Transient strain driven by a dense electron-hole plasma

We measure transient strain in ultrafast laser-excited Ge by time-resolved x-ray anomalous transmission. The development of the coherent strain pulse is dominated by rapid ambipolar diffusion. This pulse extends considerably longer than the laser penetration depth because the plasma initially propagates faster than the acoustic modes. X-ray diffraction simulations are in agreement with the observed dynamics.     

N'-alkylated guanidiniocarbonyl pyrroles: new receptors for amino acid recognition in water

[reaction: see text] N'-Substituted guanidiniocarbonyl pyrroles 7 were synthesized for the first time by activation of a Boc-protected guanidiniocarbonyl pyrrole 3 with triflic anhydride and subsequent reaction with a primary amine. These guanidinium cations are efficient receptors for the complexation of amino acid carboxylates even in water (K(assoc) > 10(3) M(-1)) as could be shown by UV titration studies.     

A pseudopotential calculation of the phonon spectrum of aluminium

The phonon dispersion curves for Al are calculated from a theory based on Coulomb interactions among the ions and a local pseudopotential interaction between ions and conduction electrons. Conduction electron correlations are taken into account according to a theory of Singwiet al. ^1,2. A dielectric constant is constructed which satisfies the compressibility sum rule. This is especially important for longitudinal phonons. The agreement of the computed frequencies with the experimental dispersion curves of Stedman and Nilsson³ is good. Sites, sizes and widths of Kohn anomalies in the dispersion curves are also calculated. As expected the theoretical curves show larger anomalies than the experimental data. Finally a comparison is made with the recent calculations of Wallace⁴.     

Raman scattering from phonons and magnons in antiferromagnetic Fe3BO6

Raman scattering by phonons and by magnon pairs has been observed in Fe₃BO₆. Of the predicted 60 Raman-active modes, 39 have been identified and classified according to their symmetries. The two-magnon band shows a strong decrease in intensity with increasing temperature, and almost vanishes close to T_N = 508 K. The origin of this effect is attributed to the existence of a nearly dispersionless magnon branch.     

Kinetics and mechanisms of homogeneous catalytic reactions. Part 4. Hydrogenation of cyclohexanone and 2-cyclohexen-1-one catalysed by the complexes [MH(CO)(NCMe)2(PPh3)2]BF4 (M = Ru,Os)

Kinetic and mechanistic studies of the homogeneous hydrogenation of cyclohexanone were carried out using the cationic complexes [MH(CO)(NCMe)₂(PPh₃)₂]BF₄ (M = Ru, Os) as the catalyst precursors, which were very efficient under mild reaction conditions in 2-methoxyethanol solution. For both complexes, the catalytic hydrogenation of cyclohexanone proceeds according to the rate law r = k[M][H₂]. The activation parameters were also calculated, the activation energy for the osmium catalyst being higher than for the ruthenium(I). All experimental data are consistent with a mechanism involving the oxidative addition of hydrogen as the rate-determining step of the catalytic cycle. Finally, the [MH(CO)(NCMe)₂(PPh₃)₂]BF₄ complexes were efficient precatalysts for the selective reduction of 2-cyclohexen-1-one to cyclohexanone; the reduction of the CO group of cyclohexanone only begins to take place when the ,-unsaturated ketone has been consumed.     

Magnon squeezing in an antiferromagnet: reducing the spin noise below the standard quantum limit

We report the first experimental demonstration of quantum squeezing of a collective spin-wave excitation (magnon) using femtosecond optical pulses to generate correlations involving pairs of spins in an antiferromagnetic insulator MnF2. In the squeezed state, the fluctuations of the magnetization of a crystallographic unit cell vary periodically in time and are reduced below that of the ground-state quantum noise.     

Efficient complexation of N-acetyl amino acid carboxylates in water by an artificial receptor: unexpected cooperativity in the binding of glutamate but not aspartate

A new tris-cation 1 binds N-acetyl amino acid carboxylates in water even at millimolar salt concentrations with Kass approximately 103 M-1 due to a clustering of electrostatic interactions. Binding is efficient enough to allow a naked-eyed detection using an indicator displacement assay. Furthermore, receptor 1 shows an unexpected 2:1 complex formation with strong positive cooperativity with glutamate but not aspartate.