Triplet Excitions in Crystalline Trans-Stilbene I. Spectroscopy of the So → T1 Transition

Abstract

The first singlet → triplet absorption of trans-stilbene has been studied between 10 and 300 K. The triplet exciton energy (energy of the lowest 0–0 line) is 17380 cm^?1 at 10 K, and does not change significantly with temperature. Vibrational A_g modes of 206. 1250 and 1570 cm^?1 are active. The Franck-Condon factor of the origin region is small. The low temperature spectra of the 0–0 and 206 cm^?1 regions show a doublet structure with a splitting of 82 ± 1 cm^?1 which is attributed to the site splitting. Further structure shown by the lines is discussed. The lines are approximately lorentzian. From curve fitting, line-widths and exciton-phonon coupling constants, increasing linearly with T, are deduced. Exciton-phonon coupling appears to be different on the two sites. The product of absorption coefficient at 4880 Å, α, by the total triplettriplet interaction rate constant has been measured at 295 K: αγ=2.3 ± 0.3. 10^?15 cm² sec^?1, corresponding for γ_total to a value of a few 10^?12 cm³ sec^?1.     

Spectres de vibration du Cristal de Biphényle à Basse Température

Abstract

Low-temperature infrared and Raman spectra of Crystalline biphenyl have been investigated in the 3100–25 cm^?1 range, and those of biphenyl-D₁₀ between 200 and 25 cm^?1. The infrared dichroism of an Oriented crystal at 77°K has been measured in the 3100–400 cm^?1 region. The assignment for the internal modes V ₅(B_2u), v ₆ (B_2u), v ₁(B_1u), v ₁₀(B_1g), v ₂(B_2g) et v ₃ (B_3g) is given.

The band splitting is analized and hte components due ot the correlation effect in the fundamentals are separated from the components due to combinations. Isotopic shifts are used to assign the nine external vibrations as well as the torsional mode. The temperature effect on the frequencies occuring below 500 cm^?1 is discussed.     

New (TMTSF)2X Derivatives: A Change in the Selenium Network Dimensionality Derived From the Molecular and Crystal Structures of (TMTSF)2(Fso3) [T=298K, 123K] and (TMTSF)2(Bro4) T=298K

Abstract

We report the first crystallographic analysis, as a function of temperature, of a TMTSF derivative. Both (TMTSF)₂(FSO₃) and (TMTSF)₂(BrO₄) are isostructural (triclinic, with space group PI) with superconducting (TMTSF)₂(ClO₄). (TMTSF)₂(FSO₃) undergoes a metal-to-insulator transition at 86-90K as observed by microwave conductivity, D.C. conductivity, and magnetic susceptibility. The crystal structure contains 2-dimensional sheets of short Se-Se contacts in the molecular stacking direction and perpendicular to the stacking direction. The temperature dependent variations in these contact distances appear to be of special importance in determining the conduction properties of these materials, and are observed to change in a surprising manner when (TMTSF)₂(FSO₃) is cooled (298 → 123K). The homoatomic Se separations within each TMTSF molecule appear to increase slightly, but not significantly. At the same time the entire 2-dimensional sheet of intermolecular (intra- and interstack) Se-Se contacts between TMTSF molecules contract quite anisotropically, which results in an increase in “dimensionality” of the Se-Se network. Hence, an increase in electrical conduction, in the absence of insulating phenomena, over the temperature range 298 → 123K is not surprising. The intermolecular Se-Se contact distances in (TMTSF)₂(BrO₄) are significantly longer than in (TMTSF)₂(FSO₃) which suggests that the room temperature electrical conductivity of the (BrO₄)^? salt may be diminished compared to the (FSO₃)^? analogue.     

NMR Studies of (TMTSF)2PF6

Abstract

Pulsed NMR determinations of the relaxation times T₂, T₂* and T₁, as a function of temperature and field, for the methyl group protons in (TMTSF)₂PF₆ are reported. Below the metal-insulator transition (T_MI)T₂* shortens while T₂ increases, indicating a line which is inhomogeneously broadened due to the onset of a SDW. The SDW also contributes to the spin-lattice relaxation rate, T₁ ^?1, in the neighborhood of T_MI. A frequency dependent maximum in T₁ ^?1 is observed near 20K and is attributed to methyl group rotation. A frequency independent maximum at 58K suggests a structural phase transition involving rearrangement of the methyl and PF₆ groups.     

Superconductivity in the Organic Charge Transfer Salts: (TMTSF)2X and (TMTTF)2X

Abstract

We report pressure dependent studies of the a-axis resistivity as a function of temperature for several members of the isostructural families of organic charge transfer salts, (TMTSF)₂X and (TMTTF) ₂X. For a typical (TMTSF)₂X material the low temperature metal-insulator transition seen at 1 bar is suppressed above some critical pressure, P_c, where a superconducting transition is observed near 1 K. We find a correlation between P_c and the ambient pressure c lattice parameter which reflects the anion size. The (TMTTF) _cX salts exhibit very different ambient pressure behaviour but we find that with the application of sufficiently high pressures (～30 kbar) their behaviour resembles that seen in the (TMTSF)₂X family but at lower pressures. In particular we find evidence of a possible superconducting transition near 4 K in (TMTTF)₂Br at 25 kbar. At this pressure the conductivity near 4 K is extremely high with a value approaching 10⁶ (Ωcm)^?1 and the resistivity ratio is about 400.     

Low-Temperature Electron Spin Resonance in (TMTSF)2PF6 in the High Pressure Metallic Phase

Abstract

The electron spin resonance of (TMTSF)₂PF₆ has been observed at low fields (H_o < 110 Oe) in the high pressure, metallic phase (p > 6.5 kbar) in the temperature range 1-4^?K. The anisotropy in the g value is similar to that observed at ambient pressure above the metal-insulator transition. The linewidth is very narrow and the spin susceptibility strongly decreases as the superconducting transition is approached from above. We interpret this as evidence for singlet-paired superconductivity. Superconductivity is observed at 1.1 K and the critical field has angular dependence in the be plane. These observations lead us to conclude that (TMTSF)₂PF₆ is a singlet paired superconductor.     

Optical Properties,Conductivity and Structure of the DTT-TCNQ CT Complex

Abstract

The crystal structure, the optical spectra and electrical conductivity of dithieno(3,2-b; 2′,3′-d)thiophene-tetracyanoquinodimethane (DTT-TCNQ) charge-transfer complex have been measured. DTT-TCNQ crystallizes in the monoclinic space group P2₁/c with a = 7.206(2), b = 7.574(2), c = 32.324(9), β - 92.18(4)°, Z - 4, and the donor and acceptor molecules are arranged in alternated stacks.

The low conductivity (a = 10^?2 S cm^?1 for the single crystal, along the stacking axis a) is characteristic of a semiconductor with activation energy of .61-.76 eV, and is related to the alternate stack structure.

Despite the poor condctivity of the DTT-TCNQ CT complex we estimated, from structure and IR data, a low degree of ionicity (p = .2–.3) which characterizes the DTT molecule as an interestign donor.

The polarized IR spectra shows the effect of vibronic activation of some of the a_g modes of TCNQ.     

Optical phonon modes and transmissivity in BaWO4 single crystal

Barium tungstate (BaWO₄) single crystal has been grown using Czochralski technique. It belongs to the scheelite structure, forming the space group I 4₁/a at room temperature and the primitive cell contains two molecular units. The polarized Raman spectra were recorded by a micro‐Raman spectrophotometer system in the backscattering geometry. All the observed Raman modes were assigned. The Raman mode at 924 cm^–1, which belongs to the totally symmetrical A_g optical modes, has the strongest intensity and its linewidth is 4.6 cm^–1. The infrared active lattice vibrations have been studied, eight optical modes were observed and assigned. The ultraviolet absorption edge is at 256 nm and the optical transparency range is up to 2500 nm at room temperature. The energy gap E_g of this crystal was obtained from the optical transmission spectra. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The field effect in amorphous chalcogenides: An investigation of localized states and electronic transport

The temperature dependence of the field effect response permits an unambiguous determination of the identity of those states responsible for electrostatic screening in the amorphous chalcogenides. We observe (1) in As₂Te₃, field effect screening by localized states at the Fermi level at low temperatures (～ 10¹⁹ cm^?3 eV^?1) and by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) at high temperatures, and a transition from p-type to two-carrier (primarily n-type) conductivity as the temperature is raised above ～320 K; (2) in As₂SeTe₂, screening by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) with strongly type conductivity; (3) in As₂Se₂Te, screening by localized states at the Fermi level (～ 10¹⁹ cm^?3 eV^?1) with strongly p-type conductivity; and (4) in Sb₂Te₃, a very high density of localized states at the Fermi level (～ 2 × 10²⁰ cm^?3 eV^?1) with both electron and hole contributions to the conductivity. Correlation with thermoelectric power results suggests that the p-type conductivity in As₂Te₃ is due to near-equal contributions from two processes: hopping in localized states plus extended state conduction. Aging and annealing behavior is described with the aid of a “chaotic potential model” that appears to be able to account for large changes in mobile carrier density that leave the conductivity unaltered.     

Thermal Expansion in TMTSF-Dmtcnq & (TMTSF)2PF6

We have measured the anisotropic thermal expansion of TMTSF-DMTCNQ and (TMTSF)₂PF₆ in the temperature range 10-300 K using an X-ray diffraction technique with a relative accuracy of 3 parts in 10⁴. Between 10 K and 300 K we find expansion in the a*, b* and c* directions to be respectively 2.5%, 0.2% and 1.7% for TMTSF-DMTCNQ and 3.7%, 1.6% and 1.3% for (TMTSF)₂PF₆     

High Electric Field Transport In (TMTSF)2 Pf6 [Bis-Tetramethyltetraselenafulvalene Hexafluorophosphate] at Low Temperatures

We obtain electron and hole mobilities from the measured Hall mobility of (TMTSF)₂ PF₆. At 4K these are greater than 10⁶cm²/Vsec and agree with the mobility calculated for acoustic phonon scattering. The initial increase of conductivity in dc fields found at low temperatures in this material can be accounted for by single particle effects, specifically heating of the carriers by the field. To explain further increases we suggest that the gap is spatially nonuniform. This could be accounted for by dislocations.     

Synthesis and Characterization of Polyphenothiazene Iodine Complexes

Abstract

N-methyl 3,6 dibromophenothiazene has been polymerized by using a nickel complex-assisted Grignard coupling. Vapor phase iodine doping of the organometallic polymerized material led to a conductivity increase of 10^?11 ohm^?1 cm^?1 to 10^?5 ohm cm^?l at room temperature. The complex thus formed exhibited a semiconductor behavior with a thermal activation energy of 0.1 eV and a strong electronic IR absorbance from 4000–400 cm^?1. However, the complexed polymer could be dissolved in liquid I₂ and cast into films with air stable, room temperature conductivities as high as 1 ohm^?l cm^?l. The mechanisms that could give rise to these differences in behavior will be discussed.     

Smooth titania nanotubes: Self-organization and stabilization of anatase phase

We present transmission electron microscopy and Raman scattering measurements showing that niobium inhibits the processes of nucleation and growth of anatase crystallites in the initial amorphous titania nanotubes and thus shifts the temperature of the complete amorphous-to-anatase phase transition to higher values up to 550 °C. Niobium dopants stabilize the anatase phase in titania nanotubes up to 650 °C. The size of anatase crystallites can reach 30–50 nm. Excess niobium atoms which are pulled off from the volume of anatase crystallite form polymeric or monomeric Ti–O–NbO groups at the interface area. Slight shift and broadening of E_g (144 cm^?1), A_1g (515 cm^?1) and E_g (630 cm^?1) modes in Raman spectra can be explained by niobium insertion into the anatase structure.     

Identification of crystal symmetry in Kramers and non‐Kramers ions by optical absorption: Divalent copper and nickel ions in diamagnetic lattices

The optical absorption spectra of Ni(II) doped hexaimidazole zinc(II) dichloride tetrahydrate (HZDT) and Cu(II) doped magnesium potassium phosphate hexahydrate (MPPH) have been studied at room temperature. Ni(II)/HZDT spectrum consists of three strong and one weak band. The calculated value of Dq is 1051 cm^‐1 and the interelectron repulsion parameters B and C are 854 and 3626 cm^‐1 respectively. The correlation of optical work with EPR has yielded the spin‐orbit interaction parameters λ and ξ as –225 and –450 cm^‐1 respectively. The symmetry around the Ni(II) ion is distorted octahedral, as suggested by EPR results. The estimated percentage covalency of the nickel‐nitrogen bond is around 30%. The optical absorption studies of Cu(II)/MPPH show three bands, which are identified as ²B_1g → ²A_1g, ²B_1g → ²B_2g and ²B_1g → ²E_g transitions. The octahedral field parameter Dq and the tetragonal field parameters have been evaluated from the observed adsorption bands. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of calcination on the crystallinity of sputtered TiO2 thin films as studied by Raman scattering

Titanium dioxide films have been deposited using DC magnetron sputtering technique onto silicon substrates at an ambient temperature and at an oxygen partial pressure of 7 × 10 ^–5 mbar and sputtering pressure (Ar + O₂) of 1 × 10 ^–3 mbar. The deposited films were calcinated at 673 and 773 K. The composition of the films as analyzed using Auger Electron Spectroscopy (AES) revealed the stoichiometry with an O and Ti ratio of 2.08. The influence of post‐deposition calcination on the Raman scattering of the films was studied. The existence of Raman active modes A_1g, B_1g and E_g corresponding to the Raman shifts are reported in this paper. The improvement of crystallinity of the TiO₂ films as shown by the Raman scattering studies has also been reported. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low frequency Raman scattering in chalcogenide glasses

Low frequency Raman spectra of chalcogenide glasses are analyzed in terms of matrix element effects and modes of a layered structure. The spectra of GeSe₂ at low temperature shows no peaks which can be assigned to layer modes. The reduced spectra indicates that the density of states exhibits nearly ω² dependence for ω < 60 cm^?1, and the coupling constant approaches ω² dependence at frequencies less than 20 cm^?1.     

The structure of 2R-(3′S-benzyloxy-5′S-hydroxy-2′R-hydroxymethyl-1′-oxacyclohexan-6′S-yl)-propionic acid-1,5′-lactone,C16H20O5

The title compound is C₁₆H₂₀O₅, MW=292.3, orthorhombic,P2₁2₁2₁,a=9.741(2),b=29.391(7),c=5.354(1) Å from diffractometer measurements,V=1532.8 ų,Z=4,D _c =1.267 g cm^?3,D _o =1.271 g cm^?3 (ether/1,1,2,2-tetrabromoethane), λ(Mo Kα)=0.71069 Å,F(000)=624,μ=1.02 cm^?1, crystal dimensions 0.23×0.23×0.40 mm,R=0.049 for 1164 observed reflections. The molecule contains a possibly significant asymmetric ether linkage between the oxacyclohexane ring and the highly anisotropic benzyl ring. The packing consists of zigzagged chains parallel to thea-axis formed by hydrogen bonds. The chains are separated by van der Waals contacts.     

Crystal Structure of a Non-Conductive Non-Stoichiometric Tetrathiafulvalenium Salt: (TTF)3(BF4)2

Abstract

(TTF)₃(BF₄)₂ crystallizes in the triclinic system, space group P1, a = 8.017(3), b = 8.601(1), c = 11.635(2) Å, α = 108.79(1), β = 100.96(2), γ = 99.09(2)°, Z = 1. The TTF entities are stacked in parallel columns arranged into parallel layers alternating with layers of BF₄ ^? anions. A TTF stack is constituted of (TTF⁺)₂ diads interspersed with TTF° monads; the TTF⁺-TTF⁺ overlap is of the ring-over-ring type while the TTF⁺-TTF⁺ overlap is of the bond-over-ring type. These features explain the low conductivity ([sgrave]_powdcr = 2 × 10^?5 Ω^?l cm^?1) of this apparently non-stoichiometric TTF salt.     

The Absorption,Fluorescence and Phosphorescence and Phosphorescence of Single Crystals of 1,2,4,5-Tetrachlorobenzene and 1,4-Dichlorobenzene at Low Temperatures

Abstract

The emission spectra of ultra pure single crystals of 1, 2, 4, 5. tetrachlorobenzene (TCB) and 1, 4-dichlorobenzene (DCB) at temperatures from 4.2°K upwards are reported. In addition, by use of the phosphorescence excitation technique the singlet-triplet absorption spectrum at 4.2°K has been obtained.

The phosphorescence emission of TCB at 4.2°K occurs predominantly from a defect origin “X” situated 48 cm^?1 below the triplet exciton band. The triplet exciton energy level is at 26676 cm^?1 from both emission and absorption studies. This is the triplet emission origin when the crystal is above 12°K. The temperature dependence of the emission intensity from the defect has an activation energy of 40±8 cm^?1. This value is consistent with our suggestion of thermal depopulation of the traps.

Single crystals of DCB show strong excimer emission and weak triplet exciton emission. There is no evidence for trapping levels. The triplet exciton emission. There is no evidence for trapping levels. The triplet exciton origin is 27890 cm^?1 from both emission and absorption studies.

Weak fluorescence (φ < 10⁰²) is detected from both TCB and DCB.

Vibrational analyses are reported for the absorption, fluorescence and phosphorescence spectra.     

The effect of temperature on the raman spectra of strontium formate dihydrate single crystals

The temperature dependence of the polarized Raman Spectra (3000 cm⁻¹ – 10 cm⁻¹, between 293 K and 10 K) of Sr(HCOO)₂ · 2H₂O(2D₂O) and partially deuterated samples is reported. From the analysis of the internal and external modes associated with water molecules and formate ions a phase transition is deduced. This is interpreted on the basis of the positional disorder of the hydrogen atoms, situated in non-equivalent lattice positions at low temperature.