On the orientational properties of some one-dimensional model systems

We use the transfer matrix formalism to examine the behavior of some anisotropic hard-core fluids, the centers of whose particles are constrained to a line. At large elongation and pressure, the compressibility factorp/p is higher than that for a system with complete aligment by a factor 1+ that depends upon the molecular geometry. For molecules with a finite radius of curvature, e.g., ellipses,=d/2, while for objects with flat sides=d; hered is the number of orientational degrees of freedom. A connection is made to some recent studies of hard ellipsoid fluids. We also model the effect of an external field on physical adsorption and show the existence of a phase transition in certain limiting situations.     

Atmospheric pressure chemical ionisation reversed-phase liquid chromatography/ion trap mass spectrometry of intact bacteriohopanepolyols

Atmospheric pressure chemical ionisation liquid chromatography/multi-stage ion trap mass spectrometry (APCI-LC/MS(n)) has been applied to the study of intact bacteriohopanepolyols. Spectral characterisation of bacteriohopanepolyols of known structure present in bacterial extracts (Zymomonas mobilis and a fermenter containing methanotrophs including Methylococcus capsulatus) has revealed greater structural detail than previous liquid chromatography/mass spectrometry (LC/MS) methods and identified characteristic fragmentations indicative of numerous biohopanoid structures. Analysis of a Recent sedimentary extract from Lake Druzhby (Antarctica) has demonstrated the power of this technique to detect biohopanoids in complex samples including at least partial characterisation of previously unknown composite structures.     

Time dependent desorption: A memory function approach

A formalism for the adsorption kinetics of systems where the desorption rate is a function of the residence time of the adsorbed particle is presented. The adsorbed density at time t is expressed simply as a convolution of a memory kernel, Q(t), and the available surface function, (t). For completely irreversible adsorption, Q(t) = 1, while for a system which approaches an equilibrium state, Q tends to zero at sufficiently large times. When the desorption rate, k _d , is constant, Q(t) = exp(–k _d t). Two models for the memory kernel are considered. In the first, the molecule is assumed to interact with the surface via two ligands which bind and debind at rates and µ respectively. In the second model, the adsorption is assumed to be partially reversible: molecules transform to a permanently bound state at a rate and desorb at a rate µ. In both models, the adsorption kinetics and memory kernels are found analytically. Strategies for determining the memory function from experimental data are discussed.     

Hydrogen bond vibrations of 2-aminopyridine.2-pyridone,a Watson-Crick analogue of adenine.uracil

The 2-aminopyridine.2-pyridone complex (2AP.2PY) is linked by antiparallel N-H.O=C and N-H.N hydrogen bonds, providing a model for the Watson-Crick hydrogen bond configuration of the adenine-thymine and adenine-uracil nucleobase pairs. Mass-selected S(1) <--> S(0) vibronic spectra of the supersonically cooled 2AP.2PY base pair analogue were measured by laser resonant two-photon ionization and emission spectroscopies. The hydrogen bond vibrations nu(2) (buckle, out-of-plane) and the three in-plane vibrations nu(3) (opening), nu(5) (shear), and nu(6) (stretch) were observed in the S(0) and S(1) states, giving detailed information on the stretching and deformation force constants of the (amide)N-H.N(pyridine) and the (amino)N-H.O=C hydrogen bonds. Density functional calculations with the B3LYP functional and the 6-311++G(d,p) and 6-311++G(2d,2p) basis sets yield ground-state hydrogen bond frequencies in close agreement with experiment.     

Experimental investigations of the nuclear level density by using heavy ion reactions

The transition of the level density parameter a _off from the low excitation energy value a _off=A/8 MeV⁻¹ to the Fermi gas value a _FG ∼ A/15 MeV⁻¹ was discovered a few years ago studying particle spectra evaporated from hot compound systems of A∼ 160. A number of experiments have been recently performed to confirm the earlier findings and extend the investigation to other mass regions and to higher excitation energies. Furthermore, precision coincidence experiments have been done in the lead region in which evaporation residues are tagged by low energy gammarays. Those experiments open the possibility of a detailed study of the level densities in nuclei where the shell effects are important.     

Cross-sectional tongue shape during the production of vowels

This study used ultrasound imaging to examine the cross-sectional shape of the tongue during the production of the ten English vowels ( see text ) in two consonant contexts--/p/ and /s/--and at two scan angles--anterior and posterior. Results were compared with models of sagittal tongue shape. A newly built transducer holder and head restraint maintained the ultrasound transducer in a fixed position inferior to the mandible at a chosen location and angle. The transducer was free to move only in a superior/inferior direction, and demonstrated reliable tracking of the jaw. Since the tongue is anisotrophic along its length, anterior and posterior scan angles were examined to identify differences in tongue shape. Similarly, the coarticulatory effects of the sibilant /s/ versus the bilabial /p/ were examined, to assess variability of intrinsic tongue shape for the vowels. Results showed that the subject's midsagittal tongue grooving was almost universal for the vowels. Posterior grooves were deeper than anterior grooves. In /s/ context, posterior tongue grooves were shallower than in /p/ context. Anteriorly, /s/ context caused deeper grooves for low vowels. Cross-sectional tongue shape varied with tongue position similarly to sagittal tongue shape.