Spectral characteristics of heterocyclic compounds with a chain structure, cooled in an ultrasonic jet

We have recorded the fluorescence excitation spectra of three heterocyclic compounds with a chain structure [BPO (2-phenyl-5-(4-diphenylyl)oxazole), POPOP (1,4-di[2-(5-phenyloxazolyl)]benzene, and TOPOT (1,4-di[2-(5-n-tolyloxazolyl)]benzene] and the fluorescence spectra of POPOP, under conditions where the molecules were cooled in an ultrasonic helium jet. A line structure is observed in the spectra of POPOP and TOPOT; for the BPO molecules, whose configuration changes considerably during electronic excitation, vibrational structure is apparent only in the low-frequency region of the excitation spectrum, and a diffuse spectrum is recorded starting from ν ₀ ⁰ + 200 cm⁻¹. For all the compounds, in the spectra we recorded vibrations with frequencies up to 100 cm⁻¹, arising due to the flexibility of the molecular structure. The rotational contours of the lines for the electronic and vibronic transitions of the POPOP molecules (T_rot = 10.5 K) and TOPOT molecules (T_rot = 15 K) are structureless and bell-shaped. The degree of polarization of the fluorescence P_fl for the jet-cooled POPOP molecules for excitation of vibrations along the absorption band up to 2000 cm⁻¹ above ν ₀ ⁰ is practically constant (∼8.4%) and matches P_fl for high-temperature vapors. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 728–734, November–December, 2006.     

Electron Energy Loss Under Scattering by Complex Organic Compound Vapors

Abstract

Low-energy electron scattering by anthracene and 1, 4-di 2-(5-phenyloxazolyl) -benzene (POPOP) vapors at low pressures is studied. The primary electron beam energy E_O was varied between 10 and 60 eV. The scattering angle was θ = 90°. It has been found that in the electron-energy-loss spectra, bands with maxima of 3·7 and 5·41 eV for anthracene, 4.02 and 7 eV for POPOP correspond to the S_O → S₁ and S_o → S₂ transitions. Low intensity electron-energy loss due to T₂ -states excitation has been observed in the region of 2·15 eV for anthracene and 2·54 eV for POPOP. From comparison with optical absorption spectra and analysis of spectra structure changes at different E_O the nature of other bands has been determined. From the obtained results it was concluded that the probability of T₁ -state excitation is low. It is shown that the main mechanism of populating the T₁ -state by electric methods of vapor pumping is the process of intersystem crossing.

The development of free complex molecule spectroscopy under electric excitation is hindered by the lack of systematic reliable information about elementary interactions of electrons and other particles with such molecules. When excited in electric discharge or by an electron beam, the complex molecule may be in both a neutral and an ionized state. The investigation of such nonequilibrium gas system presents great difficulties due to a large number of possible elementary processes responsible for excitation, ionization, recombination, dissociation of complex molecules. To develop a kinetic model of such a system, one must know a number of cross sections or rate constants of elastic and inelastic electron collisions with the complex molecule. Unfortunately, such data are practically unavailable.

The aim of the present paper is to obtain information about probabilities of complex molecule singlet and triplet states excitation in a gas phase as a result of collision with low-energy electrons (less than 50 eV).

Most accurate data on effective cross sections of molecule (atom) collisions with electrons are provided by using an electronic spectrometer in which an electron beam with small energy spread (of the order of tens of in electron-energy-loss spectra, S₂-state excitation cross sections have a considerably greater value.     

Photophysical parameters for potential vapour-phase dye-laser media

The fundamental photophysical parameters of the organic scintillators, anthracene, 9, 10 dimethyl antracene (DMA), 9, 10 diphenyl anthracene (DPA), 2-(1-napthyl)-5-phenyloxazole (α-NPO) and 2, 2′-p-phenyl bis (5-phenyloxazole) (POPOP), were investigated in the liquid and vapour phases. Their ground state, triplet-triplet and fluorescence spectra were determined and accurate extinction coefficients obtained. Photodecomposition of POPOP vapour under 337 nm excitation was investigated in detail and a photodecomposition quantum yield of 1.1×10⁻² obtained at 568 K. The POPOP triplet state in the vapour phase is quenched by the addition of 1, 3, 5, 7 cyclo-octatetraene and a quenching rate constant of 2.9×10¹⁰ M⁻¹ s⁻¹ was determined.     

Calculation of gain coefficients for vapour phase dyes

The gain coefficient for four vapour phase organic dyes has been calculated under laser and (for POPOP, flashlamp) pumped conditions and compared with various experimental results for POPOP vapour under nitrogen laser excitation. As quenching of the POPOP vapour triplet state by vapour phase 1,3,5,7 cyclooctatetraene (COT) has been observed with a known rate constant this has been applied to the calculation of the gain coefficient of POPOP/COT vapour mixtures.     

Organic vapor excitation by electron beam pumping of rare-gas—Organic vapor mixtures

Broad-band fluorescence at 4000 Å has been observed from gas phase mixtures of the dyes POPOP and α-NPO in Xe and Ar. The photon yields have been measured at 3 atm total pressure and are found to reach the following maximum values: α-NPO/Xe: 2%, α-NPO/Ar: 5%, POPOP/Xe: 9%, POPOP/Ar: 20%.     

Polarized Fluorescence of Jet‐Cooled Indole

The polarization of indole fluorescence on excitation in the P, Q, and R branches of a purely electron transition has been investigated. Relations are given to calculate the fluorescence polarization with arbitrary orientation of the dipole moments of transitions with absorption and emission and with excitation in the Q branch of the vibronic line. The coincidence of the measured (7.3%) and calculated (7.7%) values of the polarization degree confirms the reported intramolecular orientation of the dipole moment of transition. The measured fluorescence polarization degree on nonselective excitation is about 3%.     

Solvent Polarity and Excitation Wavelength Dependence of the Dual Fluorescence in N,N-Diethyl-4-Nitrosoaniline

Dual fluorescence in N,N-Diethyl-4-nitrosoaniline (DENA) has been studied employing absorption, excitation and emission spectroscopic techniques and computational methods. The absorption and fluorescence spectra of DENA were measured in solvents of various polarities at room temperature. The emission spectra of DENA were found to exhibit a single emission band in non polar solvent (cyclohexane) and in a highly polar solvent (acetonitrile). In the contrary, two emission bands were observed in medium polar solvents (tetrahydrofuran, 1,2-dichloroethane and dichloromethane) whereby the short (local excited; LE) and long (charge transfer; CT) emission maxima correspond to the emission maxima of the compound observed in cyclohexane and acetonitrile solutions, respectively. Moreover, the two emission bands have shown strong excitation wavelength dependence, and area normalization resulted in an iso-emissive point. The two emission maxima were in addition found to correspond to two excitation maxima in 3D fluorescence spectra. Further, two minima were obtained in potential energy surface calculation of DENA. From the experimental and computational results it was concluded that the dual fluorescence may be attributed to the presence of two different ground state structural conformers of DENA in equilibrium that are stabilized through solute-solvent interaction.     

Deshaping of the laser beam observed on the collisionally induced fluorescence of Na atoms in the nozzle expansion

The excitation spectrum ofD ₁ fluorescence and collisionally inducedD ₂ fluorescence of sodium in the early stage of the beam formation near the nozzle is studied. We observe strong reabsorption and imprisonment ofD ₁ photons influencing collisionally inducedD ₂ fluorescence. For high pressure of sodium vapour the excitation profile ofD ₂ fluorescence is dominated by the collisional broadening effect and shows unusual maxima and minima. These we connect with the laser beam focusing and defocusing effect, changing the number of photons penetrating the high density region.     

Excitation-wavelength Dependent Fluorescence of Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

The excitation wavelength dependence of the steady-state and time-resolved emission spectra of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in tetrahydrofuran (THF) at room temperature has been examined. It is found that the ratio of the fluorescence intensity of the long-wavelength and short-wavelength fluorescence bands strongly depends on the excitation wavelength, whereas the wavelengths of the fluorescence excitation and fluorescence bands maxima are independent on the observation/excitation wavelengths. The dynamic Stokes shift of fluorophore in locally excited (LE) and intramolecular charge transfer (ICT) states has been studied with a time resolution about 30 ps. The difference between Stokes shift in the LE and ICT states was attributed to the solvent response to the large photoinduced dipole moment of EAADCy in the fluorescent charge transfer state. On this base we can state that, the relaxation of the polar solvent molecules around the fluorophore was observed.     

Dual fluorescence and laser emissions from fluorescein-Na and eosin-B

Dual laser emissions were observed from fluorescein-Na and eosin-B in ethanolic solutions individually in the concentration range from 10⁻² to 10⁻³ mol dm⁻³ under N₂ laser excitation. The first compound was found to lase at two distinct regions with wavelength maxima around 540, 550 nm, while the second one around 558, 574 nm. Steady-state absorption, fluorescence excitation, fluorescence polarization, fluorescence emission and decays of the dyes in various solvents under varying conditions of excitation and detection systems were carried out to identify the nature of the emitting species responsible for laser emissions in two distinct regions. Both the dyes exhibited concentration and excitation wavelength dependence of fluorescence and the effects were found to be more pronounced in binary solution. The fluorescence decays of dyes were monoexponential in ethanol, while in some other solvents used, the decays showed biexponential behavior. The absorption and excitation studies using thin layers of solutions revealed the formation of dimers with the dye concentration around 1×10⁻³ mol dm⁻³. Fluorescence polarization and decay studies confirmed the presence of dimers. The two laser bands observed in the shorter and longer wavelengths were respectively ascribed to monomeric and dimeric species.     

Electronic energy transfer from Cd(53P1 to Popop vapor

Quenching of Cd(5³P₁) by POPOP and sensitization of POPOP by Cd(5³P₁) have been investigated in the vapor phase at total pressures of 10^-1 to 10^-2 torr. Quenching of Cd is observed to occur at a rate of 5.9 X 10^-10 cm³/sec and sensitization of POPOP at a rate of 4.2 X 10^-11 cm³/sec. The sensitized fluorescence rate has been found to be consistent with Forsters long-range dipole-dipole mechanism. Selection of a donor for a possible energy transfer, vapor phase, dye laser is discussed.     

基于跃迁偶极矩取向特征研究乙醚溶液中分子的各向异性度

利用偏振荧光光谱和偏振激发光谱研究了乙醚溶液中荧光分子跃迁偶极距的取向特征.实验结果表明,在垂直线偏振光照射下,乙醚溶液发射出峰值位于305 nm的荧光谱,对应的最佳激励光波长为256 nm.由偏振激发光谱分析得到荧光体的吸收跃迁偶极矩和发射跃迁偶极矩间夹角α的变化规律,揭示了荧光去偏振过程：在粘性溶液中的荧光分子具有一定的偶极取向,α随激发光波长发生变化,当激发波长接近最佳激发光波长时,吸收跃迁偶极矩和发射跃迁偶极矩间趋于平行,荧光的退偏效果较弱,偏振度最大.研究结果能为分子空间取向特征的理论研究提供参考.     

Polarized Fluorescence of Jet-Cooled indole and Carbazole and Their Complexes with Water

The spectra of the fluorescence excitation within the rotational contours of the bands of the pure electronic long-wavelength S ₀-S ₁ transitions of jet-cooled indole and carbazole molecules and their complexes with water are measured. For the carbazole-water complex, a contour with three maxima is registered, which is possibly related to the occurrence of two isomers, differing in a slight displacement of hydrogen between the nitrogen atom of the imine group of carbazole and the oxygen atom of the water molecule. The degrees of polarization of integral fluorescence upon excitation within the rotational contours of the S ₀-S ₁ electronic transition bands of the above molecules and their complexes with water are determined for the first time. The coincidence of the calculated (7.7%) and measured (7.3%) values of the degree of polarization upon excitation in the rotational Q branch of the ^b L ₁-A electronic transition of indole confirms the accepted intramolecular orientation of the transition dipole moment at an angle of 38.3° with respect to the principal axis of inertia A. Upon excitation of indole, its complex with water, and carbazole into the P and R branches, the measured and calculated degrees of polarization are also close to each other and amount to 2–3%. This confirms the occurrence of contributions to the fluorescence polarization due to the rotations of the indole molecules around the principal axes of inertia A and C.     

Use of a single-mode fiber coupler for second-harmonic-generation microscopy

We present a compact second-harmonic-generation (SHG) microscope based on a three-port single-mode fiber coupler. The fiber coupler is used to deliver a near-infrared ultrashort-pulsed laser beam as well as to collect the SHG signal in the visible wavelength range. The SHG microscope exhibits an axial resolution of 1.8 microm, which shows a slight enhancement of the optical sectioning effect compared with that for two-photon excitation at the same excitation wavelength. It is also demonstrated that SHG and two-photon fluorescence images under parallel and perpendicular laser excitation polarization can be simultaneously obtained.     

A spectropolarimetric study of theH α radiation from ellerman bombs

Polarization data are given for three whiskers recorded with the large solar vacuum telescope at the Baikal Observatory, Institute of Solar-Terrestrial Physics, Russian Academy of Sciences. The observed degree of polarization varies from 2 to 13%. The direction of the polarization plane for the mediumintensity whisker was tangential, while that for the weakest one was radial. The brightest whisker had the polarization plane direction close to radial at the center of the line but tangential at the flanks. It is assumed that the polarization is due to impact excitation of a hydrogen-atom ensemble in the solar chromosphere by an electron beam, and estimates are made of the beam energy. St. Petersburg Fine Mechanics and Optics Institute, St. Petersburg. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 41, No. 12, pp. 85–89, December 1998.     

Steady-State and Time-Resolved Fluorescence Polarization Behavior of Acenaphthene

Steady-state and time-resolved fluorescence polarization studies have been carried out on acenaphthene (ACE) in low-temperature glass solutions and at room temperature. In the low-temperature glass the fluorescence polarization values vary considerably with both emission and excitation wavelength. There is a time dependence (on the nanosecond time scale) of the fluorescence anisotropy, r(t), at 77 K, which has a strong dependence upon the excitation and emission wavelengths. Under these conditions, the time-dependent decay of the anisotropy is not attributable to chromophoric motion. The observations are consistent with emission from two closely lying and interconverting excited states. Rate constants for the photophysical processes involved have been determined by fitting the data using a model proposed by Fleming et. al. The results are discussed with particular reference to the care required in using dynamic fluorescence polarization measurements to determine energy transfer rates in systems containing this chromophore.     

Comments on the problem of anomalous fluorescence of atomic sodium vapor at a wavelength of 589.6 nm

The problem of the laser-induced fluorescence of sodium vapor at a wavelength of 589.6 nm in the case of photoexcitation of a small volume of the medium by a laser beam has been solved numerically. The photon transfer from the zone of excitation to the periphery in the direction perpendicular to the direction of propagation of the laser radiation leads to a significant extension of the volume with a high density of excited atoms. The influence of this factor on the formation of the resonance fluorescence of sodium vapor is discussed.     

Investigations of Yb-doped optical fiber using selective laser excitation

The method of tunable site-selective laser excitation was used for a study of the spectral and fluorescence characteristics of Yb³⁺ optical centers in laser optical fiber. Decay curves and positions of fluorescence spectra maxima at different wavelengths of selective laser excitation and positions of excitation spectra maxima at different fluorescence selective registration provide new information on lifetimes and Stark energy distribution among inhomogeneously broadened Yb³⁺ lines. The obtained Stark splitting energy distribution diagram demonstrates the tuneability of Yb³⁺ laser oscillation wavelength under tunable excitation of a pump laser.     

Polymer-doped ferroelectric liquid crystal: UV absorbance,fluorescence and electro-optical study

In this study, UV absorption, fluorescence and electro-optical study have been performed for pure ferroelectric liquid crystal (FLC) and its three different concentrations, i.e. 1%, 3% and 5% of fluorescent polymer in the pure FLC. We observed that there is higher value of UV absorption for fluorescent polymer-doped system in comparison to the pure system. We have found excitation wavelength, emission wavelength and quantum yield for all the mixtures. Quantum yield shows the probability of excited state being deactivated by fluorescence rather than non-radiative mechanism. The higher value of spontaneous polarization and faster response has been observed for fluorescent polymer-doped system compared to the pure FLC system. The electro-optical study also suggests that there exists an optimum concentration of fluorescent polymer for which we have found highest spontaneous polarization and least response time.     

Study of the frequency distribution of the fluorescent light induced by monochromatic radiation

The frequency distribution of the fluorescent light induced by monochromatic dye laser radiation was investigated. To exclude the influence of the Doppler width a strongly collimated atomic beam was used. The spectrum was measured by means of a piezoelectrically tunable spherical Fabry Perot. The interaction region between the laser light and the atomic beam was placed into the center of the interferometer. Thus the observed fluorescence spectrum was considerably more intense than in the case where the interferometer is used separately from the beam. The fluorescence spectrum was observed for different directions of polarization of the incident laser beam. In the case of weak excitation the spectrum consists of a sharp component essentially due to elastically scattered light. At high intensities a structure of three components is observed which is in agreement with theoretical predictions when circularly polarized light is used for excitation.