Impurity Molecules Research Articles

Use of polarons and bipolarons in logical switches based on conjugated polymers

Considering impurity molecules functioning as switches, the charge-transport process on a single conducting polymer chain is analyzed. The dynamics of polarons and bipolarons is studied with the time-dependent unrestricted Hartree-Fock approximation using a combination of the Su-Schrieffer-Heeger and extended Hubbard models, extended to include the potential due to the dopants. A Brazovskii-Kirova-type symmetry-breaking interaction is inserted in the Hamiltonian model. The polarons and bipolarons are put into motion by a spatially uniform external electric field introduced through a time-dependent vector potential. The process of charge transport through the sites with radicals that work like a switch is analyzed by the numerical resolution of the equations of motion. The use of polarons and bipolarons as information carriers is analyzed. The results show that the impurity molecule can work as a gate to the passage of charged polarons or bipolarons (i.e., it can control of charge transport). The polarons and/or bipolarons can be trapped by the impurity molecule or can pass through it, resulting in two configurations: closed and open logical switches. Such results show that polarons and bipolarons provide good candidates to logical switching in molecular circuits of conducting polymers. The energies involved are also discussed.

X-Ray Studies of Structural Changes of Impurity-Helium Solids

We have performed studies of the formation and evolution of samples of Im-Helium (Im=Ne, D2, N2) solids obtained by allowing a mixed beam of helium and impurity molecules to fall onto the surface of superfluid helium. Clusters consisting of single impurity molecules and/or small aggregates of the molecules surrounded by layers of solid helium agglomerate to form porous solids. These solids have been studied by x-ray diffraction techniques. As the solids were heated above their formation temperature of T=1.5 K, the small aggregates or clusters of impurities tended to form larger clusters, which x-ray diffraction revealed to be nanocrystallites ranging in size from 3 to 6 nm.

PHYSICOCHEMICAL PROPERTIES OF WATER CLUSTERS IN THE PRESENCE OF HCl AND HF MOLECULES. MOLECULAR DYNAMIC SIMULATION

The molecular dynamic method is used to study the physicochemical properties of water clusters containing HCl and HF molecules. These impurity molecules have the ability to undergo hydration in water vapor. In clusters with the same number of water molecules, large dipole moments are induced for HCl, and smaller ones, for HF. The diffusion coefficient of the impurity in the clusters is slightly lower than the analogous characteristic for water molecules and exhibits nonmonotonous behavior with increasing size of the aggregate.

Impurity-Helium Solids – Quantum Gels?

Impurity-Helium solids are highly porous van der Waals solids made up of impurity atoms, molecules, or clusters of atoms and molecules surrounded by thin layers of solid helium. The results of structural investigations via ultrasound and x-ray diffraction are reported. Preliminary nuclear magnetic resonance measurements on D2 impurities in these solids are discussed. Recent electron paramagnetic resonance experiments have explored exchange tunneling reactions in the Impurity-Helium solids. In particular, when D atoms are introduced into an Impurity-Helium solid containing H2, the concentration of H atoms is observed to increase, possibly as a result of the reactions D + H2 → H + HD and D + HD → H + D2.

Application of Molecular Dynamic Simulation of Water Clusters with CO and CO2 Molecules to Binary Nucleation Problems

The thermodynamic properties of pure water clusters and aqueous aggregates with either CO or CO2 molecule were calculated by the molecular dynamics method. The resulting size dependence of the surface tension of the clusters was used to determine the size of the critical seeds. The rate of homogeneous and binary nucleation in atmospheric air was estimated. The role of polar and nonpolar impurity molecules at the initial stage of steam condensation is discussed.

Stabilization of a metastable polymorph of sulfamerazine by structurally related additives

The influence of structurally related additives, namely N4-acetylsulfamerazine (NSMZ), sulfadiazine (SD) or sulfamethazine (SM), on the rate of the solvent-mediated polymorphic transformation (I→II) of sulfamerazine in acetonitrile (ACN) at 24°C was studied. The transformation rate is controlled by the crystallization rate of the more stable Polymorph II. All three impurities exhibit inhibitory effects on the crystallization of Polymorph II and hence stabilize the metastable Polymorph I in ACN suspension. The rank order of the inhibitory effect (NSMZ⪢SD>SM) is the same as the rank order of the binding energy of the impurity molecule to the surface of the host crystal. The relationship between the concentration of the impurity and the inhibitory effect was fitted to various models and was found to be best described by a model based on the Langmuir adsorption isotherm.

Spectral features of impurity luminescence in uniaxial polymer films and nematic glasses

A study is conducted of the static shift and the splitting of the photoluminescence and electroluminescence polarized bands of uniaxial impurity molecules, embedded into uniaxial polymer films with axial or in-plane orientation of the impurity subsystem or into the vitreous phase of a nematic liquid crystal. Dependences of the band shift and the splitting on the type of alignment of impurity molecules, the polarization of transition moments in absorption and emission, and the host-impurity anisotropic interactions of different ranks are established. For both types of host matrices, characteristic features of orientational statistics of impurity molecules at a high degree of orientation ordering are studied. These features are revealed in a qualitative difference between the dependences of positions and splittings of polarized impurity fluorescence bands on the order parameter of the impurity molecules axially ordered in stretched polymer films and relevant dependences in nematic glasses. Specific features of the polarized impurity-related photoluminescence and electroluminescence in the matrices under study, observed experimentally, are interpreted.

Spontaneous emission rates of single impurity molecules in birefringent crystals

We have developed a theory of the dependence of the spontaneous emission rate and, consequently, the radiative line width of a single impurity molecule on the orientation of the transition dipole moment vector in birefringent biaxial host crystal. The calculations for seven biaxial host crystals (naphthalene, anthracene, terphenyl, phenanthrene, fluorine, chrysene, and diphenyl) show that the spontaneous emission rate depends strongly on the orientation of the transition dipole moment vector with respect to the principal axes of the host crystal and has only a weak dependence on the chemistry of the host.

Light-induced temporal broadening of optical lines of individual molecules in polymers and glasses

The spectral diffusion theory developed for explaining the logarithmic temporal broadening of the optical line of an individual impurity center and based on spontaneous tunnel transitions in a polymer or glass is supplemented by taking into account the tunnel transitions occurring in the center itself upon the absorption of a photon. This light-induced additional tunneling leads to a sharp temporal broadening of the optical line from an individual impurity molecule occurring against the background of a slower logarithmic line broadening. It is shown that the inclusion of light-induced tunneling can explain the difference in the temporal line broadening of three individual Terylene molecules introduced in polyethylene, which was measured in experiments by the group headed by M. Orrit [A. M. Boiron et al., Chem. Phys. 247, 119 (1999)].

Investigation of the electron energy distribution function in hollow-cathode glow discharges in nitrogen and oxygen

The mechanism for the formation of the inverse electron distribution function is proposed and real- ized experimentally in a nitrogen plasma of a hollow-cathode glow discharge. It is shown theoretically and experimentally that, for a broad range of the parameters of an N 2 discharge, it is possible to form a significant dip in the profile of the electron distribution function in the energy range e = 2-4 eV and, accordingly, to pro- duce the inverse distribution with df ( e )/ d e > 0. The formation of a dip is associated with both the vibrational excitation of N 2 molecules and the characteristic features of a hollow-cathode glow discharge. In such a dis- charge, the applied voltage drops preferentially across a narrow cathode sheath. In the main discharge region, the electric field E is weak ( E < 0.1 V/cm at a pressure of about p ~ 0.1 torr) and does not heat the discharge plasma. The gas is ionized and the ionization-produced electrons are heated by a beam of fast electrons (with an energy of about 400 eV) emitted from the cathode. A high-energy electron beam plays an important role in the formation of a dip in the profile of the electron distribution function in the energy range in which the cross section for the vibrational excitation of nitrogen molecules is maximum. A plasma with an inverted electron distribution function can be used to create a population inversion in which more impurity molecules and atoms will exist in electronically excited states. © 2001 MAIK "Nauka/Interperiodica".

Molecular Dynamics Simulation of Model Lipid Membranes: Structural Effects of Impurities

The gel to fluid phase transition or ordered to disordered phase transition observed in biological membranes are simulated by using constant energy Molecular Dynamics. The surface part of the membrane is modelled as a two-dimensional matrix formed by the head groups of the phospholipid molecules. Head molecules which are modelled as three spheres fused with three force centers, interact with each other via van der Waals and Coulomb type interactions. The -so called- impurity or foreign molecule embedded in the surface represents the protein type molecule which is present in biological membranes and control its activity. It is modelled as a pentagon having one force centers in each corner. It also interacts with the surface molecules again via van der Waals and Coulomb type interactions. The surface density is kept constant in the simulations of the systems with or without impurity. Structural and orientational changes due to impurity were observed and proved by monitoring two-dimensional order parameter. It has been shown that melting of the surface or breakage of the ordering of the surface molecules becomes easier and ordered to disordered phase transition temperature was lowered by 100 K if the impurity is present.

Acetylated lysozyme as impurity in lysozyme crystals: constant distribution coefficient

Hen egg white lysozyme (HEWL) was acetylated to modify molecular charge keeping the molecular size and weight nearly constant. Two derivatives, A and B, more and less acetylated, respectively, were obtained, separated, purified and added to the solution from which crystals of tetragonal HEWL crystals were grown. Amounts of the A and B impurities added were 0.76, 0.38 and 0.1 mg/ml and 0.43, 0.22, 0.1 mg/ml, respectively. The HEWL concentration were 20, 30 and 40 mg/ml. The crystals grown in 18 experiments for each impurity concentration and supersaturation were dissolved and quantities of A or B additives in these crystals were analyzed by cation exchange high performance liquid chromatography. All the data for each set of 18 samples with the different impurity and regular HEWL concentrations is well described by one distribution coefficient K=2.15±0.13 for A and K=3.42±0.25 for B. According to definition of K by Eq. (1) in the text, the condition K=const is equivalent to a decrease of impurity amount in the crystal as the supersaturation increases. The observed independence of the distribution coefficient on both the impurity concentration and supersaturation is explained by the dilution model described in this paper. It shows that the impurity adsorption and incorporation rates are proportional to the impurity concentration and that the growth rate is proportional to the concentration of crystallizing protein in solution. The frequency at which an impurity molecules irreversibly join the crystal was estimated to be 3 s −1, much higher than such frequency for regular crystal molecules 5×10 −2 s −1 at 30 mg/ml lysozyme concentration. Reasons for this inequality are discussed.

A gas cell for thermalizing, storing and transporting radioactive ions and atoms. Part I: Off-line studies with a laser ion source

The application of a gas cell filled by noble gas (He or Ar) for thermalizing, storing and transporting trace radioactive ions and atoms has been studied in off-line conditions. Two-step laser resonance ionization has been used to produce ions of stable and long-lived radioactive isotopes inside the cell. The interactions of ions with impurity molecules, with noble gas atoms, with electrons and with electrical fields have been investigated in order to specify the requirements for the gas cell as a source of radioactive rare isotopes for the next generation radioactive ion beam facilities. Extraction and transport of ions by the sextupole ion guide (SPIG) without applying DC electrical field allows to get information about the gas purity and about chemical reactions which take place inside the gas cell.

Photoselection of impurity molecules in amorphous media over the zero-phonon linewidth

The shape of a multiwell hole, which is produced upon burning a number of closely spaced equidistant spectral holes in the low-temperature absorption spectrum of an impurity amorphous solid, is studied. When the bottom of such a hole is probed by the wavelength-modulated light, the transmission and luminescence of the sample become modulated. This modulation is caused by a subsystem of impurity molecules, which differs from a total ensemble by a smaller scatter in the zero-phonon linewidth and a smaller (by a factor of 2–3) mean value of this width. The study of the photophysical parameters of this subsystem can yield useful information for simulating the nanostructure of an amorphous body.

Photoselection of impurity centers in amorphous media over the hole-burning rate and the value of the induced Stark moment

It is shown that probing of the bottom of a multi-well spectral hole by frequency-modulated light allows one to study, in amorphous systems, the subensembles of impurity molecules, whose distribution over the hole-burning quantum yield and the matrix-induced Stark moment is different (narrower) from their initial distribution. This permits the study of correlation relations between these two distributions and between each of these distributions and other distributions.

On the theory of ultranarrow pure-electronic zero-phonon lines

A pure-electronic zero-phonon line, which represents an optical analog of the Mossbauer γ-resonance line, is a cornerstone of three remarkable areas of spectroscopy and optics of impurity solids, namely, ultrahigh-resolution spectroscopy, persistent hole-burning spectroscopy, and spectroscopy of single impurity molecules. In recent experiments made by the photon echo technique, a zero-phonon line width as small as 78 Hz was achieved (with the Q-factor, i.e., the ratio of the transition frequency to the line width being 2×1014:78≈ 1012), which is several tenfold lower than the frequency shift of the vibronic spectrum caused by the momentum of the absorbed or emitted photon of optical range. We estimate the position, intensity, and width of zero-phonon lines under conditions when these characteristics, as well as properties of the phonon wing, are determined not only by Stokes losses but also by the photon recoil effect. It is shown that the latter effect, within the approximation used, does not lead to strong and easily detectable phenomena. They, however, take place, and their study is becoming more topical with improving accuracy and sophistication of the experimental technique (in the photon echo configurations, etc.).

Logical switching with the use of bipolarons in conjugated polymers

Considering impurity molecules functioning as switches, the charge transport on a single conducting polymer chain is investigated. The dynamics of structural bipolaron-type defects in conjugated polymers using a modified tight-binding hamiltonian is extended to include the effects of an external electric field and the parameters of the switching molecules are studied. The electric field is used to accelerate charged bipolarons in polymers and it is introduced in terms of a time-dependent vector potential. We use the Pariser–Parr–Pople model combined with the Su–Schrieffer–Heeger model of electron–lattice coupling and a Brazovskii–Kirova-type simmetry-breaking interaction. The process of charge transfer through the sites, and that it works like a switch, is analyzed by the numerical resolution of the equations of motion within the unrestricted Hartree–Fock approximation. The switching character of the parameters of the molecules on the charge transport through the chain is determined. The energies involved are also discussed.

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

The PVA-PAAN intrapolymer complexes as sensors with optical response

Sorbtion of some impurities from aqueous solutions by polymer complex poly(vinylalcohol) with poly(acrylamide) grafted copolymers (PVA-PAAN) was studied by means of spectrophotometry and steady-state fluorescence methods. The experiments showed that the PVA-PAAN films effectively sorbs impurities molecules of average benzene ring from water. Simultaneously, the processes of polymer films dissolving in water take place. The thermal annealing (with the help of special methods) PVA-PAAN film gives a possibility to obtain non-dissolving polymer films which possess high sorbtion ability. Such films are proposed to be used as the sensors with optical response.

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule