Luminescence from the Pr 4f5d and S0 states in LiLaP4O12

The photoluminescence and excitation spectra of Pr³⁺ activated LiLaP₄O₁₂ has been investigated in the 10-300 K temperature region. At all temperatures, the luminescence consists of optical transitions emanating from both the Pr³⁺ 4f¹5d¹ and the ¹S₀ states. However, at low temperatures the emission spectrum is dominated by the intraconfiguration emission transitions emanating from the Pr³⁺¹S₀ state. With increasing temperature, there is an exchange of intensity between the two emitting states; emission transitions from the ¹S₀ state exhibit strong intensity quenching while the 4f¹5d¹→4f² emission transitions reveal intensity gain. These results are explained on the basis of thermal population of the 4f¹5d¹ state by the ¹S₀ state. The energy barrier of 0.05 eV (403 cm⁻¹) for the nonradiative process is determined from the temperature dependence of the ¹S₀ lifetime.     

Role of zinc and nickel impurities in high-temperature superconductors

The local changes produced in the electronic structure and their effect on the physical properties of the superconducting and normal phases when zinc and nickel are substituted for copper are examined on the basis of a multiband p-d model. It is shown that strong electronic correlations suppress the S=1 configuration of Ni²⁺ and cause the superposition of the S=1/2 and S=0 states of nickel. The change in the density of states in p-and n-type systems is studied, and the peculiarity of Zn impurity for p-type systems and Ni impurity for n-type systems is shown. The universal dependence of the T _c on the residual resistance in lightly doped superconductors and deviations from it in optimally doped systems are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 596–600 (April 1999)     

Effect of post annealing temperature on structural and optical properties of ZnCdO thin films deposited by sol–gel method

Ternary ZnCdO thin films oriented along c-axis have been successfully deposited on p-Si (1 0 0) substrates using sol–gel spin coating route. To optimize most suitable annealing temperature for the Zn_1−xCd_xO thin films; these films with selected cadmium content x = 0.10 were treated at annealing temperatures from 300 °C up to 800 °C in oxygen ambient after deposition. The structural and optical properties of deposited thin films have been characterized by X-ray diffraction, energy dispersive spectroscopy, atomic force microscopy, UV–Vis spectroscopy, and photoluminescence spectra. The results show that the obtained films possess high crystallinity with wurtzite structure. The crystallite size, lattice parameters, lattice strain and stress in the deposited films are determined from X-ray diffraction analysis. The band gap energy increased as a function of annealing temperatures as observed from optical reflectance spectra of samples. The presence of Cd in the deposited films is confirmed by energy dispersive spectrum and it is observed that Cd re-evaporate from the lattice with annealing. The photoluminescence measurements as performed at room temperature did not exhibit any luminescence related to oxygen vacancies defects for lower annealing temperatures, as normally displayed by ZnO films. The green yellow luminescence associated to these defects was observed at higher annealing temperatures (≥700 °C).     

Relaxation Processes at High Levels of Vibrational Excitation of Polyatomic Molecules in the Ground Electronic State

By the spectral and kinetic characteristics of the luminescence of vapors of polyatomic molecules (anthracene, anthraquinone, fluorenone) initiated by selective IR multiphoton excitation (IR MPE) of molecules in the ground electronic state S ₀ the relaxation processes proceeding under vibrational excitation of molecules to energies exceeding the energies of the lower excited electronic states have been investigated. The changes in the spectral and kinetic characteristics with increasing CO₂ laser energy density and vapor P _v and foreign gas pressure P _FG are analyzed. They are similar to the characteristics obtained for normal fluorescence of these molecules with changing vibrational energy E _vib content. On the basis of experimental data and model calculations it has been concluded that at the laser radiation densities used in the case of IR MPE the molecules reach energies considerably exceeding the energies of the electronic levels. It is shown that a nonadiabatic connection between the electronic states leads to the population of mixed electronic states isoenergetic to the vibrational levels of the ground electronic state and to emission of delayed luminescence spectrally identical to the normal luminescence of these molecules. It has been found that when high vibrational levels are populated, new relaxation channels, such as reverse electron relaxation, emission from high vibrational levels of the ground electronic state, and multiquantum vibrational energy transfer at collisions leading to a rapid establishment of vibrational equilibrium become important.     

The photoexcitation and photoluminescence of the crystalline tetrahedral complexes of manganese(II) chloride with hydrochloride of phenazone and aminophenazone

Summary The excitation and emission spectra of the complexes (PH)₂MnCl₄ and (APH)₂. MnCl₄ have been studied. All the bands characteristic of excitation transitions ofd ⁵ configuration — the manganese(II) ion being of tetrahedral symmetry — were identified. The spectrum was interpreted in terms of the Orgel's weak field matrix for quartet levelsd ⁵; and the Racah's parametersB andC, the secular determinants, the ground terms and the field strength of thsse complexes were calculated. The 10Dq value for either of the two compounds was found to be 400 cm^–1.The theories of a harmonic oscillator make possible the determination of characteristic parameters from the emission spectrum.The character and the arrangement of bands of excitation and emission spectra — green luminescence — of the crystals are indicative of tetrahedral structure of the complexes. The phenomenon of luminescence has been explained by the⁴ T ₁(G) ⁶ A ₁(S) transition.     

The Influences of Structural Evolution of Silica Gel Glass on the Photophysical Properties of Benzoic Acid

Benzoic acid was dispersed into silica gel glass at molecular level by sol-gel process. The luminescence properties of benzoic acid in gel glass heat-treated at various temperatures were studied. The changes in gel glass structure and the surrounding environment of benzoic acid resulted in different electronic transitions, and thus the different luminescence spectra. The fluorescence bands centered at 320 nm and 355 nm, respectively induced by the S_1* S₀ transition of benzoic acid and S_1n* S₀ transition of benzoate anions, respectively, were observed when the doped gel glass was heat-treated at the temperatures below 200°C and at the temperature range 300 400°C, respectively. On heating at 500°C, benzoate anions were steadily immobilized in the cages of the Si–O network with the SiO₄ tetrahedra formation and the RTP of benzoate anions is observed. It is proposed that the luminescence spectra of benzoic acid can be used as a structural evolution probe of gel glass.     

Excited high spin states of novel π conjugated verdazyl radicals: photoinduced spin alignment utilizing the excited molecular field

This paper reports the excited quartet (S = 3/2) and quintet (S = 2) states arising from the intramolecular radical-triplet pair in the purely organic π conjugated spin systems. A previous paper reported the excited quartet and quintet states of 9-anthracene-(4-phenyliminonitroxide) and 9,10-anthracene-bis(4-phenyliminonitroxide), respectively, in which iminonitroxide radicals are linked to the phenyl- or diphenylanthracene moiety (a spin-coupler) through the π conjugation. The similar excited quartet and quintet states were observed for the 9-anthra-cene-(4-phenylverdazyl) radical (1) and 9,10-anthracene-bis(4-phenylverdazyl) diradical (2) by time resolved electron spin resonance (TRESR). The TRESR spectrum was analysed by the ordinary spin Hamiltonian with the Zeeman and fine structure terms. For the quartet state of 1, the g value, fine structure splitting, and relative population of the Ms sublevels have been determined to be g = 2.0035, D = 0.0230 cm^?1, E = 0.0, P _1/2′ = P _?1/2′ = 0.5 and P _3/2′ = P _?3/2′ = 0.0, respectively, by spectral simulation. The spin Hamiltonian parameters of the quintet state of 2 were determined to be g = 2.0035, D = 0.0128 cm^?1, E = 0.0, P _2′ = P _?2′ = 0.0, P _1′ = P _?1′ = 0.37 and P _0′ = 0.26, respectively. Direct observation of the excited high spin state showed that photoinduced intramolecular spin alignment is realized between the excited triplet state (S = 1) of the phenyl- or diphenylanthracene moiety and the doublet spin (S = 1/2) of the dangling verdazyl radicals. Ab initio MO calculations (DFT) were carried out in order to clarify the mechanism of the photoinduced spin alignment.     

On quantum systems in thermal contact

It is shown that under rather general conditions two K.M.S. states ₁ and ₂ of systemsS ₁ andS ₂, respectively, can be simultaneously extended to a K.M.S. state of a system composed ofS ₁ andS ₂, provided both systems have equal temperatures. This result gives further support to the conjecture that K.M.S. states are equilibrium states. In the second part, a model of thermal coupling is constructed which satisfies the assumptions of the first part, thereby showing that the result is also valid in the interesting case of systemsS ₁ andS ₂ in thermal contact.     

Luminescent and thermochromic properties of tellurium(IV) halide complexes with cesium

The spectral?luminescent and thermochromic properties of complex compounds of the composition Cs₂TeHal₆ (Hal = Cl, Br, I) are studied. The interrelation between the geometric structure and spectral–luminescent properties is studied using the example on complex compounds of tellurium(IV) halides with cesium. The Stokes shift and the luminescence intensity of Тe(IV) ions with island octahedral coordination are found to depend on the position of the A band in the luminescence excitation spectra, the diffuse reflection, and the energy of the luminescent ³P₁ → ¹S₀ transition of the tellurium(IV) ion. The maximum luminescence intensity and the minimum Stokes shift at 77 and 300 K are observed for Cs₂TeСl₆. The geometrical and electronic factors responsible for luminescence intensification in Te(IV) complexes under study are analyzed.     

6H-SiC（0001）-6[KF（]3[KF）]×6[KF（]3[KF）]R30°重构表面的同步辐射角分辨光电子能谱研究

利用同步辐射角分辨光电子能谱（SRARPES）对6H-SiC（0001）-6[KF（]3[KF）]×6[KF（]3[KF）] R30°重构表面的电子结构和表面态进行了研究.通过鉴别价带谱中来自于体态的信息,可以推断出重构表面的费米能级位于体态价带顶之上（2.1±0.1）eV处.实验测出的体能带结构与理论计算的结果较为符合.在重构表面上发现三个表面态,分别位于结合能-0.48 eV（S₀）,-1.62 eV（S₁）和-4. 关键词： 角分辨光电子能谱 碳化硅（SiC） 电子结构 表面态     

The synthesis and photolysis mechanisms of 8‐nitroquinoline‐based photolabile caging groups for carboxylic acid

Photolabile protecting groups have been extensively studied and applied for protection of small biological molecules, which make it convenient to detect the biological processes of the caged compounds. In this study, a series of 8‐nitroquinoline‐based photolabile caging groups for carboxylic acid were synthesized with improved photolysis efficiency. Among them, 6‐bromo‐8‐nitro‐1, 2‐dihydroquinolinyl chromophore was proven the best derivative on account of its longest absorption wavelength (345 nm), highest caging ability, and quantum yield (Φ = 0.003). Moreover, density functional theory calculations were performed in order to study the photolysis mechanisms. Theoretical calculations revealed that the reaction was kinetically inert under general mild condition with the high barrier height of 34.3 kcal/mol at carbonyl migration step, while under the photolysis condition, because of the large energy gap (64.5 kcal/mol) between S₀ and S₁ states, the reaction should be accessible in the triplet ground state (T₁) through successive excitation of S₀ → S₁ states, subsequent intersystem crossing of S₁ → T₁ states, and finally returned to the stable S₀ state for product via potential energy surface crossing between T₁ and S₀ states. Copyright © 2014 John Wiley & Sons, Ltd.     

Electronic structure of HgGa2S4

The electronic structure and chemical bonding in HgGa₂S₄ crystals grown by vapor transport method are investigated with X-ray photoemission spectroscopy. The valence band of HgGa₂S₄ is found to be formed by splitted S 3p and Hg 6s states at binding energies BE=3-7 eV and the components at BE=7-11 eV generated by the hybridization of S 3s and Ga 4s states with a strong contribution from the Hg 5d states. At higher binding energies the emission lines related to the Hg 4f, Ga 3p, S 2p, S 2s, Hg 4d, Ga LMM, Ga 3p and S LMM states are analyzed in the photoemission spectrum. The measured core level binding energies are compared with those of HgS, GaS, AgGaS₂ and SrGa₂S₄ compounds. The valence band spectrum proves to be independent on the technological conditions of crystal growth. In contrast to the valence band spectrum, the distribution of electron states in the bandgap of HgGa₂S₄ crystals is found to be strongly dependent upon the technological conditions of crystal growth as demonstrated by the photoluminescence analysis.     

Coherent Raman and Infrared Studies of Sulfur Trioxide

High-resolution (0.001 cm⁻¹) coherent anti-Stokes Raman scattering (CARS) was used to observe the Q-branch structure of the IR-inactive ν₁ symmetric stretching mode of ³²S¹⁶O₃ and its various ¹⁸O isotopomers. The ν₁ spectrum of ³²S¹⁶O₃ reveals two intense Q-branches in the region 1065–1067 cm⁻¹, with surprisingly complex vibrational–rotational structure not resolved in earlier studies. Efforts to simulate this with a simple Fermi-resonance model involving ν₁ and 2ν₄ states do not reproduce the spectral detail, nor do they yield reasonable spectroscopic parameters. A more subtle combination of Fermi resonance and indirect Coriolis interactions with nearby states, 2ν₄(1=0, ±2), ν₂+ν₄(1=±1), 2ν₂(1=0), is suspected and a determination of the location of these coupled states by high-resolution infrared measurements is under way. At medium resolution (0.125 cm⁻¹), the infrared spectra reveal Q-branch features from which approximate band origins are estimated for the ν₂, ν₃, and ν₄ fundamental modes of ³²S¹⁸O₃, ³²S¹⁸O₂¹⁶O, and ³²S¹⁸O¹⁶O₂. These and literature data for ³²S¹⁶O₃ are used to calculate force constants for SO₃ and a comparison is made with similar values for SO₂ and SO. The frequencies and force constants are in excellent agreement with those obtained by Martin in a recent ab initio calculation.     

The special features of the luminescence and light resistance of europium complexes and their mixtures with lanthanum complexes in polymethyl methacrylate

The kinetics of luminescence and transformation of short-lived products of the photolysis of europium and lanthanum complexes with thenoyltrifluoroacetone and 1,10-phenanthroline and their mixtures in polymethyl methacrylate films was studied by the nanosecond laser photolysis method with recording both light emission and absorption. Fast (535 and 585 nm, ⁵ D ₁ → ⁷ F ₀, ⁷ F ₃, decay time 0.7 μs) and slow (613 nm, ⁵ D ₀ → ⁷ F ₂, luminescence rise and decay times 0.7 μs and 0.5 ms, respectively) luminescence was studied. Induced absorption with a maximum at 600 nm and decay time ∼3 ms was observed; this absorption was assigned to triplet states of the deprotonated form of thenoyltrifluoroacetone. The dependences of luminescence intensity on the concentration of the components in a mixture of complexes were analyzed, and synergistic effects of luminescence strengthening were estimated. The kinetics of a decrease in luminescence intensity during photolysis was studied. Possible mechanisms of a decrease in the relative initial process rate and an increase in the quasi-stationary value of relative luminescence intensity as the concentration of complexes in the polymer increased were discussed.     

Hydrogen bonding and excited state properties of the photoexcited hydrogen‐bonded (E)‐S‐(2‐aminopropyl) 3‐(4‐hydroxyphenyl)prop‐2‐enethioate complexes

The time‐dependent density functional theory (TDDFT) method has been performed to investigate the excited state and hydrogen bonding dynamics of a series of photoinduced hydrogen‐bonded complexes formed by (E)‐S‐(2‐aminopropyl) 3‐(4‐hydroxyphenyl)prop‐2‐enethioate with water molecules in vacuum. The ground state geometric optimizations and electronic transition energies as well as corresponding oscillator strengths of the low‐lying electronic excited states of the (E)‐S‐(2‐aminopropyl) 3‐(4‐hydroxyphenyl)prop‐2‐enethioate monomer and its hydrogen‐bonded complexes O₁‐H₂O, O₂‐H₂O, and O₁O₂‐(H₂O)₂ were calculated by the density functional theory and TDDFT methods, respectively. It is found that in the excited states S₁ and S₂, the intermolecular hydrogen bond formed with carbonyl oxygen is strengthened and induces an excitation energy redshift, whereas the hydrogen bond formed with phenolate oxygen is weakened and results in an excitation energy blueshift. This can be confirmed based on the excited state geometric optimizations by the TDDFT method. Furthermore, the frontier molecular orbital analysis reveals that the states with the maximum oscillator strength are mainly contributed by the orbital transition from the highest occupied molecular orbital to the lowest unoccupied molecular orbital. These states are of locally excited character, and they correspond to single‐bond isomerization while the double bond remains unchanged in vacuum.     

Manifestation of vibronic intensity borrowing in fluorescence spectra of metalloporphyrins

In selective laser excitation and at T=4.2 K, fine-structure spectra of the fluorescence of the complexes of Mg, Zn, In, and TiO with octaethylporphyrin are obtained. It is shown that the fine-structure fluorescence spectra of these compounds are formed to a large aegree by ribronic interaction between quasi-degenerate S₁ and S₂ electron states. An increase in the relative intensities of a number of negatively polarized fluorescence lines in displacement of the excitation line from a longwave edge of the band of an S₀→S₁ transition in the shortwave direction is revealed. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 518–522, July–August, 1999.     

Excited States of Magnesium Complexes of Porphin and Bacteriochlorin

Quantum-chemical calculations of magnesium complexes of porphin (D _4h , C _4v ) and bacteriochlorin (D _2h , C _2v ) have been carried out for three variants of Mg parameters used in the CNDO/S method. It is shown that the Mg parameters and z _Mg weakly influence the results of the calculations of these complexes in the excited S _i states localized on the macrocycles in contrast to the calculations for the S _i ^CT states. The G S _i ^CT transitions are transitions with intramolecular electron transfer (CT) from the macrocycles to Mg. The energies of the S _i ^CT states can be close to the energies of the B levels and differ by a value from 1600 to 6400 cm^–1 depending on the Mg parameters. A displacement of Mg (z _Mg) 0) from the midplane of the macrocycles and hydrogenation of the pyrrole rings decrease the energies of the lower G S _i ^CT transitions and enhance their CT character.     

Coherent population trapping involving Rydberg states in xenon probed by ionization suppression

We report the observation of pronounced coherent population trapping and dark resonances in Rydberg states of xenon. A weak two-photon coupling with radiation of = 250 nm is induced between the 5p^{6 1} S ₀ ground state of xenon and state 5p ⁵6p[1/2]₀, leading to (2+1) resonantly enhanced three-photon ionization. The state 5p ⁵6p[1/2]₀ is strongly coupled by radiation with ≃ 600 nm to 5p ⁵ ns[J _C]₁ or 5p ⁵ nd[J _C]₁ Rydberg states with principal quantum numbers n in the range 18 ?n? 23 and with the rotational quantum number of the ionic core J _C = 1/2 or J _C = 3/2. The ionization is monitored through observation of the photoelectrons with an energy resolution ΔE = 150 meV which is sufficient to distinguish the ionization processes into the two ionization continua. Pronounced and robust dark resonances are observed in the ionization rate whenever is tuned to resonance with one of the ns- or nd-Rydberg states. The dark resonances are due to efficient population trapping in the atomic ground state 5p^{6 1} S ₀ through the suppression of excitation of the intermediate state 5p ⁵6p[1/2]₀. The resolution is sufficient to resolve the hyperfine structure of the ns-Rydberg levels for odd xenon isotopes. The hyperfine splitting does not vary significantly with n in the given range. Results from model calculations taking the natural isotope abundance into account are in good agreement with the observed spectral structures. Pronounced dark resonances are also observed when the dressing radiation field with is generated from a laser with poor coherence properties. The maximum reduction of the ionization signal clearly exceeds 50%, a value which is expected to be the maximum, when the dip is caused by saturation of the transition rate between the intermediate and the Rydberg state due to incoherent radiation. This work demonstrates the potential of dark resonance spectroscopy of high lying electronic states of rare gas atoms. Received 7 May 2000 and Received in final form 25 June 2001     

Theoretical study on the isomerization mechanisms of phenylazopyridine on S0 and S1 states

In this work, some important structures tied closely to the isomerization of 2‐(phenylazo)pyridine (2‐PAPy) and 4‐(phenylazo)pyridine (4‐PAPy) on the S₀ and S₁ states were characterized in detail by using the complete active space SCF (CASSCF) theory. The isomerization mechanism was discussed on the basis of the mapped potential energy surfaces (PESs) and conical intersections (CIs). A comparison of PAPy with azobenzene was carried out to stress the effect of molecular structure on the photoisomerization details. The results indicate that the thermal isomerization for both 2‐PAPy and 4‐PAPy are mainly attributed to the inversion of CNN angle on the side of the pyridine ring. In view of the energy, an optimized CI_rot with a twisting structure supports the rotation mechanism in the photoisomerization of PAPy on the S₁ state. However, it was found that another conical intersection (CI_inv) with a planar structure is higher in energy than the corresponding trans‐FC structure, via which only the PAPy excited on S₂ state or vibrationally hot S₁ state can relax their excitation energy. Minimum energy paths (MEPs) showed that the relaxation process of cis‐PAPy being excited on the S₁ state is characterized by a smoothly falling curve, which is very similar to that of azobenzene. Furthermore, a S₁ minimum and a transition state (TS₁) were found to exist on the MEP starting from the trans‐FC point to the CI_rot for 4‐PAPy, but these two typical structures were not found on the MEP of 2‐PAPy. Compared with azobenzene, 4‐PAPy exhibits a very similar photoisomerization PES, but a subtly different one can be predicted in the case of 2‐PAPy. The present results are expected to provide useful information for the design of photoresponsive materials based on the PAPy units. Copyright © 2009 John Wiley & Sons, Ltd.