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.     

Lifetime of an emitting dipole near various types of interfaces including magnetic and negative refractive materials

A classical electromagnetic calculation of the lifetime of an emitting electric dipole near a material slab is presented. The lifetime is deduced from the imaginary part of the electric field Green's tensor associated with the stratified medium. The method is applied not only to the well known case of metallic reflectors, but also to magnetic reflectors and to negative refractive index slabs. The frequency dependence of the nonradiative decay rate at small distances is analyzed and interpreted in terms of the surface polariton modes of the slab.     

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.     

Lattice dynamics of solid α-carbon monoxide

The lattice dynamics of α-CO is studied assuming a fully-ordered antiferro P2₁3 structure, using a potential model which includes electrostatic, dispersion and exchange interactions. The coupling of translational and librational motions leads to mixed modes at finite wave vectors and to anomalous acoustic dispersion. Anharmonic frequency shifts and the damping due to phonon—phonon interactions are evaluated for the zero wave vector modes. The single-particle potential wells for the high-frequency librational modes are investigated and suggest that translation—rotation coupling plays a primary role in the molecular reorientations.     

Ultrafast Manipulation of Transient Energy Transfer Between Surface Plasmons and Resonators in Hybrid Nano-coupling System

Surface-plasmon(SP) modes triggered on metal nanostructures were strongly coupled to the local restricted electronmagnetic field supported by a Fabry-Perot(F-P) cavity. This hybrid system provided an ideal platform to study the interaction between SP and F-P resonators on nanoscales. However, the time-resolved transient energy transfer process is far from resolved. In this letter, we addressed this question by time-resolved femtosecond pump-probe technology and readily observed the transient energy transfer between SP and nanocavity resonant energy. The interaction resulted in the emergence of hybrid splitting mode and the oscillating dynamics between upper and lower polariton branch(the split hybrid states). Our work may provide a well comprehension of strong coupling between SP modes and F-P resonator modes, and lay some groundwork for many future photonic applications.     

Heat capacity and vibrational dynamics of α poly(β-benzyl-L-aspartate)

Poly(β-benzyl-L-aspartate) (PBLA) is an unusual polypeptide, which is capable of going into four different conformations, namely, left-handed α helix, right-handed α helix, ω helix, and β pleated sheet. The present work is a complete study of normal modes and their dispersion in the unusual left-handed α form. A special feature of some of the dispersion curves is their tendency to bunch in the neighborhood of helix angle. This is attributed to the presence of strong intramolecular interactions. Crossing and repulsion between the dispersion curves is also observed. The N-deuterated analogue of PBLA has been studied to check the validity of assignments and force field (Urey Bradley). Specific heat has been obtained from dispersion curves via density of states. A comparative study of left-handed and right-handed forms is presented. © 1996 John Wiley & Sons, Inc.     

Lattice dynamics of InAs under hydrostatic pressures

The lattice dynamics of InAs under variable hydrostatic pressures is investigated on the basis of an ‘11-parameter’ rigid-ion model (RIM). The calculated phonon dispersion curves are in satisfactory agreement with the neutron scattering data (available for the TA modes only) measured at room temperature and atmospheric pressure. The one- and two-phonon densities of states functions and mode Gruneisen parameters have been computed at two arbitrary hydrostatic pressures. The effect of high pressure on the phonon dispersion curves is shown to lead to a typical ‘softening’ in the transverse acoustic modes and eventually to a phase trnasformation of the compound.     

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.     

Enhanced Raman Scattering from Vibro‐Polariton Hybrid States

Ground‐state molecular vibrations can be hybridized through strong coupling with the vacuum field of a cavity optical mode in the infrared region, leading to the formation of two new coherent vibro‐polariton states. The spontaneous Raman scattering from such hybridized light–matter states was studied, showing that the collective Rabi splitting occurs at the level of a single selected bond. Moreover, the coherent nature of the vibro‐polariton states boosts the Raman scattering cross‐section by two to three orders of magnitude, revealing a new enhancement mechanism as a result of vibrational strong coupling. This observation has fundamental consequences for the understanding of light‐molecule strong coupling and for molecular science.     

Phonon dispersion in polycytidilic acid

A study of the normal modes of vibration and their dispersion in polycytidilic acid based on the Urey–Bradley force field and Raman and Fourier transform infrared spectroscopy is reported. Characteristic features of dispersion curves such as regions of high density of states, repulsion, and character mixing of dispersive modes are discussed. The predictive value of the heat capacity as a function of temperature is reported. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 504–516, 2006     

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.     

The influence of electron confinement,quantum size effects,and film morphology on the dispersion and the damping of plasmonic modes in Ag and Au thin films

Plasmons are collective longitudinal modes of charge fluctuation in metal samples excited by an external electric field. Surface plasmons (SPs) are waves that propagate along the surface of a conductor. SPs find applications in magneto-optic data storage, optics, microscopy, and catalysis.The investigation of SPs in silver and gold is relevant as these materials are extensively used in plasmonics. The theoretical approach for calculating plasmon modes in noble metals is complicated by the existence of localized d electrons near the Fermi level. Nevertheless, recent calculations based on linear response theory and time-dependent local density approximation adequately describe the dispersion and damping of SPs in noble metals.Furthermore, in thin films the electronic response is influenced by electron quantum confinement. Confined electrons modify the dynamical screening processes at the film/substrate interface by introducing novel properties with potential applications. The presence of quantum well states in the Ag and Au overlayer affects both the dispersion relation of SP frequency and the damping processes of the SP.Recent calculations indicate the emergence of acoustic surface plasmons (ASP) in Ag thin films exhibiting quantum well states. The slope of the dispersion of ASP decreases with film thickness.High-resolution electron energy loss spectroscopy (HREELS) is the main experimental technique for investigating collective electronic excitations, with adequate resolution in both the energy and momentum domains to investigate surface modes.Herein we review on recent progress of research on collective electronic excitations in Ag and Au films deposited on single-crystal substrates.     

Phonon dispersion curves and phonon lifetimes in crystalline ammonia

The theory of lattice dynamics of molecular crystals using atom—atom and multiple—multipole potentials developed in previous papers is extended to take account of anharmonic effects. Calculations of the dispersion curves and the density of states for the harmonic phonons in solid ammonia are presented. The finite phonon lifetimes due to anharmonicity are calculated for the zero wave vector modes and compared with experiment.     

Pancake Bonding: An Unusual Pi-Stacking Interaction

A category of parallel π-stacking interaction, termed pancake bonding, is surveyed. The main characteristics are: the interaction occurs among radicals with highly delocalized π-electrons in their singly occupied molecular orbitals (SOMOs), the contact distances in the π-stacking direction are shorter than the typical van der Waals distances, and the stabilization obtained by the bonding combination of the SOMO orbitals leads to direct atom-to-atom overlap with strong orientational preferences. These atypical intermolecular interactions contain a component of electron sharing between the radicals that can be viewed as covalent-like. Pancake bonded dimers characteristically have low-lying singlet and triplet states and show characteristic interlayer vibrational modes. Pancake bonded aggregates serve as molecular components in many conducting and other functional organic materials. The role of van der Waals (vdW) interactions in pancake bonded dimers, chains, and other aggregates is different from closed shell vdW aggregates: here the Pauli repulsions reduce the attractive dispersion interaction significantly. Fluxionality between π- and σ-bonded aggregates often occur in the context of pancake bonding. Both experimental and computational aspects are reviewed.     

Vibronic interactions and possible electron pairing in the photoinduced excited electronic States in molecular systems: a theoretical study

Electron-phonon interactions in the photoinduced excited electronic states in molecular systems such as phenanthrene-edge-type hydrocarbons are discussed and compared with those in the monoanions and cations. The complete phase patterns difference between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) (the atomic orbitals between two neighboring carbon atoms combined in phase (out of phase) in the HOMO are combined out of phase (in phase) in the LUMO) are the main reason that the C-C stretching modes around 1500 cm(-1) afford much larger electron-phonon coupling constants in the excited electronic states than in the charged electronic states. The frequencies of the vibrational modes that play an essential role in the electron-phonon interactions for the excited electronic states are similar to those for the monoanions and cations in phenanthrene-edge-type hydrocarbons. Possible electron pairing and Bose-Einstein condensation in the photoinduced excited electronic states as well as those in the monoanions and cations in molecular systems such as phenanthrene-edge-type hydrocarbons are also discussed.     

Localized states in polymeric molecules. I. The transfer matrix for long range interactions

We have extended the transfer matrix technique to determine the Green's function for 1-dimensional systems with long range interactions. The formalism is applied to study the density of states and impurity modes of linear chains; calculations are presented for nearest and next nearest neighbors interactions.Work partially supported by FINEP and CNPq.     

Properties and long range interactions of the calcium atom

The properties of a number of states of calcium are determined from a large basis configuration interaction calculation. The main focus is on the polarizabilities of the low lying states (the 4s2 1Se, 4s3d 1,3De, 4s4p 1,3Po, and 4s5s 1,3Se states) and the dispersion interactions of those states with the calcium ground state, the hydrogen atom, and the rare gases.     

Theory of depolarization dispersion of inversely polarized modes in heme proteins

A new polarization phenomenon observed recently in resonance Raman scattering from heme proteins is explained by vibronic interactions between split Q_x, Q_y, and B_x, B_y electronic states in porphyrin rings. Analytical formulas are presented which account well for the observed depolarization dispersion of the 1585 cm^?1 and 1310 cm^?1 a_2g modes in ferrocytochrome c.     

Dynamic light scattering in sols of a poly(N-vinylcaprolactam) and polyacrylamide mixture

The properties of semidilute solutions of a mixture of polyacrylamide and thermoresponsive polyvinylcaprolactam are studied via dynamic light scattering. It is found that the relaxation time (correlation length) distribution contains several modes. The obtained experimental data are interpreted in terms of the theory describing the dynamics of semidilute and concentrated solutions with allowance for viscoelastic interactions. The investigation of mode dispersion (the dependence of relaxation time on the wave vector of scattering) shows that the fastest and slowest modes are of the diffusion type. The fast mode is the mode of collective diffusion; it may be attributed to the fluctuation motion of polymer units inside blobs. The slowest mode corresponds to the diffusion motion of macromolecule clusters. The rate of relaxation of medium modes nonlinearly depends on the squared wave vector, and these modes are apparently viscoelastic.     

Raman dispersion and intermolecular interactions in unsubstituted thiophene oligomers

We present a critical analysis of the Raman spectra of unsubstituted oligothiophenes and rediscuss the well-known Raman dispersion of conjugated systems explicitly considering intermolecular interactions. Temperature-dependent Raman spectra and DFT calculations for dimers of different chain lengths show that the effect of intermolecular interactions on the frequency and intensity of carbon-carbon (CC) stretching modes is non-negligible. This effect has not been considered in previous works and might explain many spectral features of this class of compounds which are not completely interpreted by the usual models. Both intensities and frequencies are significantly affected by intermolecular interactions showing that molecular self-organization should be taken into account in future spectroscopic studies of conjugated molecules. In particular, the interactions among molecules cause an upward shift of the frequency of the R mode (amplitude mode) which can explain the lack of frequency dispersion with conjugation length of oligothiophenes, as experimentally observed for solid-state samples at room temperature.