Wavelength dependent photoelectron circular dichroism of limonene studied by femtosecond multiphoton laser ionization and electron-ion coincidence imaging

Enantiomers of the monoterpene limonene have been investigated by (2+1) resonance enhanced multiphoton ionization and photoelectron circular dichroism employing tuneable, circularly polarized femtosecond laser pulses. Electron imaging detection provides 3D momentum measurement while electron-ion coincidence detection can be used to mass-tag individual electrons. Additional filtering, by accepting only parent ion tagged electrons, can be then used to provide discrimination against higher energy dissociative ionization mechanisms where more than three photons are absorbed to better delineate the two photon resonant, one photon ionization pathway. The promotion of different vibrational levels and, tentatively, different electronic ion core configurations in the intermediate Rydberg states can be achieved with different laser excitation wavelengths (420 nm, 412 nm, and 392 nm), in turn producing different state distributions in the resulting cations. Strong chiral asymmetries in the lab frame photoelectron angular distributions are quantified, and a comparison made with a single photon (synchrotron radiation) measurement at an equivalent photon energy

Identifying and understanding strong vibronic interaction effects observed in the asymmetry of chiral molecule photoelectron angular distributions

Electron-ion coincidence imaging is used to study chiral asymmetry in the angular distribution of electrons emitted from randomly-oriented enantiomers of two molecules, methyloxirane and trifluoromethyloxirane, upon ionization by circularly polarized VUV synchrotron radiation. Vibrationally-resolved photoelectron circular dichroism (PECD) measurements of the outermost orbital ionization reveal unanticipated large fluctuations in the magnitude of the forward-backward electron scattering asymmetry, including even a complete reversal of direction. Identification and assignment of the vibrational excitations is supported by Franck-Condon simulations of the photoelectron spectra. A previously proposed quasi-diatomic model for PECD is developed and extended to treat polyatomic systems. The parametric dependence of the electronic dipole matrix elements on nuclear geometry is evaluated in the adiabatic approximation, and provokes vibrational level dependent shifts in amplitude and phase, to which the chiral photoelectron angular distributions are especially sensitive. It is shown that single quantum excitation of those vibrational modes which experience only a relatively small displacement of the ion equilibrium geometry along the normal coordinate, and which are then only weakly excited in the Franck-Condon limit, can be accompanied by big shifts in scattering phase; hence the observed big fluctuations in PECD asymmetry for such modes

Intense vibronic modulation of the chiral photoelectron angular distribution generated by photoionization of limonene enantiomers with circularly polarized synchrotron radiation

Photoionization of the chiral monoterpene limonene has been investigated using polarized synchrotron radiation between the adiabatic ionization threshold, 8.505 eV, and 23.5 eV. A rich vibrational structure is seen in the threshold photoelectron spectrum and is interpreted using a variety of computational methods. The corresponding photoelectron circular dichroism—measured in the photoelectron angular distribution as a forward-backward asymmetry with respect to the photon direction—was found to be strongly dependent on the vibronic structure appearing in the photoelectron spectra, with the observed asymmetry even switching direction in between the major vibrational peaks. This effect can be ultimately attributed to the sensitivity of this dichroism to small phase shifts between adjacent partial waves of the outgoing photoelectron. These observations have implications for potential applications of this chiroptical technique, where the enantioselective analysis of monoterpene components is of particular interest

Branching in the radical polymerization of acrlamide

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Molecular chirality under the reaction microscope: Mass-selective photoelectron circular dichroism of pure and multi-component enantiomeric mixtures

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Molecular Weight Control of Polyacrylamide with Sodium Formate as a Chain-Transfer Agent: Characterization via Size Exclusion Chromatography/Multi-Angle Laser Light Scattering and Determination of Chain-Transfer Constant

We discuss the synthesis and characterization of polyacrylamide (PAM) homopolymers with carefully controlled molecular weights (MWs). PAM was synthesized via free-radical solution polymerization under conditions that yield highly linear polymer with minimal levels of hydrolysis. The MW of the PAM homopolymers was controlled by the addition of sodium formate (NaOOCH) to the polymerization medium as a conventional chain-transfer agent. MWs and polydispersity indices (PDIs) were determined via size exclusion chromatography/multi-angle laser light scattering analysis; for polymerizations carried out to high conversion, PAM MWs ranged from 0.23 to 6.19 x 10(6) g/mol, with most samples having PDI approximate to 2.0. Zero-shear intrinsic viscosities of the polymers were determined via low-shear viscometry in 0.514 M NaCl at 25 degreesC. Data derived from the polymer characterization were used to determine the chain-transfer constant to NaOOCH under the given polymerization conditions and to calculate Mark-Houwink-Sakurada K and a values for PAM in 0.514 M NaCl at 25 degreesC. (C) 2003 Wiley Periodicals, Inc

Enantioselective femtosecond laser photoionization spectrometry of limonene using photoelectron circular dichroism

Limonene is ionized by circularly polarized 420 nm femtosecond laser pulses. Ion mass and photoelectron energy spectra identify the dominant (2 + 1) multiphoton ionization mechanism, aided by TDDFT calculations of the Rydberg excitations. Photoelectron circular dichroism measurements on pure enantiomers reveal a chiral asymmetry of ±4 %. This journal i

The human rights and the terrorism

Regarding Human rights has been emphasized by governments through universal declaration human rights and its violations faced the global reaction in any case. From this perspective; the necessity of observing human rights against terrorism has been emphasized by many resolutions of the General Assembly, the Security Council and many regional resolutions. But, today especially after the September 11 events and America unilateral attacks by resorting to human rights and of course by considering the lack of international agreement on the norms, strategies and principles of human rights, the campaign against terrorism has started on the basis of its obligations, regardless to existing norms. So it would be said that today; the war against terrorism more than any other terrorist attacks threatens the security and human rights. However, human rights framework has never prevented effective and legitimate actions to the terrorist attacks. In other words; in the human right framework, there is no conflict with the campaign against terrorism, it emphasizes on identifying and fighting and even it guarantees the effectiveness of such a fight. As a result, Human rights violations at the expense of security, certainly associated with the destruction of both

Chiral Asymmetry in the Multiphoton Ionization of Methyloxirane Using Femtosecond Electron-Ion Coincidence Imaging

Multiphoton photoelectron circular dichroism (MP-PECD) has been observed as an asymmetry in the angular distribution of photoelectrons emitted in the ionization of pure enantiomers of the small chiral molecule methyloxirane using a femtosecond laser operated at 420 nm. Energetically, this requires the uptake of four photons. By switching the laser between left- and right-circular polarization, and observing the differences in the full three-dimensional electron momentum distribution recorded in an electron-ion coincidence technique, the chiral (odd) terms in the angular distribution expression can be isolated. Electron events can additionally be filtered by coincident ion mass, providing mass-tagged electron distributions and quantitative measures of the MP-PECD asymmetry that help characterize the different ionization channels. For the production of ground state parent cation, the magnitude of the mean chiral asymmetry is measured to be 4.7%, with a peak magnitude exceeding 10%(Figure Presented)