There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Mise au point. Theorie des effets de couplage vibronique multimodes. Probleme a 2 etats. Couplage vibronique mettant en jeu des modes et des etats degeneres. Effets du couplage vibronique multimodes en spectroscopie. Comportement statistique des niveaux d'energie vibroniques. Intersections coniques et evolution temporelle de la fluorescence

Multimode molecular dynamics beyond the Born-Oppenheimer approximation

Recoil-ion and electron momentum spectroscopy is a rapidly developing technique that allows one to measure the vector momenta of several ions and electrons resulting from atomic or molecular fragmentation. In a unique combination, large solid angles close to 4π and superior momentum resolutions around a few per cent of an atomic unit (a.u.) are typically reached in state-of-the art machines, so-called reaction-microscopes. Evolving from recoil-ion and cold target recoil-ion momentum spectroscopy (COLTRIMS), reaction-microscopes—the `bubble chambers of atomic physics'—mark the decisive step forward to investigate many-particle quantum-dynamics occurring when atomic and molecular systems or even surfaces and solids are exposed to time-dependent external electromagnetic fields. This paper concentrates on just these latest technical developments and on at least four new classes of fragmentation experiments that have emerged within about the last five years. First, multi-dimensional images in momentum space brought unprecedented information on the dynamics of single-photon induced fragmentation of fixed-in-space molecules and on their structure. Second, a break-through in the investigation of high-intensity short-pulse laser induced fragmentation of atoms and molecules has been achieved by using reaction-microscopes. Third, for electron and ion-impact, the investigation of two-electron reactions has matured to a state such that the first fully differential cross sections (FDCSs) are reported. Fourth, comprehensive sets of FDCSs for single ionization of atoms by ion-impact, the most basic atomic fragmentation reaction, brought new insight, a couple of surprises and unexpected challenges to theory at keV to GeV collision energies. In addition, a brief summary on the kinematics is provided at the beginning. Finally, the rich future potential of the method is briefly envisaged.

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Recoil-ion and electron momentum spectroscopy: reaction-microscopes

The development of femtosecond time-resolved methods for the study of gas-phase molecular dynamics is founded upon the seminal studies of Zewail and co-workers, as recognized in 1999 by the Nobel Prize in Chemistry.1 This methodology has been applied to chemical reactions ranging in complexity from bond-breaking in diatomic molecules to dynamics in larger organic and biological molecules, and has led to breakthroughs in our understanding of fundamental chemical processes. Photoexcited polyatomic molecules and anions often exhibit quite complex dynamics involving the redistribution of both charge and energy.2-6 These processes are the primary steps in the photochemistry of many polyatomic systems,7 are important in photobiological processes such as vision and photosynthesis,8 and underlie many concepts in molecular electronics.9 Femtosecond time-resolved methods involve a pump-probe configuration in which an ultrafast pump pulse initiates a reaction or, more generally, creates a nonstationary state or wave packet, the evolution of which is monitored as a function of time by means of a suitable probe pulse. Time-resolved or wave packet methods offer a view complementary to the usual spectroscopic approach and often yield a physically intuitive picture. Wave packets can behave as zeroth-order or even classical-like states and are therefore very helpful in discerning underlying dynamics. The information obtained from these experiments is very much dependent on the nature of the final state chosen in a given probe scheme. Transient absorption and nonlinear wave mixing are often the methods of choice in condensed-phase experiments because of their generality. In studies of molecules and clusters in the gas phase, the most popular methods, laser-induced fluorescence and resonant multiphoton ionization, usually require the probe laser to be resonant with an electronic transition in the species being monitored. However, as a chemical reaction initiated by the pump pulse evolves toward products, one expects that both the electronic and vibrational structures of the species under observation will change. Hence, these probe methods can be * To whom corresondence should be addressed. A.S.: telephone (613) 993-7388, fax (613) 991-3437, E-mail albert.stolow@nrc.ca. D.M.N.: telephone (510) 642-3505, fax (510) 642-3635, E-mail dan@radon.cchem.berkeley.edu. 1719 Chem. Rev. 2004, 104, 1719−1757

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Femtosecond time-resolved photoelectron spectroscopy.

We present a modified velocity map imaging (VMI) spectrometer to be used in angle-resolved molecular photoionization studies in the gas phase with synchrotron radiation (SR) in the VUV/soft x-ray range The main modifications as compared to the original design of Eppink and Parker [A T J B Eppink and D H Parker, Rev Sci Instrum 68, 3477 (1997)] are an open repeller which allows the VMI spectrometer to be coupled to an independent dispersive electrostatic analyzer for combined operation in coincidence mode experiments, and the introduction of a coupled double Einzel lens in the flight tube in order to collect the full 4π solid angle for higher kinetic energy particles The length and position of the lenses have been optimized by a genetic algorithm to obtain the maximum kinetic energy possible without compromising the energy resolution Ray-tracing simulations and SR experiments show that the lenses can increase the kinetic energy bandwidth by a factor of up to 25 Furthermore, a remarkable impro

A refocusing modified velocity map imaging electron/ion spectrometer adapted to synchrotron radiation studies

Valence and C1s core level photoelectron spectra of gaseous alanine have been recorded with synchrotron radiation. Using ab initio Green's Function calculations of the vertical outer valence ionization energies and CMS-Xα calculations of the orbital ionization cross-sections, it is possible to account well for the features of both the new hν = 92 eV valence photoelectron spectrum and also its differences with an earlier hν = 21.2 eV spectrum. Good agreement may be achieved by considering just the contribution of a single molecular conformation. This agrees with previous experimental findings, but conflicts with calculations which suggest that a range of molecular conformations would coexist in an equilibrium sample. A study of the valence photoelectron spectrum of the amino acid threonine complements that of alanine, but unlike the latter is limited by the effects of thermal decomposition of the sample. The C1s core level spectrum of alanine is reported and its peaks are assigned to ionization of the thre...

Investigation of the Gas-Phase Amino Acid Alanine by Synchrotron Radiation Photoelectron Spectroscopy

A theoretical treatment of the photoionization of the chiral amino acid l-alanine is presented. Particular attention is paid to a previously unobserved circular dichroism which should be detectable in the photoelectron angular distribution (CDAD) from randomly oriented molecules. Numerical estimates of this difference in the differential cross-sections for left- and right-circularly polarized light range as large as 40% of the mean cross-section. Three different low-energy conformational structures are considered. Further comparisons with the experimental photoelectron spectrum suggest, however, that only one dominates in the gas phase. This concurs with other experimental data but disagrees with conclusions drawn from previous molecular orbital calculations. The magnitude of the predicted CDAD effect, especially when ionizing skeletal bonding orbitals, is sufficient to suggest that it may provide an experimental means for successfully distinguishing optical and conformational isomers.

Photoelectron Spectroscopy and Circular Dichroism in Chiral Biomolecules: l-Alanine

Simultaneous, enantiomer-specific identification of chiral molecules in multi-component mixtures is extremely challenging. Many established techniques for single-component analysis fail to provide selectivity in multi-component mixtures and lack sensitivity for dilute samples. Here we show how enantiomers may be differentiated by mass-selected photoelectron circular dichroism using an electron-ion coincidence imaging spectrometer. As proof of concept, vapours containing ∼1% of two chiral monoterpene molecules, limonene and camphor, are irradiated by a circularly polarized femtosecond laser, resulting in multiphoton near-threshold ionization with little molecular fragmentation. Large chiral asymmetries (2-4%) are observed in the mass-tagged photoelectron angular distributions. These asymmetries switch sign according to the handedness (R- or S-) of the enantiomer in the mixture and scale with enantiomeric excess of a component. The results demonstrate that mass spectrometric identification of mixtures of chiral molecules and quantitative determination of enantiomeric excess can be achieved in a table-top instrument.

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Enantiomer-specific analysis of multi-component mixtures by correlated electron imaging–ion mass spectrometry

Introduction and Applications of ab initio Methods (P. Botschwina & S. Schmatz). Photoionization, Photoelectron and Photodissocation Studies: Combining Experiment and Theory (C. Ng). The Calculation of Unimolecular Decay Rates with RRKM and ab initio Methods (T. Baer). Non-Adiabatic Processes in Ionic Dissociation Dynamics (J. Lorquet). Structure and Reactivity of Selected Distonic Radical Cations (R. Smith, et al.). Potential Energy Surfaces for Ion-Molecule Reactions (M. Gordon). Unimolecular and Bimolecular Chemistry of Gas Phase Ion (N. Nibbering, et al.). Indexes.

Ivan Powis

Papers

A refocusing modified velocity map imaging electron/ion spectrometer adapted to synchrotron radiation studies

Investigation of the Gas-Phase Amino Acid Alanine by Synchrotron Radiation Photoelectron Spectroscopy

Photoelectron Spectroscopy and Circular Dichroism in Chiral Biomolecules: l-Alanine

Enantiomer-specific analysis of multi-component mixtures by correlated electron imaging–ion mass spectrometry

The structure, energetics and dynamics of organic ions