Single Impurity Molecules Research Articles

Electronic spectra and structure of 1- and 2-fluoronaphthalene impurity centers in crystalline naphthalene

The results of experimental investigations on the low-temperature fluorescence and absorption spectra of the 1- and 2-fluoronaphthalene impurities in the crystalline naphthalene are presented and analyzed. When the 1- and 2-fluoronaphthalene concentrations are less than 1%, the absorption and fluorescence spectra are shown to be composed of two identical band series, induced by the formation of two different centers, consisting of the single impurity molecules. The impurity concentration increase of over 1% results in the appearance of the additional structure in the polarized absorption spectra near the 0-0 bands of the single centers in a form of narrow bands with similar intensity and different polarization. This structure is associated with a formation of impurity paired centers, consisting of the two resonantly interacting molecules of either 1-fluoronaphthalene, or 2-fluoronaphthalene. Besides, in the case of the 2-fluoronaphthalene, the absorption spectra contain intense peaks, located ∼ 45 cm−1 higher in frequency, relative to the 0−0 transition bands for each single-center, and consisted of complexes of narrow lines. These spectral peaks are also explained by a formation of impurity paired centers, but here interactions between the molecules are not resonant. For the pure naphthalene crystal factors, causing the observation of additional oppositely polarized weak bands in the absorption spectra at ∼ 10 cm−1 near the Davydov doublet components, are described.

Direct Observation of a Quasilocalized Low-Frequency Vibrational Mode in the Fluorescence Excitation Spectrum of a Single Impurity Molecule in a Polymer Matrix

The possibility of direct observation of a quasi-localized elementary excitation of the phonon type in polyisobutylene has been demonstrated by detecting the fluorescence excitation spectrum of a Mg‑tetrazaporphyrin single probe molecule in a wide spectral range at a temperature of 6 K. The parameters of the quasilocalized low-frequency vibrational mode have been measured—the frequency (energy) in the case, when impurity molecule is in excited electronic state (ωe = 13.94 ± 0.21 cm–1) and the halfwidth of the mode spectrum (γ = 3.82 ± 0.66 cm–1). The measured energy of the low-frequency vibrational mode coincides with the position of a boson peak maximum in the spectrum of vibrational states of the polymer, which indicates that the weak coupling approach can be applied to the considered case of the electron–phonon interaction of an organic dye molecule with the local environment in a polymer matrix.

Local-Field Effects in the Zero-Phonon Spectral Lines of Single Impurity Molecules in Solid Matrices at Low Temperatures

In this work, we study the relation between the widths of the zero-phonon lines (ZPL) of single impurity molecules (SM) in solid matrices with local effective values of the refractive index n. Since at ultralow temperatures the width of SM ZPL is determined mainly by the radiative lifetime T 1 , while the local-field effects produce the function T 1 (n ), SM spectroscopy makes it possible to measure the distribution of n local values in solids. The approach proposed here for studying local fluctuations of n has been tested on the example of doped polymers and molecular crystals.

Observation of structural relaxations in disordered solid media via spectral histories of single impurity molecules

This paper presents the technique and results of the investigation of the structural relaxation in disordered solid media (polyisobutylene, toluene, ortho-dichlorobenzene) at temperatures below the glass transition point via the spectra and fluorescence images of single impurity fluorescent molecules (terrylene, tetra-tert-butylterrylene). Application of the thermal-cycling method has made it possible to significantly extend the temperature range of investigation (from 4.5 K to the glass transition temperature). The changes observed in individual parameters of low-energy elementary excitations of the tunneling and vibrational types due to structural relaxation processes have been studied. It has been found that there is a spatial inhomogeneity in the distribution of the activation temperatures of local relaxation processes.

S-wave-scattering resonances induced by dipolar interactions of polar molecules

We show that s-wave scattering resonances induced by dipolar interactions in a polar molecular gas have a universal large and positive effective range, which is very different from Feshbach resonances realized in cold atoms before, where the effective range is either negligible or negative. Such a difference has important consequence in many-body physics. At high temperature regime, a positive effective range gives rise to stronger repulsive interaction energy for positive scattering length, and weaker attractive interaction energy for negative scattering length. While at low-temperatures, we study polaron problem formed by single impurity molecule, and we find that the polaron binding energy increases at the BEC side and decreases at the BCS side. All these effects are in opposite to narrow Feshbach resonances where the effective range is negative.

Evolution of Kondo Resonance from a Single Impurity Molecule to the Two-Dimensional Lattice

The successive spectral evolution of the Kondo resonance state was investigated from a single iron(II) phthalocyanine molecule to the two-dimensional lattice on Au(111) by interrogating the individual molecules with a scanning tunneling microscope. A sharp Kondo peak appears in the single-impurity regime, which broadens and splits as the lattice builds up. The origin of spectral evolution together with the electronic ground state of the lattice are discussed based on the competition of the Kondo effect and Rudermann-Kittel-Kasuya-Yosida coupling between the molecular spins.

Spontaneous emission rates of a single-impurity molecule in biaxial host crystals

The single molecule spectroscopy provides the possibility to measure the spectra of a single impurity molecule, whereby the influence of the inhomogeneous structure of the host are eliminated. The main object of studies in the single molecule spectroscopy is the zero-phonon line – the narrowest and the most intense line in the spectrum. We have taken into account a factor which up to now has not been considered in the theories of zero-phonon line – the influence of the anisotropy of the host crystal on the spontaneous emission rate of the single-impurity molecule and thus also on the radiative line width. The relative spontaneous emission rate of the dipole transition of single-impurity molecules in biaxial crystals is determined through a formula with two coefficients. These coefficients are calculated as the functions of the difference of the principal refractive indexes n 3 − n 2 for values of the principal refractive indexes 1.5 ⩽ n 1 ⩽ 1.7 and 1.55 ⩽ n 2 ⩽ 1.95 .

Magic Number Stabilities of Small para-H2 Clusters Doped by a Single ortho-D2

Path-integral Monte Carlo calculations have been performed to study superfluid behavior of small para-H2 clusters doped by a single impurity molecule of ortho-D2 at a low temperature of 1.6 K. It is found that the D2 molecule is located near the center of mass of the cluster and H2 molecules are distributed around the D2 molecule to form a shell structure. All the clusters considered here show significant superfluid fractions at this temperature but superfluidity is found to be noticeably quenched at specific sizes of the clusters. The different superfluid propensity depending on the cluster size is understood to be related to the magic number behavior of the para-H2 clusters.

Effects of anisotropy and the proximity to the surface on the spontaneous emission rate of pentacene in p-terphenyl crystal

Due to the anisotropy of a birefringent host crystal, the spontaneous emission rate of a single-impurity molecule depends on the orientation of the transition dipole moment relative to the host crystal. In p -terphenyl crystal, the transition dipole moment of a single pentacene impurity can have four different orientations. Here the relative spontaneous emission rate of the electric quadrupole transition is calculated depending on the unknown component of the transition dipole moment normal to the a ′/ b ′-plane. Maximum difference in the spontaneous emission rate of the electric quadrupole transition is up to 9%. For comparison, the effects caused by the proximity to the plane surface of the crystal are calculated.

Radiative line width of a single-impurity molecule in a birefringent crystal

We have taken into account a factor which up to now has not been considered in the theories of the zero-phonon line – the influence of the anisotropy of the host crystal on the spontaneous emission rate of the single-impurity molecule and thus also on the radiative line width. The anisotropy reveals itself in the dependence of the spontaneous emission rate on the orientation of the radiative transition dipole moment in the crystal and is one of the factors which may lead to the experimental variation of the line width of a single-impurity molecule. In the model where the interstitial impurity is located in the center of the crystal cell the results of the calculations in the case of a uniaxial host crystal and eight biaxial crystals have been presented. The differences in the values of SER for the electric dipole transition in the same host crystal are up to 50% and for the electric quadrupole transition −24%.

Spontaneous emission rates of a single-impurity molecule in birefringent host crystals for electric quadrupole transitions

In a birefringent host crystal, the spontaneous emission rate of zero-phonon line of a single-impurity molecule depends on the angles between the transition dipole moment and the principal axes of the dielectric (permittivity) tensor and also on the main dielectric constants of a host crystal. In this paper, the spontaneous emission rate of the electric quadrupole transition of a single-impurity molecule is calculated for uniaxial host crystals with the ordinary reflective indices n o=1.5, 2, 3 and a variable extraordinary reflective index, and for seven biaxial host crystals (anthracene, chrysene, diphenyl, fluorine, naphthalene, phenanthrene, terphenyl). For the above-mentioned biaxial crystals, differences in the values of spontaneous emission rate of the quadrupole transition in the same host crystal are up to 11%.

Calculations of spontaneous emission rates of a single-impurity molecule in uniaxial host crystals

In a uniaxial host crystal the spontaneous emission rate of the dipole transition of the zero-phonon line of a single-impurity molecule depends on the angle between the transition dipole moment and the optical axis of a crystal and as well as on the ordinary and extraordinary refractive indices. The relative spontaneous emission rate (the spontaneous emission rate divided by the spontaneous emission rate in the case when the transition dipole moment is parallel to the optical axis) is determined through a simple formula by one coefficient. Here this coefficient is calculated as a function of the extraordinary refractive index for 40 values of the ordinary refractive index on the interval from 1.25 to 3.20. For comparison, the effects caused by the nearness to the plane interface between cinnabar crystal (HgS) and air, as an example, are calculated.

Difference values of spontaneous emission rate of a single-impurity molecule of pentacene in biaxial p-terphenyl crystal

The spontaneous emission rate of a single-impurity molecule in a birefringent host crystal depends on the angles between the transition dipole moment and the principal axes of the dielectric (permittivity) tensor as well as on the main dielectric constants of a host crystal. The transition dipole moment of a single pentacene impurity in p-terphenyl crystal may have four different orientations depending on the spectroscopic sites O 1 to O 4. Here the orientation of the transition dipole moment is defined by the angle Ω between the projection of the transition dipole moment to the a ′ / b ′ -plane of the crystal and the b′-axis. The relative spontaneous emission rate is calculated for these four spectroscopic sites in dependence on unknown component of the transition dipole moment, which is normal to the a ′ / b ′ -plane. Possible differences in the spontaneous emission rate of the electric dipole transition of pentacene in p-terphenyl crystal are up to 24%.

Anisotropic and inhomogeneous superfluidity in OCS-doped helium clusters

Path integral Monte Carlo calculations have been performed to analyze the quantum solvation structure around a single impurity molecule in doped helium-4 clusters. Using a new local superfluid estimator that is based on local decomposition of the moments of inertia of the quantum fluid, we obtain the anisotropic and inhomogeneous superfluid fraction of the surrounding helium. We find that unlike previous local estimators, this provides a consistent analysis of the response of the inhomogeneous quantum fluid to impurity rotation. Application to helium clusters doped with a linear OCS molecule shows that at T∼0.3 K, the superfluid response parallel to the molecular axis is complete in both first and second solvation shells around the molecule, but that the perpendicular superfluid response is significantly depressed in the first shell. The suppression of perpendicular superfluid response in the first shell results in the reduction of the rotational constant of OCS inside the droplets that was observed in spectroscopic experiments.

Statistical Analysis of Spectra of Single Impurity Molecules and Dynamics of Disordered Solids: II. Manifestation of Interaction of Two-Level Systems with Impurity Molecules at Different Distances

Manifestations in the spectra of impurity chromophore molecules in a disordered solid matrix of the interaction of these molecules with two-level systems (TLSs) of the matrix are analyzed. The characteristic distances within the limits of which the interaction between TLSs and an impurity molecule leads, in the main, to either splitting or broadening of the spectrum, as well as the distance beginning with which this interaction is practically undetectable, are determined. Information about the effective value of the minimal distance between impurity molecules and TLSs, which is caused by nonzero sizes of the molecules and the TLSs, is presented. The analysis is performed by a statistical processing of experimental and model spectra of single tetra-tert-butylterrylene molecules in amorphous polyisobutylene at a temperature of 2 K, when the contribution of quasi-local low-frequency vibrational modes of the matrix to these spectra can be neglected.

Statistical Analysis of Spectra of Single Impurity Molecules and Dynamics of Disordered Solids: I. Distributions of Linewidths, Moments, and Cumulants

A review of recent results of studying the low-temperature dynamics of a disordered solid-state medium that were obtained by the method of single molecule spectroscopy (SMS) is presented. The studies were carried out with the use of the new approach of the statistical analysis of a large number of spectra of single molecules embedded into a medium under study as a microscopic spectral probe. The measurements were performed in the range 2–7 K. The use of SMS allowed us to avoid the information losses connected with the averaging over an ensemble of impurity molecules that is inherent in conventional spectral methods. For the example of the system studied (amorphous polyisobutylene doped with molecules of tetra-tert-butylterrylene), it was shown experimentally that, at low temperatures, the spectra of single impurity molecules in disordered media obey the Levy statistics. New microscopic information on the interaction of two-level systems and quasi-local low-frequency vibrational modes of the matrix with impurity molecules in a disordered medium was obtained.

Quasi‐localized low‐frequency vibrational modes of disordered solids: Study by single‐molecule spectroscopy

AbstractEditor's Choice of this issue of physica status solidi (b) is the article [1] by Andrei V. Naumov et al. This paper is Part II (Part I see [2]) of a study on elementary excitations in glasses, presented at the 11th International Conference on Phonon Scattering in Condensed Matter, St. Petersburg, 25–30 July 2004. For his outstanding talk, Naumov received the new physica status solidi Young Researcher Award which was bestowed for the first time at this conference.The cover picture is a sketch of a glass with a single impurity molecule and one hypothetical quasi‐localized vibrational mode. The broadening and shift of the chromophore spectral line are caused by the interaction with this mode.Andrei V. Naumov is senior scientific researcher and deputy head of the Molecular Spectroscopy Department of the Institute of Spectroscopy, Troitsk. His main research interests are experimental and theoretical studies of low‐temperature dynamics of amorphous solids (glasses, polymers etc.) via high resolution laser selective spectroscopy techniques.The second Editor's Choice is an article by E. A. Eliseev and M. D. Glinchuk [3]. Eugene A. Eliseev is scientific researcher at the Frantsevich Institute for Problems of Materials Science of the Ukrainian National Academy of Sciences, Kiev. His research areas are the theory of size and correlation effects in ferroelectric materials as well as modelling of disordered ferroelectrics properties. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A polarization method for measuring absorption spectra of single molecules

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.

Differences of the radiative line width in single-impurity molecule spectroscopy

A theory of the dependence of the spontaneous emission rate and, consequently, the dependence of the radiative line width of a single-impurity molecule on the orientation of the position vector in a birefringent host crystal is developed for the case of the electric quadrupole radiation. Depending on the orientation of the electron position vector, the differences in the values of the spontaneous emission rate of the electric quadrupole radiation may be considerable and they depend on the values of the principal refractive indices of the host crystal—the larger refractive indices lead to larger differences in the values.

Single-molecule spectra split by the interaction with two-level systems (TLSs)

At low temperatures, the fluorescence excitation spectra of single impurity molecules in amorphous or polycrystalline matrices are often split into two or more components by the coupling of the molecule to nearby tunneling systems (TLSs) of the solid. In the case of an asymmetric TLS, the two components of the molecular line will have different intensities. An expression for the ratio of the intensities is derived. Recent experimental data of terrylene in the Shpol'skiı̆ matrix n-hexadecane and TBT (a substituted terrylene) in the amorphous polymer polyisobutylene are re-evaluated with this model.