Optics and Spectroscopy (v.97, #2)

Specific features of the excitation spectrum of helium II in a plasma initiated by an electron beam by O. V. Zhigalov; Yu. A. Piotrovskii; Yu. A. Tolmachev (167-171).
The emission of a low-pressure helium plasma (P≤2 Torr) initiated by a monochromatic electron beam is investigated. It is found that an increase in the current leads to a drastic increase in the rate of charge exchange of doubly charged helium ions. The assumption is made that inelastic collisions of He++ ions with metastable helium atoms provide the main channel of charge exchange of these ions due to the reaction He+++Hem→ He+*+He 0 + .

The Kα to Lα and Kβ to Lβ intensity ratios of seven elements have been measured following photoionization at 59.5 keV by using a Si (Li) detector (FWHM=155 eV at 5.96 keV). The intensity ratios were determined by measuring K and L x-rays emitted from a standard target of a given element. The theoretical values of the Kα to Lα and Kβ to Lβ intensity ratios were calculated using theoretically tabulated values of shell/subshell photoionization cross sections, fluorescence yields, Coster-Kronig transition probabilities, and radiative decay rates for ηKLi≠ and ηKLi=0. The measured values are in good agreement with theoretical results.

The internal dynamics of the major isomer of the LiReO4 molecule, with account taken of the torsional transitions of the Li+ cation between its six energetically equivalent equilibrium positions relative to the ReO 4 anion, is analyzed in terms of symmetry-group chain methods. On this basis, the problem of separation of torsional and rotational motions, which is a nontrivial problem in this case, is solved and a rigorous model for the description of the energy spectrum in an arbitrary vibronic state is constructed.

A clearly pronounced structure of the Rayleigh wing consisting of a great number of components is measured in the range 0–100 cm−1 using four-photon polarization spectroscopy. The features observed are compared with the calculated IR spectra of H2O molecules in the gas phase taken from the HITRAN database. A good correlation between the frequency positions of the observed and calculated lines points to the possibility of hindered rotation of water molecules in liquid.

The effect of the nonplanarity of heterocyclic analogues of fluorene with the phosphorus atom on spin-orbit interactions by E. A. Gastilovich; V. A. Godik; N. V. Korol’kova; V. G. Klimenko; R. N. Nurmukhametov; S. A. Serov (187-194).
The transition dipole moments P 00 S for the transitions from the sublevels s=z,y, and x of the triplet electronic states 3 A″, which are caused by intramolecular spin-orbit (SO) interactions, are calculated. The calculation is carried out for molecules of heterocyclic analogues of fluorene containing phosphorus and nitrogen (namely, the groups P-H, P-C6H5, and N-H). The influence that the planar and nonplanar models of the molecules, as well as the substituents −C6H5 and −H at the heteroatom (HA), exert on the values of (P 00 S )2 and on the directions of the corresponding vectors is considered. The values of the component $$k_{SO^ sim } sum _s (P_{00}^s )^2 $$ of the radiative deactivation rate constant of the T 1 state are calculated. It is ascertained that, in the series of molecules with HA = N, O, P, and S, both the calculated values of k SO and those experimentally determined from an analysis of the intensity distribution in fine-structure phosphorescence spectra vary little, whereas the constant of the SO coupling in the heteroatom ςHA increases substantially along this series. The reason for the weak influence of ςHA on values of k SO—common for both planar and nonplanar molecules—is found.

A theoretical analysis of absorption spectra of uracil, thymine, and cytosine—nucleic acid bases— is carried out. Structural dynamic models of these molecules in their electronically excited states are constructed. On the basis of the calculated vibrational structure of the electronic spectra, different tautomeric forms of these molecules are determined. The possibility of modeling the influence of hydrogen bonds on the electronic-vibrational spectra is shown.

A new method for measuring low-temperature absorption spectra of single impurity molecules using a laser confocal polarized-light transmission microscope is proposed. The sensitivity of this method is of the same order of magnitude as that of fluorescence excitation spectroscopy. Combined use of both methods makes it possible to study distributions of impurity molecules by the magnitude of fluorescence quantum yield and their correlations with the linewidth distributions, as well as to determine if spectral jumps lead to changes in the fluorescence quantum yield.

Anharmonic analysis of the vibrational states of pyrimidine by the density functional method by K. V. Berezin; V. V. Nechaev; P. M. Él’kin (210-220).
The quartic force field of pyrimidine is calculated in the approximation of the hybrid density functional B3LYP/6-31G(d). On the basis of this force field, the IR spectrum of pyrimidine in the range 250–3800 cm−1 is interpreted. The Darling-Dennison and Fermi resonances are taken into account and their spectral manifestations are analyzed. A combined method for the anharmonic analysis of the vibrational states of polyatomic molecules that employs the theoretical anharmonicity constants and experimental frequencies is proposed. The method ensures a higher prediction accuracy.

A new method of extracting information on the efficiencies of quenching of an excited state and collisional reorientation from the data on the fluorescence depolarization of molecular vapors by quenchers of the excited state is proposed. The method does not involve collisional cross sections and fluorescence lifetimes. From the experimental data on the depolarization of polarized luminescence of para-quaterphenyl and 2-(4′-dimethylamine) phenyl-5-phenyloxydiazole by oxygen and nitrogen, the ratios of the constants of the excitedstate quenching and orientation randomization (fluorescence depolarization) by oxygen are obtained. For these molecules, the probabilities of the excited-state quenching q and orientation randomization s per collision with oxygen molecules are determined (q=0.25±0.04 and s=0.13±0.04 for para-quaterphenyl and q=0.2±0.04 and s=0.21±0.04 for 2-(4′-dimethylamine)phenyl-5-phenyloxydiazole). The self-quenching of fluorescence of perylene vapors, with the probability 0.28 per collision, is found.

This study continues the experimental testing of the validity of the inductive resonance theory of dipole-dipole energy transfer from the T 1S 0 transition dipole to stretching vibrations of intramolecular CH bonds of naphthalene and its hydroxy derivatives. To this end, in the series of compounds under study, the range of variation of the geometrical parameter [Φ(CH)]2 of the Förster theory, which accounts for the mutual orientation of the energy donor and acceptor, is estimated. Preliminarily, the angles between the transition dipole moments of the radiative and absorptive electronic transitions (T 1S 0 and S 0S 1; T 1S 0 and S 0S 2; S 1S 0 and S 0S 1; and S 1S 0 and S 0S 2) are measured at 77 K by the method of polarization photoselection. From the polarization measurements, the angles between the phosphorescence transition dipole moment and the plane of a molecule are determined. It was found that, upon passage from naphthalene to its β derivatives, the orientation of the dipole moment of the radiative T 1S 0 transition relative to the plane of a molecule markedly changes, with the in-plane component of the dipole moment being increased by an order of magnitude. The experimentally determined rate constants of nonradiative deactivation of the T 1 state averaged over the CH groups of the naphthalene ring system, k nr(CH), are compared with the rate constants [Φ(CH)]2 of the inductive resonance energy transfer from the dipole of the T 1S 0 transition to the dipole of the CH vibrations polarized in the plane of a molecule, calculated with regard to the orientational factor [Φ(CH)]2. This comparison showed that, in the series of compounds under study, a change in the orientation of the dipole moment of the radiative T 1S 0 transition relative to the plane of a molecule does not affect the rate of the nonradiative T 1S 0 transition. This inference is confirmed by the absence of a correlation between the rate constants k dd(CH) calculated by us (with regard to [Φ(CH)]2) and the well-known rate constants k nr(CH) of individual sublevels of the T 1 state measured at T≤1.35 K for a number of organic molecules. The possible sources of discrepancy between the experimental data that k nr(CH) is independent of [Φ(CH)]2 and the predictions of the theory are considered. A conclusion is made that the electronic-vibrational energy transfer between electric dipoles is the most probable mechanism of the T 1S 0 transitions, but the rate constant of the dipole-dipole energy transfer upon interaction of the electronic and vibrational dipoles in a molecule does not depend on their orientations.

Dynamics of fluorescence of solid solutions of quantum dots and the organic dye DODCI: Experiment and numerical simulation by S. K. Gorbatsevich; I. A. Kaputskaya; S. A. Maskevich; S. Yu. Mikhnevich; N. D. Strekal’ (238-243).
The fluorescence of solid solutions of CdSe/ZnS quantum dots and the organic dye DODCI is investigated. It is shown that nonradiative transfer of electronic excitation energy to dye molecules, which with some probability lose their acceptor properties as a result of photoisomerization or photodegradation, is responsible for a significant increase in the fluorescence intensity of a donor. The degree of polarization of the donor fluorescence attains values exceeding 0.5, which is due to the difference in the fluorescence quantum yields of donors with different orientations of the oscillator with respect to the electric vector of an excitation light wave. A numerical simulation of the experimentally observed dependences is performed.

Low-temperature time-resolved vacuum UV spectroscopy of NH4H2PO4 crystals by I. N. Ogorodnikov; V. A. Pustovarov; M. Kirm; V. S. Cheremnykh (244-250).
A complex investigation of the dynamics of electronic excitations in nonlinear optical crystals of ammonium dihydrophosphate NH4H2PO4 was performed using low-temperature vacuum UV luminescence spectroscopy with time resolution upon selective photoexcitation by synchrotron radiation. Data on the photoluminescence decay kinetics, time-resolved photoluminescence spectra (2–6.2 eV), and time-resolved photoluminescence excitation spectra (4–24 eV) were obtained for the first time for NH4H2PO4 crystals at 8 K. It is ascertained that the photoluminescence of NH4H2PO4 crystals in the vicinity of 4.7 eV has intrinsic character due to the radiative annihilation of self-trapped excitons. Possible channels of generation and decay of relaxed and unrelaxed electronic excitations in NH4H2PO4 crystals are discussed.

Na0.4Y0.6F2.2:Er3+ (NYF:Er) crystals with an erbium concentration as high as 100 at. % (Na0.4Er0.6F2.2) were grown by the Bridgman-Stockbarger method. The optical spectra were investigated at low (6 K) and room temperatures. It is shown that the absorption spectrum of NYF:Er crystals contains wide bands (790–801 and 965–980 nm) corresponding to the emission range of laser diodes. The peak absorption cross section σa for the band peaked at λ=970.4 nm is 0.15×10−20 cm2. On the basis of the analysis of the absorption and luminescence spectra at low (6 and 12 K) temperatures, the structure of the Stark splitting of erbium levels was determined as a structure of quasi-centers for which Stark components are inhomogeneously broadened. The oscillator strengths of the transitions from the ground state of erbium to excited multiplets were calculated from the absorption spectra measured at T=300 K, and the intensity parameters Ωt were determined by the Judd-Ofelt method: Ω2=1.65×10−20 cm2, Ω4=0.56× 10−20 cm2, and Ω6=1.01×10−20 cm2. These values of the intensity parameters were used to calculate the probabilities of radiative transitions and the branching ratios. The rates of multiphonon nonradiative transitions in NYF: Er were estimated. The luminescence decay kinetics for radiative levels of erbium ions upon their selective excitation by nanosecond laser pulses was studied. The intracenter lifetimes of radiative levels of erbium ions were determined from the luminescence kinetics upon selective ion excitation by low-intensity light in a sample with a low erbium concentration (0.5%). It is demonstrated that, with an increase in temperature from 6 to 300 K, luminescence from the 4 G 11/2, 2 G(H)9/2, and 4 F 9/2 levels is quenched as a result of multiphonon nonradiative transitions. Luminescence from the 4 I 9/2 level is quenched only insignificantly with increasing temperature, and no quenching of luminescence from the 4 I 11/2 and 4 I 13/2 levels is observed. The spectra of steady-state luminescence of NYF:Er(0.5–15%) crystals were investigated upon broadband excitation by UV and UV-visible lamp light and selective time-resolved laser excitation. It is shown that low-lying levels of erbium ions separated by an energy gap smaller than 2500 cm−1 are populated via cascade mechanisms. On the basis of the results obtained, it is concluded that NYF:Er 3+ crystals are promising candidates for active media of tunable diode-pumped lasers.

The dynamics of laser hole burning in a three-dimensional system of fixed isotropically oriented oscillators is simulated. The kinetic curves obtained are analyzed using methods of solving the inverse problem. The contribution of orientational effects to the total dispersion is studied and a quantitative characteristic of this contribution in the form of the standard deviation is obtained. The influence of the broadening of impurity centers on the dispersion of the burning kinetics of persistent and dynamical holes is studied. It is shown how some experimental instrumental factors can distort the results of the analysis of kinetic curves. On the basis of spectral and fluorescence experimental kinetic data, a method for determining the rate constants of the hole burning is proposed.

The reflectance spectra of a one-dimensional photonic crystal based on a silicon-air periodic structure are calculated. A map of photonic band gaps is plotted, which makes it possible to deliberately choose the geometric parameters of the structure (the thickness of silicon partitions D Si and the period A) for different ranges of the wavelength λ. To obtain structures with a photonic band gap in the range A/λ=0.15–0.5, the main region (as rule, corresponding to the lowest frequencies) can be used, and, taking into account the secondary photonic band gaps, the range A/λ can be extended to 1 and even more. In addition, it is found that, in the range D Si/A=0.4–0.9, the secondary band gaps may be wider than the main ones (on the frequency scale). The influence of the filling factor D Si/A on the formation of the edges of spectral bands is revealed.

An estimate of the ratio of the contributions of relativistic and nonrelativistic factors to the Kramers-Kronig relations for a medium with spatial dispersion is performed in a form taking into account the relativistic requirement of the finiteness of the velocity of propagation of a signal. Even under conditions of comparatively strong spatial dispersion, in the case of Wannier-Mott excitons in typical semiconductors, this ratio proves to be very small—of the order of 10−4, which justifies the conventional (nonrelativistic) form of the dispersion relations.

A procedure for analytical averaging of the attenuation and scattering cross sections for systems of chaotically oriented axially symmetric particles was developed for the first time within the framework of the modified T-matrix method and the method of separation of variables for spheroids. These approaches essentially complement each other: one is applicable to axially symmetric scatterers of different shape but is inefficient if the ratio of the maximum to the minimum size of the particles exceeds 3–5; the other is applicable only to spheroids, but the ratio of the major semiaxis to the minor one can be considerable, for example, 100 and larger.

Speckle polarization diagnostics of scattering media using partially coherent radiation by D. A. Zimnyakov; O Yang-Te; Yu. P. Sinichkin; G. G. Akchurin; V. A. Trifonov (288-298).
The paper discusses a method for probing a randomly inhomogeneous medium using partially coherent radiation and polarization filtering of the radiation scattered by the medium. The method is based on the analysis of the contrast of speckle-modulated images of the object under study as a function of the coherence length of the probe radiation. A theoretical justification of the method is given as applied to systems of discrete scatterers with the subsequent modification of the results obtained for continuously distributed scattering systems. The results of experimental testing of the developed method for scattering media characterized by nondiffusion conditions of propagation of the probe radiation are presented and compared with the results of the theoretical analysis.

The processes of formation of interference signals in the scattered coherent field with a wide angular spectrum in a scanning Michelson interferometer are considered. The effect of longitudinal correlation properties of the light-field complex amplitude on parameters of the interference oscillation envelope, upon detection of the signal in the image space and in the near-field region of diffraction, is studied.

Maxwell’s equations in the Majorana representation are generalized to the case of a chiral medium. A relation between the dynamic variables and the parameters of a chiral medium is found. An expression for the current density 4-vector is obtained for the medium under consideration.

This paper describes the calculation, fabrication, and experimental investigation of a reflection two-mirror interferometer whose front mirror is asymmetric in reflection and made as a thin-layer diffraction metal structure (Al, Ag) in combination with a dielectric coating (MgF2, ZnS). In reflection, this interferometer creates in the zero diffraction order a transmission-like pattern similar to that produced by an ordinary Fabry-Perot interferometer operating in transmitted light.

The effect of polarization nonreciprocity in sensors of angular velocity of rotation—fiber ring interferometers (FRIs)—is considered for arbitrary polarization of eigenmodes of the single-mode optical fiber of an FRI loop and arbitrary polarization state of radiation at the FRI input. A new method for detecting the polarization nonreciprocity in an FRI is proposed. Numerical estimations are made.

The diffraction efficiency and the resistance to radiation damage of gratings with a multilayer dielectric coating, which are used for compression of high-power laser pulses, are studied. The effect of smoothing of the profile of the metallized surface of the grating upon deposition of dielectric coatings on the energy characteristics of the grating at the wavelength λ=1.05 μm is analyzed. Gratings with various profiles of the metallic surface relief and with different contrast of the refractive indices of the dielectrics forming the coating are considered.

This paper reports on the results of analyzing the conditions under which quasi-periodic structures induced in AgCl-Ag photosensitive films by a linearly polarized Gaussian laser beam (λ=633 nm) can be clearly observed with the use of an optical microscope. It is shown that quasi-periodic structures with vectors KE (where E is the vector of polarization of the inducing beam) and periods d > λ are effectively formed upon excitation of waveguide TM0 modes at a large angle of incidence (φ=70°) and the p polarization of the inducing beam. Exposure of the film on a 60° glass prism to a p-polarized beam incident on the sample at an angle φ=60° from the side of the glass leads to the effective formation of primary gratings with vectors KE (due to the excitation of waveguide TM0 modes) and secondary oblique gratings. The specific features of the quasi-periodic structures and the correlation between the primary and secondary gratings are revealed and analyzed.