Higher-order centrifugal distortion effects in ONCl

One hundred and nine new rotational transitions of ONCl in the ground vibrational state have been measured. A centrifugal distortion analysis of these transitions yields the values of six sextic and one octic centrifugal distortion constants. A comparison between experimental and calculated sextic distortion constants is reported.     

Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are considered.     

Transition temperatures and structural correlations for cellulose triesters

Six homologs of the cellulose triester series were found to exhibit, in general, three second-order transitions as determined dilatometrically. A parallel study of torsional modulus versus temperature revealed one glass–rubber transition, which allowed molecular assignment of one of the above. The dependence of the remaining two transition temperatures on length of the ester group permits some speculation with regard to their origin. The glass–rubber and secondary transitions exhibit an inversion for cellulose trivalerate and higher homologs, the two secondary transitions occurring at temperatures higher than the glass–rubber transition. A possible explanation for this anomaly in terms of side-chain crystallization is discussed.     

Phase transitions in molecular crystals of carboxylic acids

A comparative analysis of phase transitions in molecular crystals of carboxylic acids (n-alkano acids) with various chain lengths, (CH₃(CH₂) _n COOH) is performed with the use of DSC. A number of new effects related to first-order phase transitions are discovered. The temperature dependence of heat capacity is quantitatively analyzed in terms of the theory of blurred (Λ-shaped) phase transitions of the first order.     

Branchings of the fully symmetrical solution of the molecular field equation in liquid crystals

A general scheme has been constructed to study the branching of the fully symmetrical solution of the molecular field equation in the case when the molecular coordinates parametrise a certain group Q. A relationship is established between these branchings and the phase transitions. Branchings corresponding to the formation of orientationally ordered and spatially modulated structures are studied explicitly. Special attention is paid to phase transitions from the isotropic liquid to nematic and cholesteric phases. Characteristics of these transitions (for example Curie temperature, order of transition, and symmetry of the phase formed) have been related to the intermolecular potential energy and the pair correlation function.     

Influence of localized excited states on the transition moment directions of charge transfer complex absorptions

The influence of localized excited (LE) states on the spectroscopy of charge transfer (CT) complexes has been examined for a series of complexes formed between methyl-substituted benzene donors and 1,2,4,5-tetracyanobenzene as acceptor in 1,2-dichloroethane and octanenitrile solvents. A molecular orbital model was used to describe the appearance of multiple CT absorption bands that occur in the spectra of these complexes. The influence of LE states in these CT absorptions was explored using time-resolved linear dichroism spectroscopy where the direction of the CT transition moment vector (TMV) was used to probe the magnitude of intensity borrowing. The TMV directions for each of the observed CT transitions within the absorption spectra were determined for several complexes. In some cases, the observed CT transitions were interpreted as being pure CT transitions; in others the observed transitions are influenced significantly by a LE transition. The correlation between the TMV directions and the transition energy suggests that the magnitude of intensity borrowing is influenced not only by the energy difference between the CT and LE transitions but also by the specific character of the transitions under consideration.     

The kinetic of luminescence exchange promotion in rigid solutions

The influence of heavy atoms (promotor) on radiative and nonradiative transitions in luminescence processes is considered. A kinetic equation of the luminescence intensity in the presence of a promotor is obtained; moments of the kinetic curves are calculated. Experimental data on phosphorescence promotion by iodine ions are discussed for a number of aromatic compounds. A promotion effect of intensification of radiative and nonradiative transitions is observed.     

Effects of resonance occupation on dynamical electronic properties of adsorbates

A great variety of phenomena encountered in the studies of adsorption systems is, in one way or another, determined by the dynamics and the energetics of electronic transitions in adsorbates. Being intrinsically of a quantum nature, these transitions reflect the properties of the unperturbed species, as well as those of the interactions between the adsorbates and substrates that lead to adsorption. A typical feature of chemisorption systems is the occurrence of adsorbate valence electronic resonances which are degenerate to the substrate valence bands. The presence of a resonance may give rise to changes in the properties of the adsorbate electronic transitions relative to the corresponding gas phase characteristics. These changes should, in turn, manifest themselves in a number of the properties of adsorbates, which can be studied by modern surface sensitive experimental methods. In this article, we first briefly review the characteristics of the adsorbate electronic transitions involving valence resonances. Using this as a prerequisite, we present examples of the physical phenomena and events, such as van der Waals scattering from adsorbates and the measurements of the adsorbate spectra by electronic spectroscopies, which can be interpreted by invoking the effects of fractionally occupied valence resonances on the electronic transitions in chemisorbed species.     

Calorimetric determination of enthalpy changes in octylcyanobiphenyl (8CB) liquid crystal transitions

A specially designed differential scanning heat-flux calorimeter is used for the determination of enthalpy changes in the transitions between mesophases of the octylcyanobiphenyl (8CB) liquid crystal. Latent heats can be separated from pre-transitional effects in certain cases and higher-order transitions can be well identified experimentally.     

Thermodynamics of activated phase transitions of 8CB: DSC and MC calorimetry

The present paper reports the heating rate effect on the phase transitions of a pure liquid crystal octylcyanobiphenyl (8CB) with use of Differential Scanning Calorimetry (DSC) and Modulation Calorimetry (MC) techniques. The DSC runs were taken at various temperature ramp rates from 20 to 0.5 K/min for heating and cooling scans. Well-defined endothermic/exothermic peaks were found at the melting/crystallization, smectic-A to nematic (SmA-N), and nematic to isotropic (N-I) transitions on heating/cooling scans, respectively. All transitions shift in temperature significantly with different ramp rates. The temperature shift of C(p) peaks between heating and cooling scans indicates the order of the transitions. In addition, all transitions follow an Arrhenius behavior. The activation energy of a transition increases as the total energy involved in the transition decreases. The respective enthalpy and entropy change of each transition provides information on the Gibbs free energy. The significance of the results is discussed in terms of the order of transitions. A comparative analysis of MC and DSC techniques highlights the significance of the two techniques. MC is a practicable tool for observing the phase dynamics whereas DSC is a good tool for studying the rate kinematics of the transitions.     

Thermal stabilities of isoelectronic,isostructural nitrates,carbonates and borates

The effects of temperature changes on an isoelectronic, iso-structural group of compounds containing an iso-electronic, iso-structural complex of ions were studied. The stability of the nitrates is discussed on the basis of their temperature of fusion and solid state transitions; the carbonates from their decomposition temperatures and solid state transitions; and the borates on solid state transitions. The order of thermal stabilities is found to be ABO₃ > A′ CO₃ > A″ NO₃ .     

Specifics of first-order phase transitions in 1,22-docosanediol and 1,44-tetratetracontanediol

A comparative analysis of phase transitions in diols with different chain lengths, (CH₂)₄₄(OH)₂ and (CH₂)₂₂(OH)₂, was performed using differential scanning calorimetry. The use of temperature hysteresis made it possible to reveal a number of new effects associated with the specifics of first-order phase transitions. The parameters of transitions were quantitatively analyzed in terms of the self-consistent field theory for diffuse (A-shaped) first-order transitions.     

Raman scattering study of high-pressure phase transition in thiourea

A high-pressure Raman spectroscopic study of phase transitions in thiourea is reported. The changes in the Raman spectra with increasing and decreasing pressure have been followed to a maximum pressure of approximately 11 GPa. We observe several changes in the spectra including splitting of modes, appearance of new modes, and sudden change in the slope of the frequency-pressure curve at several pressures. On the basis of this study, we propose the existence of three more transitions in this system to phases VII, VIII, and IX at approximately 1, 3, and 6.1 GPa, respectively, in addition to the V-VI phase transition at 0.35 GPa reported earlier. All the transitions have been found to be completely reversible. We interpret these changes in terms of symmetry-lowering phase transitions.     

Structural phase transitions in pentacosane C25H52

The pentacosane undergoes a series of structural phase transitions in addition to the melting transition according to an x-ray scattering study. A phenomenological theory is developed to describe the structural phase transitions between different phases observed in pentacosane. We present a detailed analysis of the different phases and analyze the temperature anomalies of the elastic constants and heat capacity. The thermodynamic anomalies on the different phase transitions are described by the coupling between the order parameters and the elastic strains. The theoretical predictions are found to be in good qualitative agreement with experimental results.     

用DSC研究聚砜预聚物的转变

本工作用示差扫描量热法(DSC)研究了双酚A型和酚酞型两种聚砜预聚物的转变。发现两种预聚物经适当的热处理可以在玻璃化转变温度以下和以上分别产生两个转变。这两个转变可分别归之于局部分子间短程有序和长程有序的破坏。同时,得到了这两种聚砜的玻璃化温度与数均分子量的关系式。     

INVESTIGATION OF THE TRANSITIONS OF OLIGO-POLYSULFONES BY DSC

The transitions of bisphenol A and phenolphthalein o|igopolysulfones were investigated by DSC. It is found that both the oligomers have three transitions within the temperature range (230-550K) studied. Considering the strong dependence of the appearance of the two transitions, lower and higher than T_g, on the specimens' thermal history, i. e., temperature and time of heat-treatment, and their particular molecular strueture, it is proposed that these two transitions are morphological in origin arid are due to the disordering of relatively short range order of very small regions, and to the disordering of relatively well-ordered intraor/and intermoleeular segments, respectively. Equations for linear relationship between T_g and 1/_n for both the oligomers are given in the paper.     

Determination of the electron—phonon interaction of strongly inhomogeneously broadened optical transitions by electric-field- induced changes of persistent spectral holes

A method is presented for the direct measurement of the true phonon sideband and the Debye—Waller factor of strongly inhomogeneously broadened optical transitions. The method is based on electric-field-induced filling of the center of a spectral hole. It is aplied to fluorescence transitions of dye molecules in amorphous solids, in particular to perylene in polyvinylbutyral.