ALBERT

Notes: We have obtained x-ray phase-contrast images with high spatial resolution by using extremely asymmetric Si 111 Bragg diffractions near the critical angle of the total reflection. The x-ray image could be magnified to 294 times in both vertical and horizontal directions. By using this x-ray microscopy system, we have observed clear phase-contrast images of a 0.7-μm-wide gold-line pattern. © 2001 American Institute of Physics.

Notes: We measure the diffraction peaks of InGaAsP selective metal-organic vapor-phase epitaxial layers on 1.7-μm-wide InP stripe regions between a pair of SiO2 mask stripes. This is achieved by using an x-ray microbeam with low angular divergence and a narrow energy bandwidth that was produced through two-dimensional condensation of undulator radiation x rays from a synchrotron light source using successive asymmetric diffraction. The lattice strain is investigated by changing the SiO2 mask width from 4 to 40 μm. The rocking curves reveal clear peak shifts in the InGaAsP layers from the higher angle side to the lower angle side of the InP substrate peaks as the mask width increases. © 2000 American Institute of Physics.

Notes: The excitation spectrum of the Cs2 molecule monitoring the emission from the dissociated Cs(6p2P3/2) atoms was measured by employing a pulsed supersonic molecular beam crossed with a laser beam. Strong predissociation was found in the 16 600–17 200 cm−1 region. Vibrational progressions with broad asymmetric profiles, which were found via the ionization of dissociated Cs(6p2P3/2) atoms by Kim and Yoshihara, were observed. Transitions from the ground state X 1Σg+ to the dissociative state (2)3Σu+ are allowed by the spin-orbit interaction between the (2)3Σu+ and B 1Πu states. The (2)3Σu+, (2)3Πu, and C 1Πu states are mixed by the spin-orbit interaction. By solving the coupled differential equations using the three-channel model, the transition probability to the mixed levels, (2)3Πu(v) and C 1Πu(v′) was calculated. The coupling between the (2)3Σu+ and (2)3Πu states is larger than that between the (2)3Σu+ and C 1Πu states. Accordingly, the linewidth of the (2)3Πu←X 1Σg+ transition is observed to be wider than that of the C 1Πu←X 1Σg+ transition. The sign of the Fano shape index q was shown to coincide with the sign of the vibrational overlap integral between the discrete level and the continuum. © 2000 American Institute of Physics.

Notes: We have measured the vibrational energy relaxation of azulene in the S2 state in ethane, carbon dioxide, and xenon over a wide density region by the time-resolved fluorescence spectroscopy. The reduced density of the solvent (the density divided by the critical density) has been changed from the gaslike one (lower than 0.05) to the liquidlike one (2.8 for ethane, 2.7 for carbon dioxide, and 2.0 for xenon). The density dependence of the relaxation rate is compared with that of azulene in the S0 state [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)]. Although the absolute rate largely depends on the electronic state, the density dependence of the relaxation rates is roughly the same for both electronic states, which can be considered as a side-proof of the isolated binary collision model. Possible origins for the validity of the isolated binary collision model are discussed. © 2000 American Institute of Physics.

Notes: The excitation spectra of the 15V 31 344.9 band of the CS2 V 1B2←X 1Σg+ transition and the changes in these spectra with the application of a magnetic field of up to 12 kG have been measured with sub-Doppler resolution. The radiative lifetimes of rotationally resolved single lines and single Zeeman components were measured under collision-free conditions. All of the fluorescence decays were observed to be of a single exponential. Large Zeeman splittings were observed for many lines. The only symmetry allowed spin–orbit interaction is that of the 3A2(B2) component with the 1B2 state. The 3A2(B2) component has no magnetic moment, but a magnetic moment is induced when it is mixed with the 3A2(A1,B1) components. The mixing of the 3A2(B2) and 3A2(A1,B1) components is facilitated by spin–rotation interaction and the Zeeman interaction. From analysis of the observed Zeeman splittings of the perturbed levels, the 3A2(B2) component was determined to lie 14 cm−1 below the nearly degenerate 3A2(A1) and 3A2(B1) components in the energy region where the 15V band is observed. Irregular energy shifts and splittings of rotational lines were observed, and these were attributed to (a) Coriolis interaction between the V1B2(v′(a1);K=0JM) and V 1B2(v(b2);K=1JM) levels and (b) resonant spin–orbit interaction between the rotational levels V 1B2(v′(a1);KJM) and R 3A2(v(a1);KJM). These interactions become appreciable when two levels lie close in energy. Large Zeeman splittings were observed in case (b). Many vibrational lines with irregular intensity and spacing were observed in each band. These were attributed to (c) Fermi resonance between the vibrational levels in the V 1B2 state and (d) resonant spin–orbit interaction between vibrational levels in the V 1B2 and R 3A2 states. In case (d), large Zeeman splittings were observed for a series of rotational lines in a vibrational band. The background lines were identified from observed Zeeman splittings as the transitions to levels of the R 3A2 state, which are induced by resonant spin–orbit interaction with the levels of the V 1B2 state. The intensity of the excitation spectrum of the V 1B2←X1Σg+ transition was observed to decrease as the magnetic field increases. This was attributed to a mixing of the 3A2 state with the V 1B2 state and the resulting triplet–triplet emission, which was not detected in this experiment. It was possible to evaluate the lifetime of the radiative triplet–triplet emission via deperturbation analysis of the perturbed lines. © 2000 American Institute of Physics.

Notes: Vibrational excitation to the symmetric stretching, bending and asymmetric stretching modes of OCS molecules by electron and positron impacts is investigated for impact energy from 2 to 6 eV theoretically and experimentally. Because the OCS molecule is a polar molecule, interactions that induce the vibrational excitation for the three modes are primarily long-range permanent dipole interactions. And hence, all excitation cross sections for three modes thus obtained are found to be similar in size both for electron and positron impacts, although for positron, the magnitude is generally slightly smaller by 25% than that of electron. This situation is markedly different from that of the CO2, where for the symmetric stretching mode, the difference between positron and electron impacts is a few orders of magnitude at 5 eV. Therefore, a comparative discussion of CO2 is useful for further understanding. © 2000 American Institute of Physics.

Notes: An experimental study of electron and positron scattering from CH3Cl, CH3Br, and CH3I molecules has been carried out, and total cross sections (TCSs) for both projectiles were determined. Several strong structures due to resonances in the TCSs have been observed for electron impact, while weak but not negligible structures have also been seen for positron impact. A strong variation for the dominant resonance peak seen at around 10 eV was found to depend on a type of halogen atoms, and a detailed study of this dependence on molecular species has been performed to understand the origin and nature of these resonances. The continuum multiple-scattering method has been employed for the analysis of experimental results in addition to the evaluation of the elastic cross section. For larger halogen atoms, TCSs tend to possess larger magnitudes at energies above 100 eV than for smaller halogen atoms suggesting that the halogen atoms in fact dominate the dynamics, and their magnitudes increase in the order of CH3I〉CH3Br〉CH3Cl. A comparative study of CH4 was also performed to provide insight on the effects of molecular geometrical structure and electronic state. © 2001 American Institute of Physics.

Notes: We have calculated nonlinear time correlation functions of a dilute hard-sphere gas numerically by using the Monte Carlo method, to find that the single particle dynamics in a dilute hard-sphere gas does not follow the Gaussian process. The deviation of the self-part of the dynamic structure factor from a Gaussian function is observed. This non-Gaussian character corresponds to those of Lennard-Jones liquids reported by Itagaki et al. [K. Itagaki, M. Goda, and H. Yamada, Physica A 265, 97 (1999)], if we scale the time unit by the collision frequency. Further, we trapped a particle in a harmonic well and calculated the time development of its distribution, in order to clarify the effect of collisions to the solvation dynamics. Both the Gaussian and the linear response assumptions are broken, and the deviation becomes larger as the curvature of the harmonic well gets larger. © 2001 American Institute of Physics.

Notes: We have measured the time-resolved fluorescence spectra of azulene in the S2 state in compressed gases and in liquids. We have found that the band shape of the fluorescence changes significantly in the earlier time scale after the photoexcitation when large excess energy (about 6500 cm−1) is given. The change of the band shape is similar both in the compressed gases and in the liquids, although the time scales of the change are quite different. We have measured the excitation energy dependence of the fluorescence band shape of the isolated molecule separately, and shown that the time dependence of the fluorescence band shape in gases and liquids corresponds to the vibrational energy relaxation in the S2 state. Comparing with the excitation energy dependence of the fluorescence band shape of the isolated molecule, we have succeeded in determining the transient vibrational excess energy. The vibrational energy relaxation rates in the S2 state are 1–2 times faster than those in the ground state both in compressed gases and in liquids. © 2000 American Institute of Physics.

Notes: We present a strategy for solvating biomolecules in molecular dynamics or Monte Carlo simulations. The method employs a thin layer (often monomolecular) of explicit water with additional external forces representing the electrostatics, pressure, fluctuations, and dissipations caused by the neglected bulk. Long-range electrostatic corrections are supplied through a set of variable surface charges (polarons) that recreates the mean reaction field (or dielectric properties) of an infinite solvent. We refer to this "fictitious" boundary layer as a "surface of active polarons" (or SOAP). Test simulations of the solvation free energies of 15 amino acid analogs and nine ions are in good agreement with experiment (correlation coefficients: 0.995 and 1.000, respectively) despite the use of unaltered published force-fields with only one adjustable parameter. Dynamical capabilities of SOAP are illustrated by application to a six residue peptide with a stable conformation (SYPFDV), as well as a flexible nine residue HIV-1 gp120 peptide (TLTSCNTSV from PDB 1hhg). Future extensions, calibrations, and applications are discussed briefly. © 2000 American Institute of Physics.