Luminescence properties of humic substances

A detailed analysis of luminescence properties of humic substances and fulvic acids extracted from peat soil is carried out for the first time. Their fluorescence excitation and emission spectra, fluorescence lifetimes, phosphorescence excitation and emission spectra, and phosphorescence lifetimes at room temperature are investigated. The nature of chromophores of humic substances is discussed. Presented at the International Symposium on Peat Organic Substances, May 15–19, 1995, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 489–493, July–August, 1997.     

Luminescence of complex organic molecules upon electron excitation

Pesults of investigation of the luminescence of complex organic compounds upon excitation with electrons of various energies are reported. Data on spectra of energy losses of electrons, fluorescence excitation cross sections and functions, fluorescence emission spectra, fluorescence kinetics, polarization, and energy yield, effects of impurity gases, and theoretical analysis of the processes taking place in the system are presented. Results of investigation of the luminescence of complex molecules in electric discharges (dc, pulsed, high-frequency, non-self-maintained, and plasma-beam) and in a condensed state (solutions, molecular crystals, and thin solid films) are discussed. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk 220072, Belarus. Transtated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 694–708, September–October, 1998.     

Fine fluorescence and absorption spectra of hypericin in a polymer matrix at a temperature of 4.2 K

Monochromatic photoexcitation and selective registration in bands of purely electronic S₁↔S₀ transitions resulted in fine fluorescence and absorption (fluorescence excitation) spectra of hypericin in polyvinylbutyral at a temperature of 4.2 K. Vibration analysis of the spectra is done and conclusions are made about the form of many normal vibrations. It is found that the formation of intramolecular hydrogen bonds in the hypericin structure has a substantial effect on the relative position of the electronic vibrational levels of the pigment molecule. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina ave., Minsk 200072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 539–545, July–August, 1998.     

Energy yield of fluorescence of complex organic compounds upon electron excitation

With use of electron energy loss spectra for vapors of anthracene, perylene, 1,4[di(2,5-phenyloxazolyl)] benzene, and paratherphenyl the energy yields of fluorescence are calculated for electron collision. It is shown that they are considerably smaller than those for optical excitation conditions. Deceased Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 146–148, January–February, 1998.     

Kinetics of fluorescence polarization for solid bichromophore solutions in intense pulse excitation

The kinetics of fluorescence polarization in intense pulse excitation of solid disordered solutions of bichromophores that consist of complex molecules of two types between which there can be inductiveresonance transfer of electron-excitation energy is theoretically investigated. Variants of fluorescence excitation by single pulses and pulse trains are considered. The lifetime of the fluorescence of a given solution increases with the intensity of the exciting pulses. The possibility of controlling the duration of fluorescence attenuation for donor molecules incorporated into the bichromophores by the action of luminescence radiation at the frequency of acceptor-molecule absorption on the solution is demonstrated. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii Vol. 65, No. 4, pp. 546–550, July–August, 1998.     

Theory of polarized fluorescence of polyatomic radicals upon photodissociation of complex molecules in the gas phase

The effect of rotational predissociation on the polarization response of an ensemble of photofragments is investigated within the framework of the model with free scatter of fragments for the most general case when both original molecules and fragments are asymmetric rotators. Photoinduced decomposition reactions with single- and two-photon excitation of fragment fluorescence are analyzed. Second- and third-order orientational correlation functions determining the polarization degree of fragment fluorescence are calculated. The behavior of these correlation functions depending on the shape of original molecules and fragments, directions of dipole moments, and characteristic times of the process are analyzed. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 908–914, November–December, 1998.     

Spectral characteristics of luminescence of solutions of complex molecules under conditions of electronic excitation transfer to a saturable acceptor in the case of inhomogeneous broadening of electronic energy levels

Spectral characteristics of solutions of complex molecules under conditions of inhomogeneous broadening of energy levels are considered in the case when the nonlinear dependence of the population of molecular states on the excitation intensity is determined not by saturation of molecular levels but by exchange of the electronic excitation energy with the environment. Calculations have shown that the dependence of the position of the fluorescence spectrum on the excitation frequency is nonmonotonic in solutions of the type and varies substantially with the excitation intensity. Belarusian State University, 4, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 164–168, March–April, 1997.     

Absorption and fluorescence of 9,10-diiodoanthracene in undegassed solutions

A procedure for producing 9,10-diiodoanthracene that is an alternative to the known procedure is described. The spectra of absorption and fluorescence in hexane, carbon tetrachloride, and polymethylmethacrylate are measured at 22°C. The quantum yield of undegassed liquid solutions in hexane and carbon tetrachloride in excitation at the maximum of the 0←0 band is practically the same and is equal to 0.018. In excitation in shorter-wave vibrational bands, it is different for different solvents and decreases noticeably as the frequency of the exciting light increases. The decrease in the yields is explained by the existence of a fast nonradiative process from hot (Franck-Condon) vibrational S₁-states capable of competing with vibrational relaxation. A possible scenario of this approach is discussed. Institute of Molecular Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 197–200, March–April, 1999.     

Concentration self-quenching in solid solutions of complex organic molecules under intense excitation

A theoretical study was carried out to investigate concentration self-quenching which is nonlinear in intensity in a molecular system caused by nonradiative electronic excitation energy transfer to relatively long-lived acceptors that are formed in a solution and whose function is performed by molecules that have acquired the triplet state. It is shown that at ratios of constants of radiative and nonradiative transitions typical of complex molecules at concentrations of ∼10⁻³–10⁻² mole/liter substantial (2- to 10-fold) fluorescence takes place at intensities of excitation of ∼10⁴–10³ of the intensity of saturation for a singlet subsystem. Polarization characteristics of the system are analyzed. Belarusian State University, 4, F. Skorina Ave., 220050, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 729–733, November–December, 1997.     

Franck-Condon transitions and thermodynamics of photoinduced processes of orientational relaxation in polar solutions

Polar luminescence probe+polar medium subsystems are considered. True minimization parameters of the free energy F are obtained. It is shown that Franck-Condon transitions between F-terms of different electronic states are generally not “vertical”. A complete thermodynamic classification of photoinduced processes of orientational relaxation in the excited S₁ and ground S₀ states is presented. Depending on the excitation conditions they can be either exo- or endothermic, spontaneous or induced (not spontaneous), and accompanied by an increase or decrease in entropy. Indifferent processes also take place. Regularities of shifts of fluorescence spectral bands and burned holes in time-resolved experiments are discussed. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 753–765, November–December, 1997.     

Quenching of the fluorescence of solutions of complex molecules by nonradiative transfer of electronic-excitation energy to a saturable acceptor

The study puts toward a theoretical model describing the fluorescence parameters for concentrated solid solutions consisting of complex molecules of two types (donor-acceptor) with nonradiative transfer of electronic-excitation energy with intense pulse excitation. Consideration is given to the dynamics of quenching of the luminescence of the donor molecules due to transfer of electronic-excitation energy to the acceptor molecules. Numerical results are presented in the form of approximation formulas. To whom correspondence should be addressed. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 652–656, September–October, 1999.     

Energy transfer in chemical dimers of porphyrins and chlorins by the methods of selective laser spectroscopy at 1.8–10 K

The methods of selective laser spectroscopy applied at 1.8–10 K to a glass-forming tetrahydrofuran-toluene (3∶1) mixture have revealed that in the dimers of tetrapyrrole compounds (ethane-bis-octaethylporphyrins, ethane-bis-Zn-octaethylchlorins, heterodimers of Zn-cyclopentane porphyrins) the exciton interactions (V₁₂≈118 cm⁻¹) and S-S-transfer of energy (of probability F≈11 sec⁻¹) are implemented under conditions of substantial inhomogeneous broadening (δF_1/2≈50 cm⁻¹). Upon excitation of a donor component in the dimers, one observes a strong quenching of its emission, the disappearance of the fine structure in the fluorescence spectra, a decrease in the efficiency of spectral hole burning by, approximately, an order of magnitude, and a 12-fold increase in their halfwidth. To whom correspondence should be addressed. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 500–504, July–August, 1999.     

Computer simulation of migration processes in the reaction center of PS2 during stationary laser excitation

We observed experimentally the nonlinear dependence of fluorescence of photosynthesizing organisms on the density of the flux of photons of exciting stationary radiation from an He−Cd-laser (λ=440 nm). A model of a photosynthetic unit was developed that takes account of the structure of the reacting center (RC) of PS2, the processes of migration of the energy of laser excitation from a light-collection antenna (LCA) to the reaction center with subsequent separation of charges at it, the processes of recombination of charges in the RC, and the reverse migration of energy from the RC to the LCA. Within the framework of this model we explained the experimentally observed nonlinear dependence of fluorescence on the density of the flux of exciting-radiation photons and evaluated the contribution of recombination fluorescence to the overall fluorescence of the LCA. To whom correspondence should be addressed. Reported at the Second International Scientific and Technical Conference on Quantum Electronics, Minsk, November 23–25, 1998. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 722–725, September–October, 1999.     

Infrared multiphoton excitation of polyatomic molecules

Results of investigations of the process of multiphoton excitation of polyatomic molecules by CO₂-laser radiation are presented. The mechanism of formation of the profiles of IR absorption bands of polyatomic molecules is discussed. New experimental methods of investigation of relaxation processes at high levels of vibrational excitation of molecules in the ground and triplet states are considered. For vapors of polyatomic molecules and their mixtures with foreign gases, the quantitative characteristics of the collisional exchange and the vibrational-energy transfer as well as the rates of intercombinational conversion ⇛ and triplet-triplet transfer are presented and their dependences on the vibrational-excitation level are discussed. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 675–693, September–October, 1998.     

Quasiline fluorescence and absorption spectra of NH-isomers of octamethylisobacteriochlorin

Quasiline fluoresence and fluorescence excitation spectra of NH-isomers of 2,2,7,7,12,13,17,18-octamethylisobacteriochlorin (cis- and trans-) in polycrystalline matrices in n-hexane and n-octane at 4.2 K are obtained. A vibration analysis of the spectra in combination with the data of quasiline spectra of cyano-substituted octaethylisobacteriochlorin and octamethylporphin permitted a qualitative interpretation of the vibronic spectra of NH-isomers. It is inferred that for a trans-isomer, the 0–0 transition S₂⇃S₀ is localized in the region of electron-vibrational S₁⇃S₀ transitions; the interval S₂-S₁ has a magnitude of about 1300 cm⁻¹. Reported at the VIIIth International Conference on the Spectroscopy and Chemsitry of Poprhyrins and Their Analogs, Belarus, Minsk, September 22–26, 1998. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 79–85, January–February, 2000.     

Fluorescence assay of tyrosine in blood plasma

A more simple and sensitive assay for the quantitative determination of tyrosine in blood plasma is developed on the basis of a modification of the method of S. Udenfriend (1962). A volume of 0.2 ml of plasma is used in the analysis; after its deproteinization with trichloroacetic acid, it is reacted with sodium nitride, nitrous acid, and 1-nitroso-2-naphthol at 65°C, and the content of the reaction product is measured by the fluorescence at λ_ex/λ_em=460/570 nm. The consumption of plasma and reagents is reduced by a factor of 5–10 compared to the original method; in addition, the stage of extraction with an organic solvent is excluded. The linear dependence of the fluorescence signal on the tyrosine concentration within the range of 2–60 μg/ml, high specificity, accuracy, and reproducibility of the assay are shown. The importance of tyrosine determination in monitoring of glucocorticoid therapy and protein catabolism is discussed. Institute of Photobiology, National Academy of Sciences of Belarus, 27, Akademicheskaya St., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 366–371, May–June, 1998.     

Band structure of electronic levels and the formation of the fluorescence spectra of polar compounds in polar media

The paper sets out a statistical theory of the effect exerted by interactions of constant dipoles in solutions on the fluorescence spectra of polar compounds. The theory is based on the concept of band structure of the electronic levels of a polar impurity (dissolved) molecule in polar media. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 209–216, March–April, 1997.     

Polarized fluorescence of radicals in photodissociation of disulfides in the gas phase

The polarization of the fluorescence of radicals formed in laser-induced photodissociation of some disulfides in the gas phase is measured. Experimental data obtained in the study are interpreted within the free recoil model. It is shown that polarization experiments carried out to investigate photodecay in vapors allow researchers to uniquely the intramolecular direction of the dipole moments of transitions with absorption and emission. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 54–59, January–February, 1999.     

Extra-liganding effects in Zn-octaethylporphyrin solutions in a temperature interval of 300-77 K

It is established experimentally for the first time that Zn-octaethylporphyrin in a solution of dried methyl cyclohexane is in unliganded form in the temperature interval of 293-77 K. This form is characterized by the shortest-wave position of absorption and luminescence spectra, the independence of the degrees of fluorescence and phosphorescence polarizations from the excitation wavelength, and high phosphorescence time (τ_T=126 μsec) and quantum yield (φp=0.085) at 77 K. Adding electrodonor components to the methyl cyclohexane or going over to alcohol-containing polar media leads to the formation of a monoliganded form of ZnOEP, for which we observe a bathochromic shift of ≈150–350 cm⁻¹ for Q(0,0) absorption and luminescence bands, a splitting of ≈50–100 cm⁻² for the Q(0, 0) absorption band, and phosphorescence quenching (τ_T=60 μsec, φp=0.05-0.03 at 77 K). The effects revealed are explained by a decrease in the symmetry of a ZnOEP molecule in extra liganding and by an increase in the probability of nonradiative activation for a T₁-state due to the extra ligand being involved in the exchange of electron-excitation energy and to the enhancement of a spin-orbital coupling as a result of the disturbance of the planarity, of a macrocycle. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 900–907, November–December, 1998.     

Glow discharge in helium at atmospheric pressure as a source of spectrum excitation

Self-maintained glow discharge at atmospheric pressure in a helium flow is used as an excitation source of spectra for analysis of the metals Na, Ca, Pb, Cd, and Zn in aqueous solutions of their salts. The intensities of the analytical lines of these metals are obtained as a function of the concentration, and the detection limits for each element are determined. Deceased. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Misnk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 711–715, November–December, 1997.