Absorption,fluorescence and stimulated fluorescence by polaritons in thin anthracene crystals. A re-interpretation of spectroscopic properties of anthracene

The transmission, absorption (excitation spectra) and fluorescence spectra of thin (? 47 nm) free mounted anthracene flakes have been measured. True absorption in b polarization in the region of the lowest exciton state occurs as a result of scattering by phonons. It has a minimum near the transverse exciton frequency and a maximum near the longitudinal exciton frequency, in agreement with expected polariton behaviour. Thickness dependent polariton states have finite absorption and fluorescence transition probabilities (due to crystal inhomogeneities) and are observed below the transverse exciton frequency. These polariton states represent the energy reservoir for excitation energy in the bulk of the crystal. A surface-induced exciton state is the origin of the sharp line fluorescence from pure crystals and accounts for the high efficiency of stimulated fluorescence at low temperatures. Stimulated fluorescence can also be observed from the polariton modes when excitation occurs in these modes. Resonance interactions between polariton modes and impurity levels represent an important pathway for fluorescence quenching in crystalline anthracene.     

Manipulating valence and core electronic excitations of a transition-metal complex using UV/Vis and X-ray cavities

We demonstrate how optical cavities can be exploited to control both valence- and core-excitations in a prototypical model transition metal complex, ferricyanide ([Fe(iii)(CN)₆]³⁻), in an aqueous environment. The spectroscopic signatures of hybrid light-matter polariton states are revealed in UV/Vis and X-ray absorption, and stimulated X-ray Raman signals. In an UV/Vis cavity, the absorption spectrum exhibits the single-polariton states arising from the cavity photon mode coupling to both resonant and off-resonant valence-excited states. We further show that nonlinear stimulated X-ray Raman signals can selectively probe the bipolariton states via cavity-modified Fe core-excited states. This unveils the correlation between valence polaritons and dressed core-excitations. In an X-ray cavity, core-polaritons are generated and their correlations with the bare valence-excitations appear in the linear and nonlinear X-ray spectra.

We demonstrate how optical cavities can be exploited to control both valence- and core-excitations in a prototypical model transition metal complex, ferricyanide ([Fe(iii)(CN)₆]³⁻), in an aqueous environment.     

Polariton splittings of deuterated hexamethylenetetramine

The i.r. absorption spectra and the Raman scattering spectra of polycrystalline deuterated hexamethylenetetramine (HMTD) have been recorded. The longitudinal and transverse components of all the i.r. active F₂ modes below 1200 cm⁻¹ are assigned in the Raman spectra (except ω₂₀). The observed polariton splittings have been used to determine the static dielectric constant (2.66 ± 0.035) which is the same as that for the HMT crystal. As expected, isotopic substitution does not change this macroscopic data.     

Extraordinary exciton polaritons in the optical spectra of molecular crystals

The highly anisotropic reflectivity spectra of molecular crystals (cyanine dye, TCNQ⁰) are discussed in terms of macroscopic dielectric theory. The resonance energies defined as the maxima in the ?₂-spectra depend on the relative orientation of the wavevector of the light and the transition moment. These directional effects are quantitatively explained by directional dispersion of the extraordinary polariton modes. Vibrational satellites are responsible for the detailed structure of the reflectance bands. Strong mixing of the various polariton branches shows up as additional broadening of the peaks in the ?₂-spectra.     

Nonlinear polariton effects in the phenanthrene crystal

Two-photon fluorescence excitation (TPE) and second harmonic generation (SHG) spectra of the origin region of the 3500 Å system of the phenanthrene crystal at 2 K are presented and compared for both melt-grown and sublimed crystals. The dependence of the upper Davydov component position on the photon polarization and propagation direction reported previously is confirmed for both TPE and SHG- This behavior is considered to be a manifestation of phase matching to the upper branch of the b-polariton dispersion curve and is intrinsic to the two-photon process for transitions which are allowed in both one- and two-photon absorption. Both TPE and SHG are understood as resulting from the fundamental process of polariton fusion. The appearance of the dipole-forbidden lower Davydov component in these spectra is commented on in terms of misalignment and excitation of a longitudinal exciton.     

Estimating the conditions for polariton condensation in organic thin-film microcavities

We examine the possibility of observing Bose condensation of a confined two-dimensional polariton gas in an organic quantum well. We deduce a suitable parameterization of a model polynomial Hamiltonian based upon the cavity geometry, the biexciton binding energy, and similar spectroscopic and structural data. By converting the sum-over-states to a semiclassical integration over D-dimensional phase space, we arrive at a principle of correspondence between ideal and non-ideal Bose gases that share a common critical exponent. Using our results, we can calculate the properties for a model cavity containing an anthracene thin film.     

Ultrafast dephasing time of localized surface plasmon polariton resonance and the involved damping mechanisms in colloidal gold nanoparticles

In this contribution, for the first time precise in situ measurements of the ultrafast dephasing time T₂ of localized surface plasmon polariton resonances in colloidal gold nanoparticles with the objective to identify the involved damping mechanisms are presented. T₂ is an essential parameter that does not only allow one to determine the field enhancement factor that is of great importance for many applications of nanoparticles, but also reflects the role of different dephasing mechanisms. The most essential result is the observation of a chemical interface damping which causes a dramatic shortening of the dephasing time. While T_2∞ = 9.4 fs can be obtained from the bulk dielectric function, the value shrinks to 3.7 fs if the nanoparticles are in aqueous solution.     

Robust identification of the cancer biomarker phosphocholine through partitioned envelopes in noisy magnetic resonance spectroscopic data by the non-parametric fast Padé transform

The recently introduced concept of envelope partitioning for shape estimation within the fast Padé transform (FPT) is presently further explored and solidified. Earlier, noise-free time signals were used and the results were reported for a single model order K . Currently, partitioned envelopes are computed for several values of model orders K by employing noise-corrupted time signals of increasing standard deviations, \(\sigma =\) 0.0289, 0.289, 2.89 in units of root-mean-square of the noise-free time signal. Moreover, spectra averaging is exploited to stabilize shape estimation in face of sensitivity to changes in model order. The main goal of this study is to establish the robustness of the non-parametric FPT for reconstructions of partitioned average envelopes computed with noisy time signals. Both the previous and present illustrations concern synthesized time signals typically encountered in single-voxel magnetic resonance spectroscopy (MRS), akin to in vitro encoding from malignant breast tissue. This particular problem area is chosen for a twofold reason: clinical urgency in cancer medicine, and a huge challenge to reliably identify a key cancer biomarker (phosphocholine), completely hidden underneath a dominant peak (phosphoethanolamine), with a separation of mere 0.001 parts per million of chemical shift. Upon successful benchmarking of partitioned envelopes for noisy simulated MRS time signals, the road would be paved for applications of this special shape estimation to the associated data from in vivo encodings. Partitioned envelopes are important since they offer possibilities to peer into the tightly overlapped resonances by splitting their components apart already at the level of sole total shape spectra. Although constrained by non-parametric estimations, they can still qualitatively decompose the regions of higher spectral density. This would enable subsequent focusing on the most critical spectral regions of interest when solving the local quantification problem by parametric estimations. Such a stepwise strategy is expected to be especially beneficial for multi-voxel magnetic resonance spectroscopic imaging (MRSI), where thousands of noisy spectra need to be processed. The FPT-based partitioned envelopes, followed by accurate local spectral analysis in narrow frequency intervals are poised to help MRSI become an efficient diagnostic modality for everyday clinical practice.     

Time resolved multiple scattering of resonance photons

A simple one-dimensional model is used to calculate analytically the probability that a resonance photon is scattered exactlyn times within a finite region containing absorbing atoms. The probability distributions are used to analyze time dependent fluorescence light signals from the mercury 253.7 nm line with the mean number of scatteringss _m of about three. A comparison between exact one-dimensional results with more realistic two- and three-dimensional Monte-Carlo simulations of multiple scattering shows that the one-dimensional model is a good approximation for smalls _m and isotropic scattering. Therefore it may well serve as a fast method to analyze measured signals whereas fine details need more refined calculations.     

Paramagnetic ring current in the diazepine ring of the 1H-benzo-1,5-diazepinium monocation

The signals of all of the protons in the PMR spectrum of 2,4-dimethyl-1H-benzo-1,5-diazepine hydrochloride are shifted to stronger fields by 0.5–1.0 ppm relative to the signals of the analogous protons in model compounds. This shift is explained by the considerable paramagnetic contribution of the eight π electrons of the diazepine ring of the 1H-benzo-1,5-diazepinium monocation to the magnetic susceptibility of the molecule. Calculations of the π-electron ring current and the π-electron component of the magnetic susceptibility of this monocation by the MO LCAO method showed that the ring current in the seven-membered ring is paramagnetic and depends markedly on the magnitude of the coulombic integral for nitrogen.     

Thermodynamics of exciton/polaritons in one and two dimensional organic single-crystal microcavities

We consider here the thermodynamics and phase-diagram of exciton/polaritons formed in low-dimensional organic single-crystal microcavities. Using the Dicke model for a lattice of Frenkel excitons coupled to a common cavity mode, we explore the transition between normal and condensate regimes as depending upon the exciton band-width and temperature of the polariton gas. We show that for one-dimensional conjugated polymers, the coexistence curve at low temperatures shifts towards lower critical coupling strengths as the exciton band-width is increased. We also consider the effect of orientational disorder in a two-dimensional polyacene slab on the formation of the polariton BEC. Our results indicate that while a small degree of orientational disorder will not have a profound effect on the critical exciton/photon coupling strength needed to produce the transition from normal to BEC regimes, BEC will likely be suppressed in glassy or strongly disordered molecular films.     

Molecular nanopolaritonics: cross manipulation of near-field plasmons and molecules. I. Theory and application to junction control

Near-field interactions between plasmons and molecules are treated in a simple unified approach. The density matrix of a molecule is treated with linear-response random phase approximation and the plasmons are treated classically. The equations of motion for the combined system are linear, governed by a simple Liouvillian operator for the polariton (plasmon+molecule excitation) dynamics. The dynamics can be followed in time or directly in frequency space where a trace formula for the transmission is presented. A model system is studied, metal dots in a forklike arrangement, coupled to a two level system with a large transition-dipole moment. A Fano-type resonance [Phys. Rev. 103, 1202 (1956)] develops when the molecular response is narrower than the width of the absorption spectrum for the plasmons. We show that the direction of the dipole of the molecule determines the direction the polariton chooses. Further, the precise position of the molecule has a significant effect on the transfer.     

Methodology of chemometric modeling of spectrometric signals in the analysis of complex samples

A generalized algorithm of the multivariate simulation of spectrometric data is considered for solving typical analytical problems, like the determination of the concentration of a particular analyte and the assignment of a sample to one of predefined classes. In particular, we considered preliminary data processing, exploratory analysis, optimization of a chemometric model, calculation of performance characteristics, transfer of the model to other spectrometers, and automation of chemometric processing for the routine analysis of samples. To illustrate the potential of the method, we selected a system of bovine and porcine heparin, mixtures of soy and sunflower lecithin, and a set of red and white wine samples as test samples. Partial least squares and discriminant analysis were used as chemometric methods. We used proton nuclear magnetic resonance (1H NMR) to record signals. Using the MATLAB environment, chemometric programs were developed for automated data processing in the context of problems under consideration and for the transfer of multivariate models to other spectrometers. Based on the results obtained, a methodology is proposed for the multivariate analysis of spectrometric data, which can be used in the analysis of various types of matrices and spectrometric signals.     

Vibrational spectra of sodium gadolinium tungstate NaGd(WO4)2 single crystals: Observation of spatial dispersion

Room temperature polarized Raman scattering and infrared reflectance spectra of a NaGd(WO₄)₂ single crystal have been measured. The IR spectra interpretation was aided by a Kramers-Krönig analysis, and fitted to the independent oscillator model. All 13 theoretically expected Raman-active bands have been identified and assigned, as well as 7 out of 8 expected IR active bands. Splitting of bands in both Raman and IR clearly indicates a lowering of the crystal symmetry due to occupation disorder in the 4a site, that randomly accommodates either an Na⁺ or a Gd³⁺ ion. The reflectance IR spectra reveal a spatial dispersion, namely a dependence of the transverse optical (TO) polariton frequencies, on the propagation direction in the crystal. The crystal vibrational modes are correlated to the internal modes of the tungstate group WO₄^2?, and to the internal modes of the molecular skeleton. A detailed correlation map of the symmetry analysis is presented.     

Characterization of the measurement error structure in 1D H NMR data for metabolomics studies

NMR-based metabolomics is characterized by high throughput measurements of the signal intensities of complex mixtures of metabolites in biological samples by assaying, typically, bio-fluids or tissue homogenates. The ultimate goal is to obtain relevant biological information regarding the dissimilarity in patho-physiological conditions that the samples experience. For a long time now, this information has been obtained through the analysis of measured NMR signals via multivariate statistics.NMR data are quite complex and the use of such multivariate statistical methods as principal components analysis (PCA) for their analysis assumes that the data are multivariate normal with errors that are identical, independent and normally distributed (i.e. iid normal). There is a consensus that these assumptions are not always true for these data and, thus, several methods have been devised to transform the data or weight them prior to analysis by PCA. The structure of NMR measurement noise, or the extent to which violations of error homoscedasticity affect PCA results have neither been characterized nor investigated.A comprehensive characterization of measurement uncertainties in NMR based metabolomics was achieved in this work using an experiment designed to capture contributions of several sources of error to the total variance in the measurements. The noise structure was found to be heteroscedastic and highly correlated with spectral characteristics that are similar to the mean of the spectra and their standard deviation. A model was subsequently developed that potentially allows errors in NMR measurements to be accurately estimated without the need for extensive replication.     

Virtual polariton modes of slab molecular crystals

The virtual polariton states of a slab dielectric consisting of N planes of discrete molecules are considered using the dipole approximation for intermolecular interactions. It is assumed that the sum of in-plane static interactions is large compared to the inter-plane sums. The dispersion relation of the virtual modes is solved for exactly and the conditions defined under which it reduces to the dispersion relations for symmetric and antisymmetric modes of a slab of continuous matter. Finally some comments are made on the occurrence of virtual polariton states in fluorescence and absorption processes.     

Natural abundance 17O NMR study of 2- and 4-substituted benzoyl chlorides

Natural abundance ¹⁷O NMR chemical shift data for 17 ortho and para benzoyl chlorides recorded in acetonitrile at 75°C are reported. ¹⁷O NMR data for the para substituted benzoyl chlorides are correlated with ¹⁷O NMR data for similarly substituted acetophenones and methyl benzoates. The ¹⁷O NMR signals for ortho isomers are downfield (ca 30 ppm) from their para isomers; the downfield shifts are consistent with torsion angle change. The ¹⁷O NMR data for the para isomers gave good correlations with σ⁺ constants and with dual substituent parameters (DSP).     

MSGRAPH: A program for the display of LC/MS data

The analysis of mixtures of unknowns using techniques interfacing liquid chromatography (LC) with mass spectrometry (MS) often suffers from the high abundance of background ions. To allow a quick search for components in the mixtures, a special type of display has been developed that shows all the data obtained in an analysis run simultaneously. On the screen, the m/z information from the mass scan (in the x direction) and the time (as scan number, in the y direction) are displayed with mass peaks given as single points. The relative intensity of a signal is colour coded. Thus, it is easy to recognize analyte signals as bright islands in a dark surrounding; even weak signals in the vicinity of strong background ions are identified quite clearly. Additionally, the program incorporates some features to allow a simple operation and very fast investigation of the data. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by a disk containing the program MSGRAPH, a manual, data files and additional text files.     

The 13C-NMR spectrum and stereochemistry of heteronemin

The ¹³C-NMR spectrum of heteronemin (1), a new sesterterpene from marine origin is reported. Assignment of most of the signals was accomplished by a combination of off-resonance decoupling, PRFT measurement, comparison with suitable known model compounds and LIS measurements. An all trand-anti-trans configuration is suggested from 1 according to the ¹³C-NMR data.