Applications of ESCA to polymer chemistry. IV. The composition and structure of copolymers of ethylene with tetrafluoroethylene

Molecular core binding energies have been measured by ESCA for a series of copolymers of tetrafluoroethylene and ethylene. Detailed analysis of the results demonstrates the utility of ESCA for determination of copolymer compositions and for providing information on important structural features for these copolymers.     

REMPI-TOF studies of the translational anisotropy and the polarization of the O ((1)D2) photofragment angular momentum following ozone photolysis at 298 nm

The translational anisotropy and the polarization of the electronic angular momentum of the O ((1)D2) fragment produced from the 298 nm photodissociation of ozone have been determined using resonance enhanced multiphoton ionization (REMPI) in conjunction with time-of-flight mass spectrometry (TOFMS). The translational anisotropy parameter beta, which is necessarily averaged over the O2 co-fragment rotational distribution, is measured to be 1.08 +/- 0.04. This is consistent with that expected for the (1)B2 <-- (1)A1 transition within an impulsive model if the tangential velocity associated with the zero point motion of the bend is constricted to opening the bond angle. Molecular frame polarization parameters of rank up to k = 4 have been extracted for the O ((1)D2) fragment and the calculated m(J) populations show a strong preference for the absolute value(m(J)) = 1 states. A small coherence term is also observed, a manifestation of the nuclear geometry of the dissociating molecule and the existence of possible non-adiabatic processes in the exit channel. The orientation associated with the mapping of the photon helicity onto the O ((1)D2) electronic angular momentum distribution was observed to have been quenched. However, the parameter gamma1', which describes the contribution to the orientation from a coherent superposition of a parallel and perpendicular excitation where the photofragment angular momentum lies perpendicular to both the recoil velocity and to the transition dipole moment, was determined to be -0.06.     

Multi-mode absorption spectroscopy, MUMAS, using wavelength modulation and cavity enhancement techniques

Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band $b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-}Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band b¹\varSigma _g⁺? X³\varSigma _g^-b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-} of molecular oxygen at 760 nm. Direct MUMAS and WMS-MUMAS signals were recorded using a White cell for air and pure oxygen for pressures in the range 0 to 1 bar. CEAS-MUMAS signals were recorded with and without WMS in an open enhancement cavity containing laboratory air. Enhancement of the signal-to-noise ratio has been obtained demonstrating the potential for increased detection sensitivity for gas-sensing applications of MUMAS.     

Site control of InAs quantum dot nucleation by ex situ electron-beam lithographic patterning of GaAs substrates

Conventional electron-beam lithographic patterning of GaAs substrates followed by reactive-ion etching of small holes has been successfully used to control the nucleation of InAs dots. We have observed >50% single dot occupancy for holes wide and deep and show that the dot occupancy and dot size can be varied by changing the size of the holes. Luminescence from an array of these site-controlled dots has been demonstrated. Thus this use of substrate patterning is a viable technique to controllably place single dots at pre-determined positions in devices.     

Postselective two-photon interference from a continuous nonclassical stream of photons emitted by a quantum dot

We report an electrically driven semiconductor single-photon source capable of emitting photons with a coherence time of up to 400 ps under fixed bias. It is shown that increasing the injection current causes the coherence time to reduce, and this effect is well explained by the fast modulation of a fluctuating environment. Hong-Ou-Mandel-type two-photon interference using a Mach-Zehnder interferometer is demonstrated using this source to test the indistinguishability of individual photons by postselecting events where two photons collide at a beam splitter. Finally, we consider how improvements in our detection system can be used to achieve a higher interference visibility.     

Anticrossing of spin-split subbands in quasi-one-dimensional wires

In quantum Hall systems, both anticrossings and magnetic phase transitions can occur when opposite-spin Landau levels coincide. Our results indicate that both processes are also possible in quasi-1D quantum wires in an in-plane B field, Bparallel. Crossings of opposite-spin 1D subbands resemble magnetic phase transitions at zero dc source-drain bias, but display anticrossings at high dc bias. Our data also imply that the well-known 0.7 structure may evolve into a spin-hybridized state in finite dc bias.     

Recent progress and future directions in protein-protein docking

This article gives an overview of recent progress in protein-protein docking and it identifies several directions for future research. Recent results from the CAPRI blind docking experiments show that docking algorithms are steadily improving in both reliability and accuracy. Current docking algorithms employ a range of efficient search and scoring strategies, including e.g. fast Fourier transform correlations, geometric hashing, and Monte Carlo techniques. These approaches can often produce a relatively small list of up to a few thousand orientations, amongst which a near-native binding mode is often observed. However, despite the use of improved scoring functions which typically include models of desolvation, hydrophobicity, and electrostatics, current algorithms still have difficulty in identifying the correct solution from the list of false positives, or decoys. Nonetheless, significant progress is being made through better use of bioinformatics, biochemical, and biophysical information such as e.g. sequence conservation analysis, protein interaction databases, alanine scanning, and NMR residual dipolar coupling restraints to help identify key binding residues. Promising new approaches to incorporate models of protein flexibility during docking are being developed, including the use of molecular dynamics snapshots, rotameric and off-rotamer searches, internal coordinate mechanics, and principal component analysis based techniques. Some investigators now use explicit solvent models in their docking protocols. Many of these approaches can be computationally intensive, although new silicon chip technologies such as programmable graphics processor units are beginning to offer competitive alternatives to conventional high performance computer systems. As cryo-EM techniques improve apace, docking NMR and X-ray protein structures into low resolution EM density maps is helping to bridge the resolution gap between these complementary techniques. The use of symmetry and fragment assembly constraints are also helping to make possible docking-based predictions of large multimeric protein complexes. In the near future, the closer integration of docking algorithms with protein interface prediction software, structural databases, and sequence analysis techniques should help produce better predictions of protein interaction networks and more accurate structural models of the fundamental molecular interactions within the cell.     

Time-resolved detection of the CF3 photofragment using chirped QCL radiation

This paper demonstrates how a quantum cascade laser (QCL) in its intrapulse mode can provide a simple method for probing the products of a photolysis event. The system studied is the 266 nm photodissociation of CF3I with the CF3 fragments subsequently detected using radiation at approximately 1253 cm(-1) generated by a pulsed QCL. The tuning range provided by the frequency down-chirp of the QCL operated in its intrapulse mode allows a approximately 1 cm(-1) segment of the CF3 nu3 band to be measured following each photolysis laser pulse. Identification of features within this spectral region allows the CF3 ( v = 0) number density to be calculated as a function of pump-probe delay, and consequently the processes which populate and deplete this quantum state may be examined. Rate constants for the population cascade from higher vibrational levels into the v = 0 state, k 1, and for the recombination of the CF3 radicals to form C2F6, k2, are measured. The returned values of k1 = (2.3 +/- 0.34) x 10(-12) cm(3) molecule(-1) s(-1) and k2 = (3.9 +/- 0.34) x 10(-12) cm(3) molecule(-1) s(-1) are found to be in good agreement with reported literature values.