Optical homogeneous linewidths of resorufin in ethanol glass: An apparent contradiction between hole-burning and photon-echo results?

Spectral holes have been burnt in the S₁ ← S₀ 0-0 transition of resorufin in two phases of ethanol glass between 0.3 and 4.2 K. The holewidths differ by a factor of 8 and follow a T^1.3 dependence. The homogeneous linewidths (Γ_hom) in one of the phases are about halfthose derived from a photon-echo experiment by Fayer and co-workers. Holes burnt between ≈600 and 573 nm yield the same holewidth. It is concluded that hole-burning measures Γ_hom in this amorphous system.     

Thermal broadening of optical homogeneous linewidths in organic glasses and polymers studied via photochemical hole-burning

Photochemical hole-burning was used to study optical dephasing in the S₁ ← S_o OO transitions of dimethyl-s-tetrazine, chlorin and free-base porphin in several organic glasses and polymers. The homogeneous width (Γ_hom) seems to follow a T^1.3±0.1 law between 0.4 and 20 K for all guest—host pairs, and extrapolates the lifetime-limited values of each guest for T→0. For a given temperature, Γ_hom, appears to be related to the number of OH groups in the glass, and to the size of side-groups on the polymer chain.     

Structural changes in low temperature water

Previous experiments on the neutron structure factor S(q) for heavy water near 11.2°C have been extended so that the next term in the Taylor expansion may be examined. An improved result for the isochoric temperature derivative of S(q) at 11.2°C is obtained. These data and those for the (ΔT)² coefficient in the Taylor expansion may be explained on a simple basis: namely that OH and HH distances between neighbouring molecules vary as (T  t₀)⁻ⁿ where (T  T₀) ≈ 1200n for T ≈ 300 K.     

Hole-burning spectroscopy on organic molecules in amorphous silica

Amorphous silica samples of high optical quality, doped with organic molecules, were prepared at room temperature by the sol-gel method. Spectral hole-burning experiments were performed in the S₁←S₀ absorption bands of chlorin (2,3-dihydroporphyrin) and of oxazine-4 perchlorate. Homogeneous linewidths of 380 and 750 MHz were obtained at 1.7 K. The temperature dependence between 1.7 and 5.7 K was T^145±0.07 for chlorin and T^127±0.05for oxazine-4.     

Zeeman effect of the S1 ← S0 transition of the two tautomeric forms of chlorin: a study by photochemical hole burning in an n-hexane host

Zeeman experiments are reported on the S₁ ← S₀ O-O transitions of chlorin (7,8-dihydroporphin) and its photoisomer in an n-hexane crystal by photochemical hole burning at 4.2 K. The holes shift with a quadratic field dependence of -39.0 MHz/T² (chlorin) and ?245.3 MHz/T² (photoisomer). For chlorin A = ¦<S₁¦L_z¦S₂ > ¦=4.5. Single-site absorption and fluo spectra are reported. PPP calculations were performed.     

Thermally induced incomplete high-spin (5T2) ⇌ low-spin (1A1) transition in solid bis[2-(2-pyridylamino)-4-(2-pyridyl) thiazolato]iron (II)

The ⁵⁷Fe Mössbauer effect in two samples (A and B) of [Fe(papt)₂] and in its solvates with CHCl₃ and C₆H₆ has been studied between 4.2 and 343 K and clearly indicates a temperature induced high-spin (⁵T₂) ? low-spin (¹A₁) transition in these compounds [paptH = 2-(2-pyridylamino)-4-(2-pyridyl) thiazole]. At 343 K, sample B shows a doublet with ΔE_Q = 2.03 mm s^?1 and δIS = +0.87 mm s^?1, characteristic of a ⁵T₂ ground state. At 257 K, a second doublet, typical for a ¹A₁ ground state, is observed and its intensity increases as the transition progresses but levels off below ～ 100 K. At 4.2 K, 83% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.56 mm s^?1 and δ^IS(¹A₁ = +0.32 mm s^?1. In an applied magnetic field, V_zz(¹A₁) < 0 and η ≈ 0.7 have been determined, whereas for the ^sT₂ ground state, V_zz(^sT₂) > 0, η ≈ 0.75, and an internal hyperfine field H_n ≈ ?13 kG have been observed. Similar results have been obtained with the other samples.Debye-Waller factors f5_T2 and f1_A1 were determined from the saturation corrected areas in the Mössbauer spectra, assuming Curie-Weiss dependence of the magnetic susceptibility for the ⁵T₂ and constant υ_cff for the ¹A₁ ground state. The temperature dependence of ?In f1_A1 closely follows the Debye model with Θ1_A1 = 165 K, whereas the same applies to ?ln f5_T2 only above ～ 210 K and Θ5_T2 = 134 K. The nature of the observed transition is discussed and the data presented are shown to be incompatible with a model based on a Boltzmann distribution between the two states.     

Thermodynamic properties of second generation poly(phenylene-pyridyl) dendrimer

The temperature dependence of heat capacity C _p ^o = f (T) of second generation hard poly(phenylene-pyridyl) dendrimer (G2-24Py) was measured by a adiabatic vacuum calorimeter over the temperature range 6–320 K for the first time. The experimental results were used to calculate the standard thermodynamic functions: heat capacity C _p ^o (T), enthalpy H ^o(T)–H ^o(0), entropy S ^o(T)–S ^o(0) and Gibbs function G ^o(T)–H ^o(0) over the range from T → 0 K to 320 K. The standard entropy of formation at T = 298.15 K of G2-24Py was calculated. The low-temperature heat capacity was analyzed based on Debye’s heat capacity theory of solids. Fractal treatment of the heat capacity was performed and the values of the temperature characteristics and fractal dimension D were determined. Some conclusions regarding structure topology are given.     

The thermodynamic properties of bis(η6-o-xylene)chromium(I) fulleride over the temperature range from T → 0 to 340 K

The temperature dependence of the heat capacity of bis(η⁶-o-xylene)chromium(I) fulleride, [(η⁶-(o-xylene))₂Cr]^+?[C₆₀]^??, over the temperature range 6–340 K was measured on an adiabatic vacuum calorimeter. The low-temperature (20 K ≤ T ≤ 50 K) heat capacity was subjected to multifractal processing; conclusions about the heterodynamic character of the structure were drawn. The experimental data were used to calculate the standard thermodynamic functions C _p ^° (T), H ^°(T)-H ^°(0), S ^°(T), and G ^°(T)-H ^°(0) over the temperature range from T → 0 to 340 K and estimate the standard entropy of fulleride formation from simple substances at 298.15 K. The standard thermodynamic characteristics of [(η⁶-(o-xylene))₂Cr]^+?[C₆₀]^?? were compared with those of the initial fullerene C₆₀.     

Anomalous charge transport in CeB6

The comprehensive study of conductivity σ, Hall coefficient R_H and Seebeck coefficient S has been carried out on high-quality single crystals of CeB₆ in a wide range of temperatures 1.8-300 K. An anomalous behavior of all transport characteristics (σ, R_H, S) was found for the first time in the vicinity of T^*≈80 K. The strong decrease of conductivity σ as well as the unusual asymptotic behavior of Seebeck coefficient S(T)∼−ln T observed below T^* allowed us to conclude in favor of crossover between different regimes of charge transport in CeB₆. The pronounced change of Hall mobility μ_H, which diminishes from the maximum value of 20 cm²/(V s) at T^* to the values of ∼6 cm²/(V s) at T∼10 K, seems to be attributed to the strong enhancement of charge carriers scattering due to fast spin fluctuations on Ce-sites. The low-temperature anomalies of the charge transport characteristics are compared with the predictions of the Kondo-lattice model.     

The 1B1u ← 1A1g 0—0 transition in crystal benzene

The transition linewidth ΔE in crystal C₆H₆, C₆D₆ and sym-C₆H₃D₃ has been measured as a function of temperature T from 4.2 to 135°K, and it extrapolates to a common value of ΔE_o = 50 cm^? at O°K. In C₆H₆ ΔE = (50 + 7T^{$\text{1}\text{2}$}) cm^?1, indicative of strong exciton—phonon coupling, and there is a line shift of +40 cm^?1 per substituent deuteron. Fluorescence excitation spectral data are used to separate the ¹B_1u(= S₂) decay rate k_H = 9.4 × 10¹² sec^?1, derived from ΔE₀, into S₂S₁ internal conversion (rate ≈ 6.6 × 10¹² sec^?1) and S₂S_x (channel 3) internal conversion (rate ≈ 2.8 × 10¹² sec^?1. A similar value of k_H = 9.9 × 10¹² sec^?1 is obtained from the S₂S_o fluorescence quantum yield of liquid benzene.     

A molecular beam study of the trapping desorption of I2 from a LiF(001) surface

Rotational, vibrational and translational Boltzmann distributions of I₂ desorbing from LiF(001) were measured using laser-induced fluorescence. The I₂ rotational temperature is lower than the surface temperature (T_S) at T_S > 300 K while the vibrational and translational temperatures are ≈T_S. An upper limit on the I₂-LiF(001) potential well depth was found to be 0.45 ± 0.03 eV.     

5T2-1A1 Spin transition and residual paramagnetism in bis(2,4-bis(2-pyridyl)thiazole)iron(II) complexes: mössbauer effect and magnetism

The ⁵⁷Fe Mössbauer effect in [Fe(pythiaz)₂] (BF₄)₂ (I) and [Fe(pythiaz)₂] (C&O₄)₂ (II) has been studied between 298 and 4.2°K (pythiaz = 2,4-bis(2-pyridyl)thiazole). At 298°K compound I shows a doublet with ΔE_Q(⁵T₂) = 1.29 mm sec^?1 and δ^1S(⁵T₂) = +0.93 mm sec^?1 characteristic of a ⁵T₂ ground state. At 236°K, a second doublet, typical for a ¹A₁ ground state appears. The transition ⁵T₂ å ¹A₁ progresses as the temperature is lowered but levels off below ≈ 120°K. At 4.2°K, 59% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.59 mm sec^?1 and δ^1S(¹A₁) = +0.26 mm sec^?1. In an applied magnetic field, V_zz(¹A₁) < 0 has been determined Similar results have been obtained with compound II.Debye-Waller factors f5_T2 and f1_A1. were determined from the Mössbauer spectra under the assumption of Curie-Weiss dependence of the magnetism for the ⁵T₂ and constant μ_eff for the ¹A₁ ground state. The resulting temperature dependence of f1_A1 is highly unusual thus suggesting complicated magnetic behaviour of both ground states in the transition region. Two mechanisms for the nature of the transition are discussed, a “spin-flip” mechanism being the physically more reasonable one. The assumption of a simple Boltzmann distribution (“spin equilibrium”) may be ruled out for the solid but could be encountered in solutions.     

Thermodynamic Properties in Gold Phenylacetylide in the Range 0-330 K

The temperature dependence of the heat capacity of gold phenylacetylide in the range 13-330 K was measured in an adiabatic vacuum calorimeter with an accuracy of 0.3%. These data were used for calculating the thermodynamic functions C _p0(T), H ⁰(T) - H ⁰(0), S ⁰(T) - S ⁰(0), and G ⁰(T) - H ⁰(0) for the range 0-330 K. The standard entropy of formation _f S ⁰ of gold phenylacetylide from the elements at T 298.15 K and p 101.325 kPa was calculated. The thermodynamic properties of gold phenylacetylide and related silver and copper compounds were compared.     

Optically detected magnetic resonance study of the lowest excited triplet state of aromatic thioketones: Xanthione

The aromatic thioketone xanthione has been investigated by means of the optically detected magnetic resonance (ODMR) technique in a n-hexane matrix at ≈ 1.1 K. It was established that the short-lived red emission, which is characteristic for many thione molecules, is phosphorescence. At high temperatures (77 K) this phosphorescence originates mainly (>80%) from the T₁^z (n, π*) sublevel (k_z^(r) >k_x^(r), k_y^(r). At low temperature (1.1 K) the intersystem crossing following S₂ (π, π*) ← S₀ excitation is a highly spin-sublevel selective process which populates predominantly the T₁^x and T₁^y, levels. Hence, the slow spin—lattice relaxation phosphorescence at low temperature originates from these sublevels. A value of 0.0611 cm^?1 was obtained for the zero-field parameter |E|/hc. A lower limit of 0.66 cm^?1 has been found for the zero-field parameter |D|/hc. This value is considerably larger than those observed for ketones, and it is shown that spin—orbit coupling contributes strongly to the zero-field splitting.     

The heat capacity and thermodynamic functions of Ca0.5Zr2(PO4)3 crystalline phosphate from T → 0 to 650 K

The temperature dependence of the heat capacity C _p ^o = f(T) of crystalline calcium-zirconium phosphate was studied over the temperature range 7–650 K by precision adiabatic vacuum and dynamic scanning calorimetry. The experimental data were used to calculate the standard thermodynamic functions C _p ^o (T), H ^o(T) ? H ^o(0), S ^o(T), and G ^o(T) ? H ^o(0) at temperatures from T → 0 to 650 K and the standard entropy of formation of Ca_0.5Zr₂(PO₄)₃ at T = 298.15 K. The data on low-temperature (30 K ≤ T ≤ 50 K) heat capacity were used to calculate fractal dimension D. Conclusions about the character of the topology of the structure of the phosphate were drawn.     

Temperature dependence of vibrational energy transfer in the “low” pressure competitive two-channel 1,1-cyclopropane-d2 system

Intermolecular energy transfer has been studied in the two-channel competitive isomerization of 1,1-cyclopropane-d₂, both in the neat system and in the presence of helium bath gas at values of k/k^? centered around 0.02. The competing path ways differ in threshold energy by ≈ 0.6 kcal. The temperature range 773 K to 973 K was covered. Several methods of treating the data, whether by isotopic ratios of rate constants or by temperature dependence of fall off, are each independent of a knowledge of collision cross sections. Used in conjunction, they provide measurements of these quantities. Cyclopropane is an operationally strong collider (β_ω = 1) for itself at 773 K with an average down step size, <ΔE/s> >/ 10 kcal mole ^?1 (>/ 3500 cm^?1). At 973 K the substrate is no longer a strong collider; β_ω declines to ≈ 0.55 with <ΔE/s> ≈ 5.2 kcal mole^?1. For helium the corresponding quantities are β_ω ≈ 0.078 <ΔE/s> ≈ 1.1 kcal mole ^?1 declining to β_ω ≈ 0.010 and <ΔE/s> ≈ 0.53 kcal mole^?1. The several methods of measuring these quantities give excellent independent agreement. Comparison with the earlier theoretical formulation of Tardy and Rabinovitch gives good agreement, the temperature dependence of β_ω for the weak collider, helium, follows the relation β_ω ∝ T^?m, where m /s> 2.     

Low-angle electron diffraction from high temperature polystyrene crazes

Low-angle electron diffraction (LAED) was used to study the microstructure of crazes produced at different temperatures T and strain rates in thin films of monodisperse polystyrene (PS). At a slow strain rate of 4.1 × 10^?6 s^?1 both the fibril diameter D and the fibril spacing D₀ of crazes in 1800k molecular weight PS remained constant with temperature up to T ≈ 70°C and then sharply increased as T approaches T_g. At a higher strain rate of ～ 10^?2 s^?1, both D and D₀ increase only slightly with T. The values of D and D₀ over a range of temperature are in very good agreement with those values obtained in bulk samples using small-angle x-ray scattering. The crazing stress was measured as a function of temperature in the thin films of the 1800k molecular weight PS strained at the same slow strain rate used for the LAED measurements. These measurements were analyzed using a simple model of craze growth to reveal the temperature and strain rate dependence of the craze surface energy Γ. At room temperature Γ ≈ 0.076 J/m² (versus Γ ≈ 0.087 J/m² predicted) and was observed to remain constant up to T ≈ 70°C and then decrease by approximately a factor of two at T = 90°C. This decrease in Γ is believed to result from chain disentanglement to form fibril surfaces at sufficiently high temperatures and occurs in the same temperature range in which the craze fibril extension ratio λ was observed to increase.     

The thermodynamic properties of star-shaped fullerene-containing poly-N-vinylpyrrolidone

The temperature dependence of the heat capacity of star-shaped fullerene-containing poly-N-vinylpyrrolidone was studied over the temperature range 6–390 K by precision adiabatic vacuum and dynamic scanning calorimetry. The temperature intervals and thermodynamic characteristics of phase transitions were determined. The low-temperature dependence of the heat capacity of the substance was analyzed according to the Debye theory of the heat capacity of solids and its multifractal generalization. The data obtained were used to calculate the standard thermodynamic functions C _p ^o (T),H ^o(T)-H ^o(0), S ^o(T), and G ^o(T)-H ^o(0) of fullerene-containing poly-N-vinylpyrrolidone from T → 0 to 390 K. The standard entropy of formation of the polymer from simple substances and the entropy of its synthesis from poly-N-vinylpyrrolidone and fullerite C₆₀ at 298.15 K were calculated. The thermodynamic characteristics of fullerene-containing poly-N-vinylpyrrolidone are compared with those of the polymer-analogue without C₆₀.     

Thermodynamics of undecanolactone,tridecanolactone, and pentadecanolactone from 0 to 340 K

The heat capacities C_p^o of undercanolactone, tridecanolactone, and pentadecanolactone have been measured between 10 and 370 K in a vacuum adiabatic calorimetric cryostat within about 0.2 per cent. The temperatures and enthalpies of physical transitions have been also estimated. The enthalpies of combustion of the compounds have been measured in an isothermal calorimeter with an accuracy of 0.05 per cent. From the results the functions {H (T) ? H (0)}, S^o(T), and {G^o(T) ? H^o(0)} have been calculated over the range 0 to 340 K, and the values of ΔH_f^o, ΔG_f^o and ΔS_f^o have been evaluated at T = 298.15 K.     

Electron spin relaxation of new organic conductors: fluroranthenyl radical cation salts

Crystals of conducting fluoranthenyl (FA) radical cation salts (FA₂)⁺X^? with X^? = PF^?₆, AsF^?₆ show ESR linewidths (ΔH_pp) of ≈15 mOe at T = 293 K at microwave(9.5 GHz) and radio frequencies (230-60 MHz). ΔH_pp is independent of orientation. We analysed longitudinal and transverse relaxation and their dependence on temperature. These salts are suitable as magnetic field probes.