Theory of the vibrational line shape of an adatom at a bridge site

The homogeneous vibrational line shape of an adatom on a surface is determined by its interactions with other localized and delocalized modes of the adsorbate and substrate. These interactions include anharmonic bonds between the adatom and the surface, and also between the substrate atoms themselves at the adsorption site. The line width and frequency shift of the perpendicular vibrational mode of an adatom at a bridge site is calculated as a function of temperature, due to coupling to phonon modes of a semi-infinite elastic continuum. A self energy formulation is employed, using perturbative methods in a quantum field theory. The effects of anharmonicity of both the adsorption bonds and of the interatomic bonds between substrate atoms at the bridge site, on the vibrational line shape, are compared. Energy relaxation by two-phonon emission is found to describe well the temperature-dependent line width in the O/Cu(110) system.     

Vibrational absorption spectrum of solid CO in the first harmonic region. Two-phonon transition

Infrared absorption spectrum of CO crystal at low temperature, recorded in the Δυ = 2 region, shows a strong Δυ = 2 zero phonon line, a broad phonon side band and a small line which is attributed to the simultaneous vibrational transitions between two molecules. The absorption coefficient and line frequency of this two-phonon line is calculated by a theory taking account both the induced dipole moment between molecular couples and the Fermi resonance between the ν = 2 level and the two-phonon level. It is shown that most of the two-phonon line intensity comes from the Fermi resonance effect. Good agreement with experiment is obtained. The linewidth is given by the convolution product of two 0 → 1 line shapes.     

Temperature dependence of the exciton—phonon coupling in the strong coupling limit: Results for solid nitrogen

High resolution (ΔE = 0.75 meV) absorption profiles of the vibronic bands in the range of the w¹Δ_u ← X¹Σ⁺_g and a¹II_g ← X¹Σ⁺_g exciton progressions at hv ≈ 8.9 eV in solid N₂ have been measured in the temperature range between 6 K and 30 K. These excitations are strongly localized so that the observed temperature dependence of the fine structure, consisting of a zero phonon line and a phonon side band, can be described very well in the model of strong exciton—phonon coupling at point defects. The experimental results for the w¹Δ_u transition are found to be consistent with the assumption of a Debye spectrum for the phonon density of states and we derive a value for the Debye temperature of θ = 78 K, which is in very good agreement with that derived from other measurements.     

Exciton dynamics in molecular crystals from line shape analysis. Time response function of singlet excitons in crystal anthracene

From an accurate measure of the temperature dependence of the line shape function ω ?₂ (ω) information is obtained about the time behaviour of the response function of singlet excitons of small wavevector encompassed by the (0-0) band of the 4000 A Å transition in crystal anthracene. An apparatus to determine the reflection band profile with high accuracy needed to give correct ω ?₂ (ω) data is described. Although the data analysis is not without problems, there is strong evidence that the time behaviour of excitons in this transition is characterized by a stochastic collision time τ_c. The temperature dependence of τ_c is consistent with a model in which the intermolecular phonons are weakly coupled with the exciton created by either b or a polarized light. Phonon annihilation is predominant for the b-polarized transition but both phonon creation and annihilation are active for these polarized transition. The similar values of the exciton—phonon coupling function for both polarizations may indicate either the importance of higher multipole terms for that function or strong interband scattering. The relationships between τ_c and parameters from other experimental results on singlet excitons in crystal anthracene are considered. The results may allow for a better understanding of the mechanism of exciton—phonon coupling in crystals.     

Infrared spectroscopic study of the local structural changes across the metal insulator transition in nickel-doped GdBaCo2O5.5

Phonons in GdBaCo₂O_5.5 have been identified using infrared spectroscopy and their mode assignments have been carried out using ab initio lattice dynamical calculations. Metal insulator transitions in undoped and nickel-doped GdBaCo₂O_5.5 have been probed using infrared absorption spectroscopy. The phonon modes corresponding to the bending mode of the CoO₆ octahedra/pyramids are seen to soften, broaden and develop an asymmetry across the insulator-metal transition pointing to extensive electron phonon interaction effects in these systems. Correlated changes of the phonon line shape parameters associated with the transition indicate a suppression of T_MIT with increased nickel doping of the cobalt sublattice. Temperature dependence of the octahedral stretching mode frequencies in undoped GdBaCo₂O_5.5 points to distinct structural distortions accompanying the high temperature metallic transition.     

Discrete resonance raman scattering and vibrational relaxation in Ca2 in solid Kr

Raman scattering is found to be strongly enhanced when the exciting laser is in resonance with discrete vibronic transitions of Ca₂ in solid Kr. Relaxation of the high vibrational levels is competitive with the rate of local phonon relaxation, and excitation in the phonon sideband produces no relaxed zero phonon line (ZPL) re-emission from levels above υ = 6.     

Phonon Raman spectra, molecular motions, and phase transitions of dimethylacetylene crystal

The phonon Raman spectra of crystals of dimethylacetylene, perdeutero-dimethylacetylene and their mixtures have been investigated down to 20°K. The high temperature phase shows only one line and the low temperature phase only two. Neither of these is related to the R_z (methyl) rotation, which seems to remain practically free. However, the latter motion is found to be restricted in a newly discovered metastable phase, which is reported here first, displaying 5–6 phonon bands. The well-known λ-point transition appears little related to methyl rotations. The internal rotation of dimethylacetylene seems to stay practically free down to 20°K in the stable phases.     

Homogeneous IR line shape of matrix isolated molecules in the debye approximation

The homogeneous line shape of vibrational transitions of matrix isolated molecules is calculated in the Debye approximation for lattice vibrations. The interaction between matrix and molecule is assumed anharmonic and expressed in space time phonon correlation functions. Excluding local modes and libration of the molecule, analytical expressions for the temperature dependence of line width and line shift are derived which permit verification by experiment.     

Electronic spectroscopy of nonalternant hydrocarbons inside helium nanodroplets

We have recorded the electronic spectra of three polycyclic aromatic hydrocarbons (acenaphtylene, fluoranthene, and benzo(k)fluoranthene) containing a five-member ring and their van der Waals complexes with argon and oxygen with a molecular beam superfluid helium nanodroplet spectrometer. Although the molecules, which differ by addition of one or two fused benzene rings to acenaphtylene, have the same point group symmetry, the spectral lineshapes show distinct differences in the number of zero phonon lines and shapes of the phonon wings. Whereas the smallest molecule (acenaphtylene) has the most complicated line shape, the largest molecule (benzo(k)fluoranthene) shows different lineshapes for different vibronic transitions. The van der Waals complexes of fluoranthene exhibit more peaks than the theoretically allowed number of isomeric complexes with argon/oxygen. The current models of molecular solvation in liquid helium do not adequately explain these discrepancies.     

Electronic polarization spectroscopy of metal phthalocyanine chloride compounds in superfluid helium droplets

We report the electronic polarization spectroscopy of two metal phthalocyanine chloride compounds (MPcCl, M=Al,Ga) embedded in superfluid helium droplets and oriented in a dc electric field. For both compounds, the laser induced fluorescence spectra show preference for perpendicular excitation relative to the orientation field. This result indicates that the permanent dipoles of both compounds are predominantly perpendicular to the transition dipole. Since the permanent dipole derives from the metal chloride, while the transition dipole derives from the phthalocyanine chromophore, in the plane of phthalocyanine, this qualitative result is not surprising. However, quantitative modeling reveals that this intuitive model is inadequate and that the transition dipole might have tilted away from the molecular plane of phthalocyanine. The out of plane component of the transition dipole amounts to approximately 10% if the permanent dipole is assumed to be approximately 4 debye. The origin for this tilt is puzzling, and we tentatively attribute it to the transition of nonbonding orbitals, either from the chlorine atom or from the bridge nitrogen atom, to the pi* orbitals of the phthalocyanine chromophore. On the other hand, although unlikely, we cannot completely exclude the possibility that both our high level density functional theory calculation and ab initio results severely deviate from reality. The droplet matrix induces redshifts in the origin of the electronic transition and produces discrete phonon wings. Nevertheless, in dc electric fields, all phonon wings and the zero phonon line demonstrate the same dependence on the polarization direction of the excitation laser. Although electronic excitation does couple to the superfluid helium matrix and the resulting phonon wings add complications to the electronic spectrum, this coupling does not affect the direction of the electronic transition dipole. Electronic polarization spectroscopy in superfluid helium droplets is thus still informative in revealing the permanent dipole and its relation relative to the transition dipole.     

Polarized Absorption,Spontaneous and Stimulated Blue Light Emission of J‐type Tetraphenylbutadiene Monocrystals

Blue amplified spontaneous emission at room temperature is demonstrated from the exposed face of the strongly emitting organic semiconductor 1,1,4,4‐tetraphenyl‐1,3‐butadiene in single crystal form. The symmetry of the crystal and calculation of lattice sums indicate the J‐type organization of the molecular transition moments. The minimum in the lowest exciton dispersion branch, from which emission takes place, is found at the edge of the Brillouin zone leading to a dominant vibronic emission since the zero‐phonon line is forbidden. The observed gain narrowed line is attributed to the vibronic replica which becomes amplified with increased pumping. The reported emission is along the normal to the exposed crystal face, important for the development of vertical cavity geometry lasers based on organic single crystals. The threshold excitation fluence of 400 μJ cm^?2 is comparable to other organic crystalline systems, even if the amplification path is much reduced as a consequence of the vertical geometry. Considering these relevant aspects, the optical characterization of this material is provided. The polarized absorption spectra are reported and the properties of the lowest‐energy excitonic state investigated. Calculation of the electronic transitions for the isolated molecule, lattice sums for the transition at lowest energy, and the symmetry of the crystal allow attributing the largest face of the samples and the observed optical bands in the spectra. Polarized time‐resolved spectra are also reported allowing to identify the intrinsic excitonic emission.     

Fluorescence-resolved hole burning of squaraine dyes in polymer matrices at low temperature

A new technique that combines nonphotochemical hole burning with multichannel detected fluorescence line narrowing has been used to obtain vibrationally resolved fluorescence spectra of squaraine chromophores in polymer matrices at 1.4 K. At a fixed excitation frequency, the intensities of the zero-phonon lines decay with time due to nonphotochemical hole burning, leaving behind a broader background attributed to emission from molecules excited into phonon sidebands. Subtracting the spectrum of the hole-burned sample from the initial one leaves predominantly a zero-phonon line excited spectrum exhibiting enhanced vibrational structure. Spectra of the same squaraine in polystyrene and polyethylene matrices show differences in the frequencies and intensities of the phonon sidebands, indicating differences in the frequencies and strengths of the matrix modes coupled to the electronic transition of the chromophore. The phonon densities of states inferred through different measurement techniques are compared and related to electronic dephasing rates.     

Electrical transport properties of liquid alloys

Two complementary mechanisms have been proposed for relatively high temperature superconductor MgB₂. While the first is the electron–phonon mechanism of BCS theory, advocated strongly by Pickett and co-workers, the second, by Bianconi et al., invokes Feshbach shape resonances. While we cannot presently discount the second mechanism, and while both proposals exploit the multiband nature of the electronic structure of MgB₂, we show here that five body-centred cubic (bcc) transition metals, whose superconducting transition temperature correlate intimately with elastic constants and therefore are plainly BCS-like in character, lie on a curve which has MgB₂ at the high T _c end. Any alternative mechanism to electron–phonon interaction in MgB₂ will need to account quantitatively for this circumstance.     

Relationship between phonon damping and pure dephasing of localized electronic excitations

Effects of quadratic electron—phonon interaction on the zero-phonon line shape of impurity-ion electronic transitions in crystals and on the spectrum of lattice phonons are investigated. Both subsystems (impurity ion and phonons) are treated on an equal fooling, employing the method of double-time thermal Green functions in the random-phase approximation. It is shown that in the case of weak dephasing due to destructive interference effects, a relationship can be established between the pure-dephasing rate of the electronic excitation and the temperature-dependent part of the electron—phonon-limited phonon damping. The phonon linewidth is shown to decrease to a lesser extent than the electronic linewidth when interference effects become enhanced, thus providing information on the dynamical broadening processes even if the electronic pure-dephasing width is too small to be observed.     

Luminescent properties of pure cubic phase Y2O3/Eu3+ nanotubes/nanowires prepared by a hydrothermal method

One-dimensional pure cubic Y(2)O(3)/Eu(3+) nanocrystals (NCs) were synthesized by a hydrothermal method at various temperatures. The NCs prepared at 130 degrees C yielded nanotubes (NTs) with wall thickness of 5-10 nm and outer diameter of 20-40 nm. The NCs prepared at 170 and 180 degrees C yielded nanowires (NWs) with diameters of approximately 100 and approximately 300 nm, respectively. Their luminescent properties, including electronic transition processes, local environments surrounding Eu(3+) ions, electron-phonon coupling, and UV light irradiation induced spectral changes have been systematically studied and compared. The results indicate that the Y(2)O(3)/Eu(3+) NTs and NWs have strong red (5)D(0)-(7)F(2) transitions. The fluorescence lifetime of (5)D(1)-(7)F(1) hardly changes in different samples, while that of (5)D(0)-(7)F(2) decreases a small amount in Y(2)O(3)/Eu(3+) NTs. The (5)D(0)-(7)F(2) lines originate from the emissions of Eu(3+) ions occupying one C(2) site, like that in the bulk powders. The phonon sideline with a frequency shift of 40-50 cm(-1) appears at the low-energy side of the (7)F(0)-(5)D(0) zero phonon line. The relative intensity of the sideline to zero phonon line increases by varying from NTs to NWs, and the spectral position of the phonon sideline shifts red. The UV light irradiation induced spectral change in the charge-transfer band was studied. The results indicate that the spectral change is dependent on sample size and is wavelength selective. A detailed model was proposed to explain the light-induced spectral change.     

Density functional theoretical study of lattice-specific heat and thermal properties of magnesium nitride

A comprehensive first principle study of thermodynamic properties of MgN is reported within the density functional theory scheme. The ground state properties such as lattice constant, Bulk modulus etc. of MgN in rock-salt (RS) phase have been determined. The thermodynamical properties have been analyzed in the light of phonon density of states of MgN and its constituent atoms. The variation of lattice-specific heat with temperature obeys the classical Dulong–Petit’s law at high temperature while at low temperature it obeys Debye T ³ law. The phonon spectrum shows the presence of all positive phonons and zero phonon density of states at zero energy confirming a dynamically stabilized structure of MgN in RS phase.     

Single‐Molecule Spectroscopy on a Ladder‐Type Conjugated Polymer: Electron–Phonon Coupling and Spectral Diffusion

We employ low‐temperature single‐molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl‐substituted ladder‐type poly(para‐phenylene) (MeLPPP) to investigate the electron–phonon coupling to low‐energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron–phonon coupling to low‐frequency vibrations of the surrounding matrix of the chromophores, and that low‐energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line‐broadening mechanism of the zero‐phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material.     

Debye equation of state for fluid helium-3

An equation of state for 3He using the Helmholtz potential function has been developed. The lower limit of the equation of 0.01 K is safely above the superfluid transition at 0.0026 K. The upper limit of 60 K is approximately the upper limit of available 3He property measurements. The new state equation form is based on Debye function which goes smoothly to zero in the limit of zero temperature and reduces to the ideal gas in the limit of zero density and/or very high temperature. The equation combines (a) necessary temperature-independent compressibility terms at the lowest temperatures, (b) terms describing the linear specific heat of a Fermi fluid below 1 K, (c) terms describing the phonon excitations which begin above about 1 K, and (d) terms which attempt to fit the conventional critical point thermodynamics at 3.3157 K and 114 604 Pa. State properties, e.g., p-V-T relations, specific heats, thermal expansion, sound velocity, etc., are determined from the Helmholtz energy by standard thermodynamics. Transport properties, e.g., thermal conductivity and viscosity, are not obtained in this work.     

Influence of temperature and pressure on shape and shift of impurity optical bands in polymer glasses

The shape, broadening, and shift of optical absorption spectra of molecular impurity centers in polymer glasses are considered in terms of inhomogeneous energy distributions and coupling of electronic transitions to vibrations. Persistent spectral hole burning was applied for frequency-selective probing of zero-phonon lines. The shift and broadening of spectral holes were studied between 5 and 50 K and by applying a hydrostatic He gas pressure up to 200 bar. Broadband absorption spectra were recorded between 5 and 300 K in poly(methyl methacrylate) and polyethylene. In addition to "normal" thermal broadening, due to the first- and second-order electron phonon coupling, several narrowing components were predicted on the basis of frequency dependent hole behavior. Thermal expansion of the matrix and the relaxation of local strains, previously accumulated on cooling below the glass temperature can lead to shrinking of the inhomogeneous width. A Voigt treatment of absorption band shapes reveals that the Gaussian component can indeed suffer remarkable narrowing. Inhomogeneous band shapes and the frequency-dependent thermal and baric line shifts were rationalized with the aid of a pair of two-body Lennard-Jones potentials. The shift of potential well minima is a crucial factor influencing solvent shifts, inhomogeneous band shapes, pressure shift coefficients, and quadratic electron phonon coupling constants.