Quantitative fluorescence excitation spectra of synthetic eumelanin

Previously reported excitation spectra for eumelanin are sparse and inconsistent. Moreover, these studies have failed to account for probe beam attenuation and emission reabsorption within the samples, making them qualitative at best. We report for the first time quantitative excitation spectra for synthetic eumelanin, acquired for a range of solution concentrations and emission wavelengths. Our data indicate that probe beam attenuation and emission reabsorption significantly affect the spectra even in low-concentration eumelanin solutions and that previously published data do not reflect the true excitation profile. We apply a correction procedure (previously applied to emission spectra) to account for these effects. Application of this procedure reconstructs the expected relationship of signal intensity with concentration, and the normalized spectra show a similarity in form to the absorption profiles. These spectra reveal valuable information regarding the photophysics and photochemistry of eumelanin. Most notably, an excitation peak at 365 nm (3.40 eV), whose position is independent of emission wavelength, is possibly attributable to a 5,6-dihydroxyindole-2-carboxylic acid (DHICA) component singly linked to a polymeric structure.     

Polarized fluorescence excitation spectra of adenines

Polarized fluorescence excitation spectra of adenine and N⁶,N⁶-dimethyladenosine, in glassy ethanol solution at 77 K, reveal the existence of the third weak ππ^* transition at about 220 nm, which has been expccted theoretically, but not so far confirmed experimentally. Relative directions between ππ^* transition moments are discussed.     

Ionization constants and fluorescence excitation and emission spectra of the isomeric benzo[a]pyrene phenols

The acid dissociation constants, for the ground and lowest excited singlet states of the benzo[a]pyrene phenols, are reported, and correlated with current thought on carcinogen activation. The corrected fluorescence excitation and emission spectra of these compounds and their anions are recorded. The shifts caused by ring hydroxylation of the parent compound, and the relative spectral band intensities for each phenol are compared to those of pyrene, in a brief assessment of spectral transition polarization in the phenols.     

Polarized two-photon fluorescence excitation spectra of indole and benzimidazole

Polarized two-photon fluorescence excitation spectra of indole in hexane, benzimidazole in isopropanol, and benzimidazole cation in methanol-H₂SO₄, all at 0.2 M and 25°C are reported for the excitation range 470–600 nm, the region of their L_b, and L_a bands. Relative two-photon absorptivities are deduced by correcting for different fluorescence response and are compared to toluene's L_b band. The indole integrated absorptivity is about 10 times greater than that of toluene. The L_a band of indole appears less dominant than in one-photon but still outweighs the L_b band by a factor of 4. The two-photon polarization spectrum for indole indicates that the L_a origin lies ≈500–1000 cm⁻¹ above the L_b origin in hexane. The benzimidazoles absorb only about twice as strongly as toluene and show strong vibronic peaks; the L_a, bands are only faintly seen. Two-photon properties calculated from INDO/S CI wavefunctions with doubly excited configurations are in good agreement with those of indole, but predict the benzimidazole TPA to be several times stronger than observed. For the cation, the predicted results are nearly two orders of magnitude too high.     

Optically excited emission and excitation spectra of solid xenon

The luminescence of solid xenon at 20 K has been excited optically in the wavelength range 1550Åto 1150Å. The 1760Å emission band due to a ${}^3{\sum }_u^{+}{\to }^1{\sum }_g^{+}$ transition of the $X{e}_2^{*}$ molecule is found in agreement with α^? or e^?-excitation. The excitation spectrum yields a relatively high quantum efficiency of the Γ(3/2) exciton series indicating efficient self-trapping. The Γ(1/2) exciton series and conduction band states have a low quantum efficiency.     

Vacuum ultraviolet absorption and fluorescence excitation spectra of Br2

Absorption and fluorescence excitation spectra of Br₂ in the region 125–170 nm have been recorded using tunable synchrotron radiation. Computer simulations of the absorption and dispersed fluorescence spectra have allowed identification of the upper electronic state responsible for the main fluorescence excitation system (150–167 nm), as the D(0⁺_u) ion-pair state. A potential function for this state is presented which accounts for vibrational levels up to ν′ ≈ 170 and a pronounced inflection on the inner wall of this potential, due to an avoided crossing, is identified at T_e + 15000 cm⁻¹. The mean radiative lifetime of the D(0⁺_u) state has been determined as ≈ 9 ns. An analysis of the 320–360 nm structured continuum fluorescence, from the D(0⁺_u) state to a lower repulsive state, is also given.     

Light emitting diode excitation emission matrix fluorescence spectroscopy

An excitation emission matrix (EEM) fluorescence instrument has been developed using a linear array of light emitting diodes (LED). The wavelengths covered extend from the upper UV through the visible spectrum: 370-640 nm. Using an LED array to excite fluorescence emission at multiple excitation wavelengths is a low-cost alternative to an expensive high power lamp and imaging spectrograph. The LED-EEM system is a departure from other EEM spectroscopy systems in that LEDs often have broad excitation ranges which may overlap with neighboring channels. The LED array can be considered a hybrid between a spectroscopic and sensor system, as the broad LED excitation range produces a partially selective optical measurement. The instrument has been tested and characterized using fluorescent dyes: limits of detection (LOD) for 9,10-bis(phenylethynyl)-anthracene and rhodamine B were in the mid parts-per-trillion range; detection limits for the other compounds were in the low parts-per-billion range (< 5 ppb). The LED-EEMs were analyzed using parallel factor analysis (PARAFAC), which allowed the mathematical resolution of the individual contributions of the mono- and dianion fluorescein tautomers a priori. Correct identification and quantitation of six fluorescent dyes in two to six component mixtures (concentrations between 12.5 and 500 ppb) has been achieved with root mean squared errors of prediction (RMSEP) of less than 4.0 ppb for all components.     

Synchronous fluorescence and excitation emission characteristics of transformer oil ageing

This paper describes the evaluation of synchronous fluorescence spectroscopy (SFS) and excitation emission matrix fluorescence (EEMF) spectroscopy as means of monitoring transformer oil degradation. When accelerated thermal ageing method is used, the onset of degradation of transformer oil on 17th day and transformer oil with polypropylene and cellulosic paper on 23rd and 27th days is sensitively reflected in the SFS and EEMF fluorescence spectral characteristics.     

Absorption,excitation and fluorescence spectra of lead activated ammonium chloride

A systematic study of absorption, excitation and fluorescence spectra of ammonium chloride activated by varied amounts of lead concentration have been made for the first time. Absorption bands at 6.08, 5.69, 4.73 and 4.57 eV at room temperature and fluorescence bands at 3.62 and 2.85 eV at liquid nitrogen temperature have been observed. These investigations provide an experimental evidence for the existence of two types of centres involved in the luminescence process.     

Two-photon and multi-photon excitation of naphthalene in the vapor phase,studied by excitation and fluorescence spectra

Naphthalene vapor is irradiated by μsec dye laser pulses of 150 kW peak power and a spectral bandwidth of 0.3 nm. A two-photon excitation spectrum is detected by monitoring the near UV fluorescence as a function of laser wavelength which is tuned between 570 and 610 nm. The fluorescence obtained by irradiation into the strongest band of the two-photon spectrum could be spectroscopically resolved using a bandwidth of 80 cm^?1. The spectrum exhibits vibrational structure which lies on a strong non-resolved background. From information in both spectra it can be definitely concluded that vibronic levels of B_3u × b_3u species in the lowest singlet state are predominantly excited in a two-photon process. The non-resolved background in the fluorescence spectrum is attributed to subsequent excitation of the two-photon state by a third photon. Further stepwise excitation in the strong radiation field of the laser is also taken into account.     

The effect of fluctuations on emission spectra: practical distinctions between raman and fluorescence spectra

By the use of stochastic theory it is clearly shown that two types of emission should to fluctuations such as would occur during interactions with their environment are resonantly driven by light. In our approach the electromagnetic field is treated in a classical manner and decay is introduced by a Wigner-Weisskopfansatz. The two types of emission should occur for a three-level system and relaxation to other levels is not involved in the mechanism. Also included is a discussion of operational definitions of fluorescence (and other spontaneous emissions) and of the relationship between absorption and scattering in terms of dephasing or T₂ processes, especially as this relates to conventional emission phenomena.     

Multivariate methods on the excitation emission matrix fluorescence spectroscopic data of diesel-kerosene mixtures: a comparative study

Quantitative determination of kerosene fraction present in diesel has been carried out based on excitation emission matrix fluorescence (EEMF) along with parallel factor analysis (PARAFAC) and N-way partial least squares regression (N-PLS). EEMF is a simple, sensitive and nondestructive method suitable for the analysis of multifluorophoric mixtures. Calibration models consisting of varying compositions of diesel and kerosene were constructed and their validation was carried out using leave-one-out cross validation method. The accuracy of the model was evaluated through the root mean square error of prediction (RMSEP) for the PARAFAC, N-PLS and unfold PLS methods. N-PLS was found to be a better method compared to PARAFAC and unfold PLS method because of its low RMSEP values.     

Resolved fluorescence emission spectra of PRODAN in ethanol/buffer solvents

The fluorescence steady-state emission spectra of lipophilic fluorescence probe PRODAN in ethanol/buffer solvents of different concentrations (0.3, 0.9, 3 mol L(-1) ethanol) were extensively studied and analytically described. The complex experimental spectra, corrected for background effects, were fitted by two Gaussian curves. The energy separation of two maxima, (0.147+/-0.002) eV at 37 degrees C and (0.143+/-0.003) eV at 25 degrees C, was independent of ethanol concentration. The blue shifts observed for both maxima were linearly dependent on solvent polarity. The linear dependences of fluorescence's intensities on PRODAN concentration in all ethanol/buffer solvents indicate that no PRODAN self-quenching takes place even at the highest measured PRODAN concentrations.     

Laser-induced fluorescence excitation spectra of CCl2 and CFCI radicals in the gas phase

Laser-induced fluorescence excitation spectra in the gas phase have been obtained for CCl₂ and CFCI radicals. The observed spacings in the excitation spectra reported here agree well with previous absorption work in the matrix.     

Studies of lifetimes of rotationally cooled NO2 using time-resolved fluorescence excitation spectra

A taunble, pulsed dye laser with output in the region of 5750 to 6000 Å was used to excite rotationally cooled NO₂ which was produced by expansion in conjunction with argon carrier gas through a supersonic nozzle. The resulting time-resolved fluorescence excitation spectra were used the lifetimes of various vibronic bands of NO₂ (²B₂). The lifetimes measured were in the region of 15 to 40 μs which were shorter than those obtained from cell experiments. For each individual excitation wavelength, only a single exponential decay was observed from very early times through 250 μs.     

Responses of epidermal phenolic compounds to light acclimation: in vivo qualitative and quantitative assessment using chlorophyll fluorescence excitation spectra in leaves of three woody species

Chlorophyll fluorescence (ChlF) excitation spectra were measured to assess the UV-sunscreen compounds accumulated in fully expanded leaves of three woody species belonging to different chemotaxons, (i.e. Morus nigra L., Prunus mahaleb L. and Lagerstroemia indica L.), grown in different light microclimates. The logarithm of the ratio of ChlF excitation spectra (logFER) between two leaves acclimated to different light microclimates was used to assess the difference in epidermal absorbance (EAbs). EAbs increased with increasing solar irradiance intercepted for the three species. This epidermal localisation of UV-absorbers was confirmed by the removal of the epidermis. It was possible to simulate EAbs as a linear combination of major phenolic compounds (Phen) identified in leaf methanol extracts by HPLC-DAD. Under UV-free radiation conditions, shaded leaves of M. nigra accumulated chlorogenic acid. Hydroxybenzoic acid (HBA) derivatives and hydroxycinnamic acid (HCA) derivatives greatly increased with increasing PAR irradiance under the low UV-B conditions found in the greenhouse. These traits were also observed for the HCA of the two other species. Flavonoid (FLAV) accumulation started under low UV-A irradiance, and became maximal in the adaxial epidermis of sun-exposed leaves outdoors. A decrease in the amount of HCA was observed concomitantly to the intense accumulation of FLAV for both leaf sides of the three species. Judging from the logFER, under low UV-B conditions, larger amounts of HCA are present in the epidermis in comparison to FLAV for the three species. Upon transition from the greenhouse to full sunlight outdoors, there was a decrease in leaf-soluble HCA that paralleled FLAV accumulation in reaction to increasing solar UV-B radiation in the three species. In M. nigra, that contains large amounts of HCA, the logFER analysis showed that this decrease occurred in the adaxial epidermis, whereas the abaxial epidermis, which is protected from direct UV-B radiation, continued to accumulate large amounts of HCA.     

Luminescence of lead chloride and lead bromide single crystals: I. The excitation and emission spectra

The luminescence of very pure PbCl₂ and PbBr₂ single crystals has been studied at liquid nitrogen and liquid helium temperature. At 10 K, PbCl₂, as well as PbBr₂, exhibits three emission bands in the spectral region investigated, viz., one in the ultraviolet part of the spectrum, a second one in the blue and a third one in the red. In addition, PbCl₂ shows a violet emission at 5 K. If, at 80 K, PbCl₂ and PbBr₂ crystals were exposed to ultraviolet radiation for one hour or more, a yellow emission appeared.Thin film absorption spectra of both lead halides are discussed. The band in the low energy region of the absorption spectra is attributed to the creation of a cation exciton.The excitation spectra of the different emissions have been measured. The thin film absorption spectra and the luminescence excitation spectra are strongly correlated. All emissions are preferentially excited in the low energy region of the absorption spectrum, i.e., the region of the cation exciton absorption band. The ultraviolet emission of both lead halides is discussed and attributed to the recombination of cation excitons.     

Dual fluorescence of the isoquinolinium cation in methanol: time-resolved emission spectra and semiempirical calculations

The fluorescence behavior of the N-ethyl isoquinolinium (iso-NEQ) cation in viscous methanol is reported and discussed on the basis of time-resolved fluorescence spectra, polarization measurements and semiempirical calculations. Dual fluorescence results from two nearby excited states (S₁ and S₂) above 300 nm only at low temperature. The detection of an isoemissive point verifies this two-state model of the emission. We use the numerical self-consistent reaction field (SCRF) to include solute–solvent interactions for the calculations of the absorption transitions of iso-NEQ in methanol. We also involved the so-called supermolecular approach with three methanol molecules to reproduce the absorption transitions. The latter results are in good agreement with our experimental data.