescence spectra obtained for excitations into S1 and S2 states. This finding indicates a correlation between the site distributions in S1 and S2. A similar idea has been put forward earlier for tetraphenylporphyrin [I. Lee, G. J. Small, and J. M. Hayes, J. Phys. Chem. 94, 3376 (1990)]. We propose that the correlation is due to isotropic polarizability in the molecular plane; calculations confirm such hypothesis. For porphycene, an additional factor that can contribute to the effect is a rapid trans-trans tautomerization that leads to the rotation of x and y in-plane axes of the molecule. FLN spectra reveal significant band broadening for excitation into S2. This suggests that the site correlation is not of 1:1 type or that at 4.2 K the site exchange dynamics is frozen in comparison with the situation at higher temperatures.

Investigation of the local environment of Eu3+ in a silicophosphate glass using site-selective spectroscopy and Molecular Dynamics simulations

Silicophosphate glasses (SiO2-P2O5) doped with Eu3+ ions were synthesized by the sol-gel process. Optical properties of these glasses were investigated by means of emission spectra and lifetime measurements. The Fluorescence Line Narrowing (FLN) technique was also used to explore the local structure around the Eu3+ ions in this host and to understand the role of phosphate as a codopant. As it is the case for aluminum, the ability of phosphate to avoid the rare earth clustering was investigated, and the role of this codopant in modifying the local order around the rare earth ion was evidenced. The analysis of the FLN spectra and lifetime measurements is consistent with this interpretation. Molecular Dynamics simulations were performed to evaluate and confirm these structural features. Two classes of europium sites were distinguished in agreement with the experimental characterization.

Electron-Vibrational Coupling and Fluorescence Spectra of Tetra-, Penta-, and Hexacoordinated Chlorophylls c1 and c2.

Chlorophylls (Chls) are a group of pigments related to light absorption, excitation energy, and electron transfer in photosynthetic complexes. Given the importance of intramolecular nuclear motion for these electronic processes, many experimental studies were performed in order to relate its coupling to electronic coordinates of these pigments, but a detailed analysis is still lacking for isolated Chls c1 and c2. To gain insight into the intramolecular motion and fluoroscence spectra of these two pigments in tetra-, penta-, and hexacoodinated states, we performed a quantum chemical study based on density functional theory and multimode harmonic approximation with displaced, distorted, and rotated normal modes. In order to benchmark the employed methods, we simulated the high-resolution fluorescence spectra of tetracoodinated Chls a, b, and d and compared them with available experimental spectra obtained with fluorescence line-narrowing techniques. Although the experimental spectra were obtained for ligand coordinated Chls, qualitatively good agreement was found between the simulated and experimental spectra. Almost all resonances were reproduced in the spectroscopically interesting region from 200 to 1700 cm(-1). The significance of mode distortion and rotation for the simulated spectra is discussed. The fluorescence spectra of Chls c1 and c2 consist of a group of peaks in the 200-450 cm(-1) spectral range, a group of weak peaks from 700 to 1000 cm(-1), and a large group of strong peaks from 1100 to 1600 cm(-1). Ligand effects are also addressed, and a mode is identified as a sensitive probe for the coordination state of Chls c1 and c2.

Fluorescence spectroscopy study of mesoporous SiO2 particles containing Gd2O3:Eu3+

Site-selective and fluorescence line narrowing (FLN) laser spectroscopy was used to study the structure of Eu3+-doped Gd2O3-based material embedded in the mesopores of submicron-sized monodisperse spherical amorphous SiO2 particles that are of interest for biomedical applications. The phase composition of the material in the pores was determined and the transformations that occur under heat treatment were studied. The room-temperature homogeneous linewidth of 5D0 − 7F0 transition of Eu3+ ions in different Gd2O3/SiO2 phases was measured with FLN technique. The radiative lifetime of the 5D0 excited state of Eu3+ ions was measured with time-resolved fluorescence technique; the local field effects on radiative transitions in the composite material are discussed.

Demonstration and interpretation of significant asymmetry in the low-resolution and high-resolution Q y fluorescence and absorption spectra of bacteriochlorophyll a

Low- and high-resolution absorption and fluorescence emission Q(y) spectra of bacteriochlorophyll a (BChl a) were recorded, along with homogeneous band line shapes, revealing significant asymmetry between the absorption and emission profiles that are interpreted using a priori spectral calculations. The spectra were recorded in a range of organic solvents facilitating both penta- and hexa-coordination of Mg at ambient and cryogenic temperatures. Detailed vibrational structure in the ground electronic state, virtually independent of Mg coordination, was revealed at 4.5 K by a hole-burning fluorescence line-narrowing technique, complementing the high-resolution spectrum of the excited state measured previously by hole burning to provide the first complete description of the Q(y) absorption and fluorescence spectra of BChl a. Spectral asymmetry persists from 4.5 to 298 K. Time-dependent density-functional theory calculations of the gas-phase absorption and emission spectra obtained using the CAM-B3LYP density functional, curvilinear coordinates, and stretch-bend-torsion scaling factors fitted to data for free-base porphyrin quantitatively predict the observed frequencies of the most-significant vibrational modes as well as the observed absorption∕emission asymmetry. Most other semi-empirical, density-functional, and ab initio computational methods severely overestimate the electron-vibrational coupling and its asymmetry. It is shown that the asymmetry arises primarily through Duschinsky rotation.

Fluorescence line narrowing spectroscopy of Eu 3+ in zinc–thallium–tellurite glass

The environment of Eu 3+ in zinc–thallium–tellurite glass of the molar composition 60TeO 2–30TlO 0.5–9.9ZnO–0.1Eu 2O 3 was investigated by laser-induced fluorescence line narrowing (FLN) techniques using Eu 3+ as a local site probe. From the site selective luminescence spectra of Eu 3+ at 7 K, the energies of the Stark components of the 7 F 1 and 7 F 2 states were recorded and then the crystal field parameters B nm were calculated assuming a C 2 v site symmetry. The ratios B 22/ B 20 and B 44/ B 40 for each excitation energy within 7 F 0– 5 D 0 transition were obtained and compared with the values calculated for Eu 3+ in other types of glasses.

Wavelength-dependent electron–phonon coupling in impurity glasses

By applying an advanced hole-burning fluorescence line-narrowing technique, an unexpectedly strong (several-fold) increase of the electron–phonon coupling strength on excitation wavelength through the inhomogenously broadened absorption origin band was first observed for low-temperature organic glass samples doped with biologically relevant chlorin and chlorophyll a molecules. The dependence, which suggests a direct correlation between solvent shift and the electron–phonon coupling, demonstrates great sensitivity of the zero-phonon transitions localized on single chromophores as optical probes of the structure and dynamics of local subnanoscopic environments.

Mirror symmetry and vibrational structure in optical spectra of chlorophyll a

The absorption and fluorescence emission spectra of chlorophyll a in different organic solvents where the central Mg atom is either penta- or hexacoordinated have been studied using conventional and selective spectroscopy methods at ambient and cryogenic temperatures. A breakdown of the basic model mirror-symmetry rule in relation to the lowest-energy Q(y) transitions was observed due to Franck-Condon and Hertzberg-Teller interactions. Detailed vibrational structure in the ground electronic state, virtually independent of the Mg coordination state, was revealed by hole-burning fluorescence line-narrowing technique. The total Huang-Rhys factor associated with the linear vibronic coupling strength of the solvent collective vibrations and the local chlorophyll a intramolecular vibrations is equal to 0.53+/-0.07 in fluorescence and to 0.39+/-0.05 in absorption. The electron-phonon coupling part was also found to depend on the excitation wavelength within the inhomogeneously broadened absorption origin band, its average value being S(ph) approximately = 0.38. All these numbers qualify for the weak vibronic coupling. A comparison of the conjugate Q(y) absorption and fluorescence emission spectra as well as the temperature dependence of the absorption spectra allowed unambiguous locating of the still controversial Q(x) absorption band position for penta- and hexacoordinated chlorophyll a species. The basic experimental findings have been qualitatively supported by semiempirical quantum chemical calculations.

Fluorescence line narrowing and decay dynamics of Er 3+ ions in silicon-rich silicon oxide multilayers

Erbium doped silicon-rich silica layer, known for their potential application for active devices, have been investigated by means of site selective laser spectroscopy and fluorescence dynamics at low temperature. It is pointed out that only one erbium local surrounding is identified within the relaxation limit introduced by the non-resonant fluorescence line narrowing technique. Furthermore, radiative relaxation of erbium ions exhibits a non-exponential behavior tentatively explained by diffusion-limited energy transfer between erbium ions and towards unknown traps.

Electron–phonon and vibronic couplings in the FMO bacteriochlorophyll a antenna complex studied by difference fluorescence line narrowing

The electron–phonon and vibronic couplings governing the spectral properties have been studied in the Fenna–Matthews–Olson (FMO) bacteriochlorophyll a (BChl a)–protein complex at 4.5 K using a spectrally selective difference fluorescence line-narrowing technique. The complex is a part of the light-harvesting system of green photosynthetic bacteria. Its lowest-energy absorption band, peaking at 826 nm and responsible for the fluorescence, is believed to be due to Q y transitions of largely isolated molecules. One of the main merits of the used method compared with the more common fluorescence line narrowing is that the zero-phonon lines (ZPL) resonant with the excitation laser can be accurately measured, allowing precise determination of the Huang–Rhys (HR) factors, the main characteristics of the linear electron–phonon and vibronic coupling strengths. Over 60 individual vibrational modes of intra- and intermolecular origin have been resolved in the energy range of 45–1600 cm −1. The small HR factors for these modes, ranging between 0.001 and 0.018, add up to a value of S vib=0.38±0.07. The effective HR factor for the phonons, S ph, was found clearly wavelength-dependent, varying from ∼0.7 at short wavelengths to ∼0.3 at the long-wavelength tail of the absorption spectrum. Coupling between the BChl a molecules is likely responsible for this wavelength dependence.

Inhomogeneity of Ho3+ activated SrLaGa3O7 and SrLaGaO4 crystals studied by fluorescence line narrowing technique

Spectroscopic properties of Ho3+ in SrLaGa3O7 (SLG) and SrLaGaO4 (SLO) are studied using non-resonant fluorescence line narrowing technique. Spectrally-different ions in inhomogeneous absorption line of holmium are selectively excited. The low temperature emission and fluorescence decays from the 5S2 level of Ho3+ ion after pulsed 5I8 → 5S2 excitation transition are investigated. The observed line-narrowed fluorescence is used to analyze the origin of spectral line broadening phenomenon in SLG and SLO hosts. Based on the experimental results an influence of the statistical distribution of La3+ and Sr2+ ions in crystal lattice on the spectroscopic properties of Ho3+ is discussed.

Investigation of site-selective symmetries of Eu3+ ions in KPb2Cl5 by using optical spectroscopy

Site-selective time-resolved spectroscopy of Eu3+ in KPb2Cl5 has been investigated by using fluorescence line narrowing technique. A crystal field analysis and simulation of the experimental results has been performed in order to parametrize the crystal field at the Eu3+ sites. Three symmetry independent crystal field sites for the rare-earth ion in this crystal were found. A plausible argument about the crystallographic nature of these sites is given.

Spectroscopic effects of disorder in laser materials

The inhomogeneous broadening of optical transitions in solids is of fundamental importance in understanding the interaction of optical centres with their environment. In perfect crystals optical linewidths are determined by excited state lifetime and other d-phasing mechanisms such as the electron-phonon interaction. However, in real crystals random differences of impurities in crystallographically equivalent sites result in overlapping transitions and inhomogeneously broadened Gaussian lines. The suppression of inhomogeneous broadening of solid-state spectra using optical hole-burning and fluorescence line-narrowing techniques is discussed in terms of the determination of the homogeneous widths, multi-site geometries and the distributions of spectroscopic parameters that accompany disorder in a number of laser crystals and glasses.

Fluorescence line-narrowing spectroscopy of a sodium phosphotantalate glass doped with Eu3+

Abstract A new sodium phosphotantalate glass of molar composition 4.4Na2B4O7-84.3NaPO3-10.8Ta2O5-0.5Eu2O3 was studied using laser-induced fluorescence line-narrowing techniques. From the site-selective luminescence spectra of Eu3+ at 77 K, the energies of the Stark components of the 7F1 and 7F2 multiplets were obtained and a crystal-field analysis was carried out assuming a C 2v site symmetry. The dependence of the crystal-field parameters B kq on the excitation energy is similar to that observed in other oxide glasses. The behaviour of the crystal-field ratios B 22/B 20 and B 44/B 40 agrees reasonably well with the values calculated using the geometric model proposed by Brecher and Riseberg and with previous observations made for different metaphosphate glasses. On the other hand, the crystal-field strength parameter 5 is distinctly lower than for these glasses.

In situ detection of ALA-stimulated porphyrin metabolic products in Escherichia coli B by fluorescence line narrowing spectroscopy

In a recent work [Photochem. Photobiol. B: Biol. 50 (1999) 8] the successful photodynamic inactivation of Escherichia coli bacteria by visible light was reported based on δ-aminolevulinic acid (ALA)-induced endogenous porphyrin accumulation. In this work, the identification of these porphyrin derivatives in intact bacteria was performed by low-temperature conventional fluorescence and fluorescence line narrowing (FLN) techniques. Conventional fluorescence emission spectroscopy at cryogenic temperatures revealed the presence of the free-base porphyrins, identified earlier by high-performance liquid chromatography analysis of disintegrated bacterial cells after ALA induction; however, emission maxima characteristic for metal porphyrins were also observed. We demonstrated that the primary reason for this signal is that metal porphyrins are formed from free-base porphyrins by Mg2+ ions present in the culturing medium. Incorporation of Zn ions originating from the glassware could also be supposed. In the FLN experiment, the energy selection effect could be clearly demonstrated for (0,0) emissions of both the free-base and the metal porphyrins. The comparison of the conventional emission spectra and the bands revealed by the FLN experiment show that the dominant monomeric structural population is that of metal porphyrins. The intensity and the shape of the FLN lines indicate an aggregated population of the free-base porphyrins, beside a small monomeric population.

Energy migration of the local excitation at the Eu3+ site in a Eu–O chemical cluster in sol-gel derived SiO2:Eu3+ glasses

By using the fluorescence line-narrowing technique, we observed a broad fluorescence band in the vicinity of a resonant line of the D05→7F0 transition in an Eu3+-doped SiO2 glass synthesized by a sol-gel process. The comparison with a similar line in an Al2O3–SiO2:Eu3+ sol-gel glass revealed the existence of a chemical cluster of Eu3+ and O2− in the tetrahedral SiO4 network. The broad fluorescence band was attributable to an energy migration among the Eu3+ ions for the site-selectively received excitation energy. Also, based on Yokota–Tanimoto’s energy diffusion model, the fluorescence decay curves for the D05→7F2 transition were closely correlated with the energy migration and gel–glass transformation. The gel-shrinkage and reduced interatomic distance between Eu3+ ions due to a thermal treatment at higher temperature definitely resulted in a decrease in the associated lifetime of the initial decay.

Site-selective spectroscopy of Eu 3+ doped lead germanate glasses

A series of lead germanate glasses of molar compositions ( x−1)PbO–(100− x)GeO 2–1Eu 2O 3 with x=30, 40 and 50 were prepared and studied by laser-induced fluorescence line narrowing (FLN) techniques. The energies of the Stark levels of the 7 F 1 and 7 F 2 multiplets were obtained from experimental FLN spectra and a crystal field (CF) analysis was carried out for each excitation energy. The dependence of the CF B kq parameters on the excitation energy is qualitatively similar to the behaviour observed for Eu 3+ in other oxide glasses. The CF calculated ratios B 22/ B 20 and B 44/ B 40 agree reasonably well with the trend predicted by the Brecher and Riseberg geometric model. The CF strength parameter S decreases with increasing lead content in the glass host. This behaviour is explained on the basis of the tendency of lead to form strongly directional Pb–O bonds. On the basis of the present CF analysis, we find no evidence of changes of the local structure at the Eu 3+ sites on increasing the amount of lead and also of 6-fold co-ordinated germanium.

Energy transfer between Eu 3+ ions in sol–gel derived silica glasses

Time-resolved fluorescence line narrowing technique has been used to analyse the evolution of the 5D 0→ 7F 0 emission of Eu 3+ ions in sol–gel derived silica glasses. These samples were doped with 2.5 mol% of Eu 3+ and heat treated at different temperatures. Background fluorescence, coming from Eu 3+ ions excited by energy transfer processes, was observed in these experiments. Using a previously proposed equation model, a study of the multipole character of the interaction between Eu 3+ ions was performed. The annealing process carried out in the samples causes important changes in the efficiency of energy transfer process and in its multipole character, varying from S=6 (in a sample treated at 400°C) to S=10 (in a sample treated at 600°C), indicating that the environment of Eu 3+ ions has been modified.