Optics and Spectroscopy (v.106, #3)

Effect of the orbital structure and symmetry of electronic states on nonradiative S-T intersystem crossing: Dibenzofuran by E. A. Gastilovich; N. V. Korol’kova; V. G. Klimenko; R. N. Nurmukhametov (311-319).
The nonradiative S-T intersystem crossing S 1(ππ*) ⇝ T 1(ππ*) in dibenzofuran (DB(O)) molecules has been theoretically investigated within the model of vibronically induced spin-orbit (VISO) coupling of electronic states, where the vibronic perturbation takes into account all out-of-plane vibrational modes of a molecule. It is established that the S-T intersystem crossing S 1(1 A 1) ⇝ T 1(3 B 2) involves also the intermediate (T m )T 2(3 A 1) and T 3(3 B 2) triplet states. The calculated rate constant K ST = (4.5–4.7) × 107s−1 of the nonradiative transition is in agreement with the known experimental data. The manifestation of approximate (belonging to the D 2h group) symmetry of singlet and triplet molecular states in VISO couplings has been studied. An effect of the heavy (oxygen) atom of a DB(O) molecule on K ST is established.
Keywords: 33.20.Wr; 33.50.Hv

The manifestation of spontaneous Raman scattering upon the three-photon ionization of a samarium atom has been investigated. The dependence of the Sm+ yield on the laser frequency, along with the peaks due to the two-photon excitation of bound states from different levels of the 4f 66s 2 7 F ground term, exhibits a strong peak due to single-photon excitation of the 4f 55d6s 2 7 D°3 state from the 4f 65d6s 9 H 2 excited state. The 4f 65d6s 9 H 2 state was populated as a result of spontaneous Raman scattering. The large amplitude of the observed peak indicates a high efficiency of this process. The estimations performed indicate that the probability of resonant ionization through an excited state populated due to spontaneous Raman scattering can be comparable with the probability of three-photon ionization through the two-photon intermediate resonance. The necessary conditions for implementing this channel of resonant three-photon ionization are formulated.
Keywords: 32.80.Rm

Superradiant decay of localized excitations in a linear atomic chain by E. A. Gavrilenko; A. I. Zaitsev (328-333).
The superradiant decay of localized excitations in a linear chain of two-level atoms has been investigated. It is shown that the excitation energy is not completely emitted in the cooperative stage of the process. The residual excitation forms waves propagating from the regions of initial localization to the chain edges.
Keywords: 42.50.Md

Identification of singlet excited electronic states of hydroxybenzoic acid isomers by E. E. Tseplin; S. N. Tseplina; G. M. Tuĭmedov; O. G. Khvostenko (334-342).
The photoelectron spectra of benzoic and para-, meta-, and ortho-hydroxybenzoic acids and their UV absorption spectra in ethanol are obtained. The photoelectron spectra in the energy region up to 15.5 eV are interpreted based on the B3LYP/6-311+G(d, p) calculations. Based on the TDDFT B3LYP/6-311+G(d, p) calculations and taking into account the obtained interpretation of photoelectron spectra, the UV absorption bands are assigned to particular singlet transitions. For each transition, the electron configuration that makes the main contribution to the transition is determined. It is shown that the transition energy depends on the energy gap between the corresponding occupied and unoccupied molecular orbitals.
Keywords: 33.20.Lg; 33.60.+q; 31.15.E-

The polarized spectra of Raman scattering by lattice vibrations in solid solutions of paradibromobenzene with parachloronitrobenzene in the β phase are studied at temperatures of 297 and 77 K. It is found that the spectrum of the β phase at room temperature differs from the spectra of the α and γ phases, which agrees with the X-ray diffraction data. The crystals have a rodlike structure in the region of parachloronitrobenzene concentrations of 25–50% and 70–85% and do not have this structure at concentrations of 50–70% at room temperature. The analysis of the spectra at 77 K shows that the structure of the β phase consists of regions with the β phase structure and regions with the paradibromobenzene structure. The frequency spectra of the mixed crystal are calculated by the Dean method.
Keywords: 78.67.Bf; 63.22.Kn; 78.30.-j

The low-frequency Raman spectra of paradichlorobenzene nanofilms with a thickness of about 30 μm, 1 μm, and 400 nm are experimentally measured. With decreasing film thickness, the spectral lines shift to lower frequencies and broaden and some additional lines appear, the intensity of which increases with decreasing film thickness. The film structure is modeled using the method of molecular dynamics, and the histograms of lattice vibration spectra are calculated by the Dean method. It is found that a decrease in the film thickness leads to an increase in the lattice parameters and to a higher orientational disorder both inside the film and, especially, at the film surface. The film structure is similar to the structure of the α-paradichlorobenzene single crystal. To correctly interpret the spectra of lattice vibrations, it is necessary to take into account surface vibrations and existence of vacancies in the structure.
Keywords: 78.67.Bf; 63.22.Kn; 78.30.-j

The spectra of dual fluorescence of 3-hydroxyflavone molecules excited by 44-ps pulses in the region of the S 1 and S 2 absorption bands are measured with a picosecond resolution. The dynamics of the spectra directly demonstrates the time development of the proton transfer from the carboxyl to the carbonyl group of the molecule. Upon excitation into the main absorption band, the transfer process occurs for about 210 ps. The excitation into the region of the S 2 band results in a faster (∼170 ps) process, and the relative contribution made to the total spectrum by the long-wavelength band, which belongs to the proton-transfer state, is higher in this case for all the time ranges of luminescence recording. The data obtained directly point to an additional channel of proton transfer via the S 2 state. The probability of this process is estimated to be 0.84 × 1012 s−1.
Keywords: 33.50.Dq

The spectral and time characteristics of the dual fluorescence of a 3-hydroxyflavone probe in a solution with a fluorescence quencher are studied in the temperature range of 20–80°C. At room temperature, the fluorescence consists of two bands that belong to the normal and tautomeric forms of the luminophore, while heating of the solution results in the appearance of an additional fluorescence band belonging to the anionic form of the luminophore. The additional band is strongly overlapped with the band of the normal form, and its intensity rapidly increases with temperature to exceed the intensity of the normal band. The introduction of the TEMPO spin quencher of excited states into the solution completely quenches the fluorescence of the anionic form, which allows one to record the pure dual fluorescence of 3-hydroxyflavone in the entire temperature range studied. The detection of probe signals in the pure form is important for applications of proton transfer molecular probes using the intensity ratio of the fluorescence bands as the main sensitive parameter.
Keywords: 33.50.Dq

The [M(N_N)(Hdphpm)]ClO4 and [(M(N_N))2(μ-dphpm)](ClO4)2 complexes (M = Pd(II), Pt(II); (N_N) is ethylenediamine (En) and orthophenanthroline (Phen); Hdphpm and dphpm2− are the mono- and bisdeprotonated forms of 4,6-diphenylpyrimidine) are obtained and characterized by 1H NMR spectroscopy and electronic absorption and emission spectroscopy. The magnetic nonequivalence of protons of (N_N) ligands is explained by a difference in the trans-effect of the carbanion and pyrimidine parts of the cyclometalated ligand. The long-wavelength absorption bands and the vibrationally structured luminescence bands of ethylenediamine complexes are attributed to optical transitions in the {M(Hdphpm)} and {M2(μ-dphpm)} metal-complex fragments. The complexes with orthophenanthroline exhibit two low-energy optical transitions involving π* orbitals localized on the cyclometalated and chelating ligands; the difference in their energies depends on the metal and is much larger for Pt(II) than for Pd(II). It is found that the replacement of Pd(II) by Pt(II) in the [(M(phen))2(μ-dphpm)]2+ complexes changes the direction of the photoexcitation energy degradation due to the energy transfer between the {M2(μ-dphpm)} bridging fragment and peripheral phenanthroline ligands.
Keywords: 33.50.Dq

Luminescence properties of phosphors based on Tb3Al5O12 (TbAG) terbium-aluminum garnet by Yu. Zorenko; V. Gorbenko; T. Voznyak; T. Zorenko; B. Kuklinski; R. Turos-Matysyak; M. Grinberg (365-374).
The processes of excitation energy transfer in phosphors based on single-crystal Tb3Al5O12:Ce (TbAG:Ce) and Tb3Al5O12:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb3+ cations) to Ce3+ and Eu3+ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb3+ ions to Ce3+ and Eu3+ ions and from Ce3+ ions to Eu3+ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb3+ cation sublattice.
Keywords: 71.35.-y; 78.47.+p; 78.55.Hx

Negative circular polarization of InP QD luminescence: Mechanism of formation and main regularities by I. V. Ignatiev; S. Yu. Verbin; I. Ya. Gerlovin; R. V. Cherbunin; Y. Masumoto (375-387).
The spectrum and kinetics of the circular polarization of InP quantum dot (QD) photoluminescence have been experimentally investigated under different conditions of optical excitation and at different bias voltages applied to the sample. It is established that, at a bias of about −0.1 V, the degree of photoluminescence polarization is negative and reaches −50% in limiting cases. It is concluded that the negative polarization is formed in QDs containing one recident electron per dot and is mainly caused by the optical orientation of the electron spin. It is shown that all experimentally observed regularities are well described in the framework of the model assuming the energy relaxation of photogenerated electron-hole pairs accompanied by the electron- hole spin flip-flop process.
Keywords: 73.21.La; 78.67.Hc

The optical absorption spectra of crystals with garnet structure (A3B2C3O12), containing V or Cr impurity ions, before and after oxidation annealing, γ irradiation, and coactivation with Ca and Mg ions have been experimentally and theoretically investigated. It is shown that V3+ and Cr4+ ions are incorporated into both octahedral and tetrahedral sites of the garnet lattice. The absorption spectra and energy level diagrams of impurity ions are identified.
Keywords: 42.70.-a; 73.20.Hb; 76.30.Fc; 78.00.00

Fluorescent probe 4-dimethylaminochalcone: Solvatochromism according to the data of semiempirical and quantum-chemical calculations by N. G. Bakhshiev; S. K. Gularyan; G. E. Dobretsov; A. Yu. Kirillova; V. Yu. Svetlichnyi (395-397).
The solvatochromic regularities observed in solutions of 4-dimethylaminochalcone (DMCh) are investigated in terms of two independent theoretical approaches, i.e., semiempirical and quantum-chemical, and the results of these investigations are directly compared with each other for the first time. It has been shown that the values of the absolute solvation shift of the long-wavelength DMCh electronic absorption band determined by different methods are in a good quantitative agreement both with each other and with experimental data. This fact can be considered as a substantial additional justification of the conclusion on the predominantly nonspecific origin of intermolecular forces determining the mechanisms by which the DMCh fluorescent probe functions in biological systems.
Keywords: 33.50.Dq

It is shown analytically that, in a trap filled by a Bose-Einstein condensate of atoms with a negative S-scattering length, there exists an instability caused by the nonlocal interaction of atoms. For this effect to be efficiently discovered experimentally, it is necessary to considerably decrease the absolute value of the scattering length using the Feshbach effect.
Keywords: 03.75.Hh

The analysis of the stability of the generationless regime in a laser with a smooth transverse inhomogeneity of the resonator length is performed. Localized structures of radiation are considered that are possible in this laser when the nonlinearity of the gain and of the refractive index of the medium is taken into account.
Keywords: 42.65.Tg

The dynamics of polarized solitary optical waves in a medium characterized by nonlinear susceptibilities of the third and fifth orders (χ3 > 0, χ5 < 0) is studied by numerical methods. The interaction of scalar pulses with the maximum amplitude leads to the creation of a coupled state of two solitary waves which oscillates during the propagation into the depth of the nonlinear medium. As a result of the inelastic collision of two circularly polarized steady-state pulses, a series of secondary pulses of different degrees of ellipticity appears.
Keywords: 42.65.Tg; 42.65.Sf; 42.65.Jx; 42.81.Dp

The generation of zero and high-order harmonics in the spectrum of a laser pulse propagating through a medium containing quantum particles whose constant resonance transition dipole moment is nonzero is studied theoretically. The consideration is performed in the approximation of slowly varying envelopes modified for the case of the medium with the nonzero permanent dipole moment. It is shown that this modification requires consideration of antiresonance terms, in particular, the Bloch-Siegert shift in equations. The conditions are revealed for the efficient optical rectification and excitation of the second harmonic at a quasi-monochromatic signal applied to the medium.
Keywords: 42.50.Md; 42.65.Tg; 42.81.Dp

The mode of operation of a three-mirror system for the case where one of the cavities is filled with a gain medium, while the other contains two cells with gain and absorbing media, has been investigated. The behavior of the amplitude and phase noises of the gain and absorbing media in both arms of the three-mirror system has also been analyzed. It is shown that the presence of a gain cell narrows the absorption line and increases the resonance absorption contrast.
Keywords: 42.50.Lc; 42.60.Da

For the problem of the reflection and transmission of monochromatic radiation by a moving smooth inhomogeneity of parameters of a motionless transparent medium, transport equations are derived and an approximate solution to these equations is obtained for the case of weak reflection. Estimates of the effects for quartz in the transparency region are given.
Keywords: 42.65.Tg; 03.50De

Ellipsometry of GeO2 films with Ge nanoclusters: Influence of the quantum-size effect on refractive index by D. V. Marin; E. B. Gorokhov; A. G. Borisov; V. A. Volodin (436-440).
Using the methods of scanning and spectral laser ellipsometry and Raman scattering spectroscopy, GeO2 films containing Ge nanoclusters with a Ge/GeO2 mole ratio of 1: 1 are studied. A substantial difference is found between the experimental spectral dependence of the complex permittivity of the films and the one calculated for the effective medium in the Bruggeman model. The distinction can be qualitatively explained by the influence of the quantum-size effect. With the use of theoretical models for quantitative analysis, this approach will make it possible to determine the phase composition and dimensions of the nanoclusters of germanium in a contactless way without destructing the film.
Keywords: 78.20.Ci

The collinear acoustooptic diffraction of arbitrarily polarized radiation is studied. It is shown that the spectrum of diffracted light at the exit of an acoustooptic cell generally consists of four components, which have different frequencies and polarizations. Beats of these components lead to the modulation of the light passed through an analyzer installed at the exit of the system. Dependences of the amplitudes of components of the passed radiation on the frequency and power of an acoustic wave are studied for different orientations of the polarizer and analyzer.
Keywords: 42.25.Fx

Narrow resonances of the intensities and oscillation frequencies of counterpropagating waves in the vicinity of the center of the quantum transition in a pure-isotope ring gas laser have been studied. It is shown that the origin of resonances is related to the presence of sources causing unequal losses and/or frequencies of the counterpropagating waves in the laser cavity. The resonance change of intensities, which is accompanied by the resonance behavior of the medium dispersion for each of the waves, is not related to resonance changes of the saturated gain coefficients of the nonlinear medium. The resonances are caused by the redistribution of energy between the waves. The character of resonance changes of frequencies and intensities depends on the nature of nonreciprocity present in the cavity.
Keywords: 42.55.-f; 42.60.Jf

Spectral methods for determining the content of chlorophyll in vegetation subjected to anthropogenic action are considered. The regularities in the changes in the spectral radiance coefficients of vegetation in certain intervals of the visible range are considered in relation to the degree of remoteness of the pollution source.
Keywords: 42.25.Bs

Linear-wavenumber spectrometer for high-speed spectral-domain optical coherence tomography by V. M. Gelikonov; G. V. Gelikonov; P. A. Shilyagin (459-465).
An equidistant (in the wavenumber) spectrometer based on a diffraction grating, a compensation prism, and a CCD linear array is developed and implemented for spectral-domain optical coherence tomography. A criterion is introduced for estimating the level of residual nonequidistance. This criterion allows one to determine the threshold compensation level necessary for obtaining the spectrally limited spatial resolution. The method is tested in spectral-domain optical coherent tomography systems at wavelengths of 1270 and 830 nm.
Keywords: 42.15.Eq; 42.25.Fx; 42.25.Hz; 42.30.Va; 42.30.Wb; 42.62.Be