Spontaneous resonance hyper-Raman scattering of intense laser fields

The features of multicomponent spectral structure of the spontaneous hyper-Raman (SHR) scattered light from a four-level system irradiated simultaneously by three near-resonant laser fields are investigated.     

A new resonance scattering spectral method for the determination of trace amounts of iodate with rhodamine 6G

Under the conditions of 0.04 mol L⁻¹ HCl-8.0 × 10⁻⁴ mol L⁻¹ KI, there is a fluorescence peak at 540 nm and a synchronous fluorescence peak at 540 nm for rhodamine 6G (RhG). When there is IO₃⁻, it reacts with exceed I⁻ to form I₃⁻. And I₃⁻ and RhG combine into ion association particles. The particles exhibit three resonance scattering peaks at 320, 400 and 595 nm. And there is fluorescence quenching at 540 nm. Iodine concentration is proportional to the intensity of the resonance scattering intensity at 400 nm in the range of 1.0-20 × 10⁻⁷ mol L⁻¹. And a new resonance scattering spectral (RSS) method has been described for the determination of IO₃⁻ in salt samples. The spectral results have been verified that the formation of (RhG-I₃)_n association particles and solid-liquid interfaces are the main factor that cause the fluorescence quenching and resonance scattering effects.     

Acidity effects on the fluorescence properties and adsorptive behavior of rhodamine 6G molecules at the air-water interface studied with confocal fluorescence microscopy

The effects of acidity on fluorescence originated from rhodamine 6G (R6G) molecules adsorbed at the air-water interface of extremely low-concentration aqueous solutions have been studied with confocal fluorescence microscopy. Similarities and differences in the observed acidity effects between R6G molecules at the interface and those in the bulk solution have been discussed. With increasing the subphase-pH from 1 to 6, height and frequency of photon bursts as well as intensity of the interface-originated fluorescence change in two steps, while bulk fluorescence changes in one step and a little change in the number of adsorbed R6G molecules is verified with surface tension measurements. The results suggest that there is an interface-specific equilibrium among the chemical forms of R6G molecules. Chemical forms contributing to the interface-originated fluorescence above pH 5 are discussed.     

Intensity enhancement and selective detection of proximate solvent molecules by molecular near-field effect in resonance hyper-Raman scattering

A new molecular phenomenon associated with resonance hyper-Raman (HR) scattering in solution has been discovered. Resonance HR spectra of all-trans-beta-carotene and all-trans-lycopene in various solvents exhibited several extra bands that were not assignable to the solute but were unequivocally assigned to the solvents. Neat solvents did not show detectable HR signals under the same experimental conditions. Similar experiments with all-trans-retinal did not exhibit such enhancement either. All-trans-beta-carotene and all-trans-lycopene have thus been shown to induce enhanced HR scattering of solvent molecules through a novel molecular effect that is not associated with all-trans-retinal. We call this new effect the "molecular near-field effect." In order to explain this newly found effect, an extended vibronic theory of resonance HR scattering is developed where the vibronic interaction including the proximate solvent molecule (intermolecular vibronic coupling) is explicitly introduced in the solute hyperpolarizability tensor. The potential of "molecular near-field HR spectroscopy," which selectively detects molecules existing in the close vicinity of a HR probe in complex chemical or biological systems, is discussed.     

On the aggregative properties of rhodamine 6G in ethanolic solutions

A new experimental technique has been developed whereby the observation of the absorption bands due to the tetrameric species of rhodamine 6G in ethanolic solutions could become possible for the first time. The excitonic treatment of the electronic data backed by the ¹H NMR and vibrational studies suggest a coplanar structure for the dimers whereas the interaction between the parallel planes at a twisted angle of θ = 108° can constitute the tetrameric aggregates. The nature of the transitions involved in the absorption spectra are shown to be vibronic. The symmetry considerations have therefore been taken into account in order to ascertain the validity of the proposed structure for the tetramers.     

Resonance Raman scattering of rhodamine 6G as calculated by time-dependent density functional theory: vibronic and solvent effects

The geometries, UV-vis absorption spectra, and resonance Raman (RR) intensities have been determined for the S1 and S3 excited states of rhodamine 6G (R6G) in vacuum and ethanol by means of DFT/TDDFT methodologies with the aim of better understanding the structures and properties of the excited states. The RR spectra have been simulated from the vibronic theory of RR scattering as well as within the short-time approximation, while the solvent effects have been modeled using the polarizable continuum model. The S1 and S3 states of R6G present UV-vis absorption bands with similar vibronic structure, i.e., a shoulder at smaller wavelengths, although this shoulder is relatively more intense and more sensitive to the solvent in the case of S3. These differences are corroborated by the larger geometry relaxations upon excitation for S3 and the fact that the charge transfer of S3 is reduced in ethanol. Moreover, the differences between S1 and S3 are magnified when considering the RR spectra. On one hand, the RR spectrum of R6G in resonance with the S0 --> S1 transition presents many transitions of which the relative intensities strongly vary when the excitation wavelength gets closer to the maximum of absorption. The RR spectrum of R6G in resonance with S1 is however little influenced by the solvent. On the other hand, the RR spectrum of R6G in resonance with the S0 --> S3 transition displays only a few bands, strongly depends on the solvent, and is little affected when changing the excitation wavelength within the limits of the absorption band. As a consequence, the short-time approximation is suitable to reproduce the RR spectrum of R6G in resonance with S3 for a broad range of excitation wavelengths, whereas the vibronic theory approach is needed for describing the RR spectrum of R6G in resonance with S1 close to resonance.     

Concentration effects on the1H NMR parameters of rhodamine 6G

Kaliningrad State University. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 4, pp. 756–760, July–August, 1995.     

Probing the Raman scattering tensors of individual molecules

Single-molecule experiments provide new views into the mechanisms behind surface-enhanced Raman scattering. It was shown previously that spectra of individual rhodamine 6G molecules adsorbed on silver nanocrystal aggregates present stronger fluctuations in two low-frequency bending modes, at 614 and 773 cm(-1). Here we use polarization spectroscopy to show that these bands are enhanced by a resonant process whose transition dipole is rotated by 15+/-10 degrees with respect to the molecular transition dipole. We also show that the polarization function remains stable over the whole time scale of a measurement, indicating that molecular reorientation with respect to the surface is unlikely. Together these findings provide further support to the involvement of a charge-transfer resonance in the enhancement of the low-frequency bands and allow us to suggest a model for the orientation of rhodamine 6G molecules at Raman hot spots.     

Spectral properties of rhodamine 6G in smectite dispersions: effect of the monovalent cations

Montmorillonite monoionic forms with alkali metal and NH(4)(+)-cations were prepared by ion exchange. The hydration properties and binding of the ions to montmorillonite surface and the swelling properties of the mineral specimens were analyzed. Whereas Na(+)- and Li(+)-ions were fully hydrated over a large range of conditions, large size K(+), NH(4)(+), and mainly Rb(+) and Cs(+) ions were apt to bind directly to the oxygen atoms on the mineral surface. The forms with large ions exhibited reduced hydration and swelling and the absence of macroscopic swelling of the respective aqueous colloids. The interaction of laser dye rhodamine 6G (R6G) in montmorillonite colloids was investigated by absorption and steady-state fluorescence spectroscopies. Significant effects of the properties of both the inorganic ions and swelling properties of colloidal dispersions on R6G molecular aggregation were observed. Large amounts of the molecular aggregates were formed in the colloids of Na(+)- and Li(+)-montmorillonites. The aggregates absorbed light at significantly lower wavelengths (~460 nm) with respect to the light absorption by monomers (535 nm). Fluorescence spectroscopy provided a key evidence for the assignment of the type of the aggregates: The emission of the aggregates at relatively low energies proved these assemblies are rather a mixed H-/J-type than ideal H-aggregates. The presence of parent inorganic cations of larger size led to a significant lowering of the amount of the R6G aggregates in favor of the monomers. Investigations of the evolution of the dye aggregation with time indicated basic features of dye aggregation reaction: The size of parent inorganic ions did not affect the reaction mechanism, but rather limited the extent of the reaction. Probably the forms with large inorganic ions, such as Rb(+) and Cs(+), did not provide sufficient surface for the formation of the large size assemblies of the dye. This property can be explained in terms of strong association of the large alkali metal ions to clay mineral surface, as well as to reduced swelling in the colloidal systems of respective forms.     

Experimental study on polarized surface enhanced resonance Raman scattering of rhodamine 6G adsorbed on porous Al2O3 substrates

The polarization properties of surface enhanced resonance Raman scattering (SE(R)RS) of rhodamine 6G molecules, adsorbed to a hexagonally ordered gold nanostructure, are studied with the purpose to discriminate between adsorption sites with different plasmonic properties. The nanostructure is based on a self-organizing hexagonally ordered porous Al(2)O(3) substrate sputter-coated with gold. Each hexagonal subunit has D(6h) symmetry, where the symmetry center may act as an isotropic site, whereas the six narrow gaps between the individual Au hemispheres may act as hot-spots. The variation of the depolarization ratio (DPR), measured in resonance for the eight most prominent vibrational modes of the xanthene moiety, is analyzed by rotating the sample. According to theory, the DPR of the SE(R)RS signal obtained from molecules physisorbed in the isotropic sites deviates from the DPR originating from molecules physisorbed in the hot-spots in two ways: 1. The DPR associated with the isotropic sites depends differently on the rotation angle than the DPR associated with the hot-spots. 2. The DPR of the SE(R)RS signal obtained from molecules physisorbed in the isotropic sites depends on the nature of the Raman modes, whereas it for molecules physisorbed in the hot-spots is independent of the nature of the Raman modes. By applying the latter in the analysis of the polarized SE(R)RS data, we conclude that the dominating SE(R)RS signal comes from molecules adsorbed in the hot-spots. However, since the DPR's obtained for Raman modes of different symmetry are slightly different, the SE(R)RS signal must contain an additional contribution. Our analysis shows that the small mode-dependent SE(R)RS signal most likely comes from molecules adsorbed in the isotropic sites. The general result that can be derived from the present study is that by measuring the polarization properties in SE(R)RS and SERS it is possible to discriminate between adsorption sites with different plasmonic properties present in a highly symmetric nanostructure, even when the magnitude of the different contributions are highly different. The consequence of the insufficient spatial resolution with respect to a detailed mapping of the substrate often encountered in unpolarized SE(R)RS and in two-photon luminescence microscopy may thereby be circumvented.     

Resonance Raman scattering of rhodamine 6G as calculated using time-dependent density functional theory

In this work, we present the first calculation of the resonance Raman scattering (RRS) spectrum of rhodamine 6G (R6G) which is a prototype molecule in surface-enhanced Raman scattering (SERS). The calculation is done using a recently developed time-dependent density functional theory (TDDFT) method, which uses a short-time approximation to evaluate the Raman scattering cross section. The normal Raman spectrum calculated with this method is in good agreement with experimental results. The calculated RRS spectrum shows qualitative agreement with SERS results at a wavelength that corresponds to excitation of the S(1) state, but there are significant differences with the measured RRS spectrum at wavelengths that correspond to excitation of the vibronic sideband of S(1). Although the agreement with the experiments is not perfect, the results provide insight into the RRS spectrum of R6G at wavelengths close to the absorption maximum where experiments are hindered due to strong fluorescence. The calculated resonance enhancements are found to be on the order of 10(5). This indicates that a surface enhancement factor of about 10(10) would be required in SERS in order to achieve single-molecule detection of R6G.     

Ab initio study of optical properties of rhodamine 6G molecular dimers

Equilibrium atomic geometries of rhodamine 6G (R6G) dye molecule dimers are studied using density-functional theory. Electron-energy structure and optical properties of R6G H and J dimers are calculated using the generalized gradient approximation method with ab initio pseudopotentials. Our theory predicts substantial redshifts or blueshifts of the optical absorption spectra of R6G dye molecules after aggregation in J or H dimers, respectively. Predicted optical properties of R6G dimers are interpreted in terms of interatomic and intermolecular interactions. Results of the calculations are discussed in comparison with experimental data.     

Spectroscopic studies of rhodamine 6G dispersed in polymethylcyanoacrylate

We report here electronic absorption, fluorescence and resonance Raman studies of rhodamine 6G laser dye dispersed in the polymethylcyanoacrylate matrix. In the electronic absorption and fluorescence spectra of dispersed rhodamine 6G, band maxima are red shifted compared to solution. Raman spectra show some new bands. These spectral changes arise due to matrix effect and interaction between rhodamine 6G and the host material involving amine group of rhodamine.     

Influence of lithium iodide on association of rhodamine 6G molecules in mixtures of isopropanol with CCl4

The spectral-luminescent properties of rhodamine 6G on addition of lithium iodide to the solutions were studied in mixtures of polar (isopropanol) and nonpolar (carbon tetrachloride) solvents. It was found that when LiI is added, the complex associates of the dye molecules formed in solutions containing more than 90 vol. % of CCl₄ dissociate into rhodamine 6G monomers. The enthalpy of association of rhodamine depends on the concentration of the salt in the system. The volume and the geometry of the complexes have been evaluated from the data of the polarization characteristics of the fluorescence of the associates.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 3, pp. 368–372, May–June, 1987.The authors wish to express their gratitudes to Associate Professor B. M. Uzhinov for his participation in the discussion of the results.     

Luminescence properties of rhodamine 6G intercalated in surfactant/clay hybrid thin solid films

To develop the solid-state laser oscillator based on laser dye compounds, the incorporation of rhodamine 6G (R6G, a laser dye) in cetyltrimethylammonium (CTA+) cationic surfactant/montmorillonite clay hybrid (HpC) thin solid films was investigated. The R6G/HpC samples were prepared by immersing the HpC films into a R6G aqueous solution with various concentration. X-ray diffraction patterns of the films of HpC, measured before and after the intercalation of R6G, proved the coexistence of both the dye and surfactant in clay interlayer spaces. All prepared thin films exhibited luminescence. It indicates that CTA+ molecules play a role as a partial suppressor of the aggregation of R6G molecules which prevents fluorescence. Moreover, the luminescence property of the present thin films was observed to be dependent on the co-intercalated degree of R6G molecules, indicating that the R6G intercalating in HpC interlayer space molecules exist as two or more luminescence species in the clay interlayer space.     

Stochastic behavior of the dynamic stokes shift in rhodamine 6G

Transient behavior of the fluorescence spectrum has been investigated in rhodamine 6G in ethanol at low temperatures by means of a time-resolved luminescence technique. The width of the fluorescence spectrum as well as its peak position has been found to change with time. These characteristics are shown to be well explained by a stochastic model.     

Resonance hyper-Raman excitation profiles of a donor-acceptor substituted distyrylbenzene: one-photon and two-photon states

Resonance Raman and resonance hyper-Raman spectra of the "push-pull" conjugated molecule 1-(4'-dihexylaminostyryl)-4-(4"-nitrostyryl)benzene in acetone have been measured at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra), resonant with the first two bands in the linear absorption spectrum. The theory of resonance hyper-Raman scattering intensities is developed and simplified using assumptions appropriate for intramolecular charge-transfer transitions of large molecules in solution. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles, all in absolute intensity units, are quantitatively simulated to probe the structures and the one- and two-photon transition strengths of the two lowest-energy allowed electronic transitions. All four spectroscopic observables are reasonably well reproduced with a single set of excited-state parameters. The two lowest-energy, one-photon allowed electronic transitions have fairly comparable one-photon and two-photon transition strengths, but the higher-energy transition is largely localized on the nitrophenyl group while the lower-energy transition is more delocalized.     

Aggregation states of rhodamine 6G in electrospun nanofibrous films

Novel fluorescent composite nanofibrous films of rhodamine 6G (Rh6G) and polyacrylonitrile (PAN) are first prepared by electrospinning. The aggregation states of Rh6G in electruspun nanofibrous films are studied as a function of concentrations and characterized by UV–vis absorption spectroscopy and emission and excitation fluorescence spectroscopy. We have also used casting films as reference material to compare the effect of incorporation of Rh6G in electrospun nanofibrous films and casting films. The large specific surface area of the nanofibers and fast evaporation of the solvents in the electrospinning process reduced the aggregation of Rh6G. The appearance of fluorescent J-type dimers, even at higher dye concentration in elctrospun films, demonstrates that the electrospun films are an ideal material for incorporation of fluorescent dyes.     

Calculation of intensity in the resonance Raman and two-photon absorption spectra of polyatomic molecules

A direct method for calculating the resonance Raman and two-photon absorption spectra of polyatomic molecules is described in detail The method is based on the adiabatic model and uses Herzberg-Teller’s approximation. Relations ruling out direct summation over vibrational quantum numbers of excited electronic states and representing the matrix elements of the Green function of a multidimensional oscillator as functions of vibration frequencies and Dushinsky transformation parameters are derived. The relations are convenient for constructing algorithms. Translated from Zhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 248–255, March–April, 1997.     

Study of the adsorptive behavior of water-soluble dye molecules (rhodamine 6G) at the air-water interface using confocal fluorescence microscope

A confocal fluorescence microscope was applied to directly study the characteristic behaviors of adsorbed molecules at the air-water interface for a water-soluble chromophore, rhodamine 6G (R6G), in its extremely low-concentration region (below 10(-10) M). Significant photon bursts were observed only from the surface, and their width, height, and frequency were found to depend on the bulk concentration, suggesting the inhomogeneous distribution of R6G molecules at the air-water interface. This property of the adsorbed molecules is different from that of the bulk one. The influence of the ionic strength on photon bursts from the interface was investigated. It was found that the addition of NaCl to the R6G solution caused a decrease of the fluorescence signal. A change in the size of the aggregate and in the fluorescence quantum yield of the adsorbed molecules was suggested to be responsible for this experimental result.