Pulse compression in radio frequency photoinjectors-applications toadvanced accelerators

While RF photoinjectors are an excellent source of high brightness electron beams, there are constraints to tying together the expected emittance and peak current performance of a given photoinjector system. These constraints, which arise from the complicated dynamics of the electrons due to the interplay of RF and space-charge forces within the photoinjector, tend to favor lower peak current operation. For some ultimate uses of photoinjector beams, such as linear collider test beams, wakefield accelerators, and free-electron lasers (FEL's), one may desire much higher peak currents. In this case, an inexpensive and reliable method for producing extremely short high-current electron bunches is to use magnetic compression. We examine this scheme analytically and by computer simulation. Many applications are illustrated, including the TESLA Test Facility/FEL injector, ultra-high current beams for plasma wakefields and generation of femtosecond electron pulses for injection into short wavelength laser-based accelerators. It is shown that the injection timing jitter associated with the laser can be nearly eliminated using this scheme, making it an indispensable component in many of the advanced accelerator injectors we consider     

Ion mobility-mass spectrometry applied to cyclic peptide analysis: Conformational preferences of gramicidin S and linear analogs in the gas phase

In this paper, we present an investigation of the gas-phase structural differences between cyclic and linear peptide ions by matrix-assisted laser desorption ionization-ion mobility-mass spectrometry. Specifically, data is shown for gramicidin S (cyclo-VOLFPVOLFP where phenylalanines are D rather than L-type amino acids and the O designates the non-standard amino acid ornithine) and five linear gramicidin S analogues. Results are interpreted as evidence for a beta-sheet (or beta-hairpin) conformational preference in both linear-protonated and sodiated-cyclic gramicidin S gas-phase peptides, and a preference for the protonated-cyclic peptide to adopt a collapsed, random coil-type conformation. A comparison with solution-phase circular dichroism measurements is performed, and structures similar to those observed in the gas phase appear to be favored in low-dielectric solvents such as 2,2,2-triflouroethanol. The utility of ion mobility-mass spectrometry (IM-MS) as a means of rapidly distinguishing between linear and cyclic peptide forms in also discussed.     

Dynamic light scattering and rheology studies of aqueous solutions of amphiphilic sodium maleate containing copolymers

Light scattering techniques, video particle‐tracking microrheology, and bulk rheology were employed to examine the structure and dynamics of a series of alternating sodium maleate copolymers with moderately hydrophobic comonomers (diisobutylene, styrene, and isobutylene) in aqueous solutions. The scaling dependence of the specific viscosity (η_sp) on the polyelectrolyte concentration (c) was studied with and without added salt; similar trends were found in both conventional rheology and particle‐tracking microrheology measurements, showing good performance of the technique with flexible polyelectrolytes. Furthermore, with dynamic light scattering performed in high added salt conditions, we examined the behavior of the amplitude of the fast mode, which is in agreement with scaling predictions. In contrast, the slow modes are not understood and display three separate behaviors for the wavevector q dependence of the decay rate (Γ), depending on the comonomer; superdiffusive (Γ ～ q^2.7, isobutylene) possibly because of sticky aggregates, wavevector independent (Γ ～ q⁰, styrene) most likely because of coupled polyion‐ion diffusion and diffusive (Γ ～ q^2.0, diisobutylene) presumably because these aggregates are not sticky. The hydrophobicity of the comonomer appears to switch the aggregation process between “open,” “closed,” and “non” association for isobutylene, diisobutylene, and styrene respectively. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 774–785, 2007     

Solvent effects on simple electron transfer reactions: A comparison of results for homogeneous and heterogeneous systems

Solvent effects on the rate constants for both homogeneous and heterogeneous electron transfer reactions have been analyzed on the basis of current models which consider the role of dynamic relaxation processes in determining the magnitude of the pre-exponential factor. A statistical method for separating the effects of the solvent longitudinal relaxation time τ_L from those of the solvent permittivity parameter γ is described and applied to 15 sets of experimental data for which results are available in at least four solvents. The degree to which the explained variation in the logarithm of the rate constant could be attributed to either of these effects varied all the way from 0 to 100% depending on the degree of reaction adiabaticity and the relative sizes of the inner and outer sphere components of the Gibbs energy of activation. Data for the limiting cases in which there is no τ_l dependence in the pre-exponential factor or in which the pre-exponential factor is proportional to τ_L⁻¹ were analyzed further to obtain the size-distance parameter and the components of the pre-exponential factor relevant to the encounter pre-equilibrium model. These parameters have been discussed with respect to current developments in electron transfer theory. Problems in estimating the longitudinal relaxation time in the solvent, required for the analysis, are also considered.     

Preparation of azulene-derived fulvenedialdehydes and their application to the synthesis of stable adj-dicarbaporphyrinoids

A "2 + 2" strategy for synthesizing adj-dicarbaporphyrinoid systems has been developed. In a model study, an azulenylmethylpyrrole dialdehyde was condensed with a dipyrrylmethane in the presence of HCl, followed by oxidation with ferric chloride, to give a modest yield of an azuliporphyrin. Fulvene aldehydes were prepared by reacting an indene-derived enamine with azulene aldehydes in the presence of Bu(2)BOTf, and azulene dialdehydes similarly reacted to give fulvene dialdehydes. The dialdehydes were condensed with dipyrrylmethanes in TFA/dichloromethane to afford good to excellent yields of dicarbaporphyrinoids with adjacent indene and azulene subunits. These 22-carbaazuliporphyrins exhibited significant diatropic character, and this property was magnified upon protonation. These characteristics are attributed to tropylium-containing resonance contributors that possess 18π electron delocalization pathways. Protonation studies demonstrated that C-protonation readily occurred at the interior indene carbon, but deuterium exchange also occurred at the internal azulene CH as well as at the meso-positions with TFA-d. Reaction of a carbaazuliporphyrin with silver(I) acetate in methanol or ethanol solutions also gave unusual nonaromatic dialkoxy derivatives.