ALBERT

Notes: We have identified an optically enhanced magnetic phase transition in the newly synthesized organic molecular charge transfer salt, (BEDT–TTF)3Ta2F11 (BEDT–TTF 3/4 bis-ethylenedithiolotetrathiafulvalene) by ESR absorption measurements in the X band microwave region. At room temperature, only a doublet state ESR absorption is observed, but below 30 K several triplet ESR absorptions appear. The orientation dependence of the ESR absorption under illumination at energies near the band gaps in the material (640 nm, T=12 to 5 K, H0〈0.34 T) indicates that there are rapid spin exchange processes with times τe〈10−8 s near 7 to 5 K along certain crystallographic directions with a temperature dependence suggesting spin-lattice relaxation times which proceed via Van Vleck "direct processes.'' This, to our knowledge, is the first case where the magnetic properties of a charge transfer salt are altered by the interaction with photons of energy equal to the band gaps in a low dimensional solid, providing a new, interesting way to investigate these materials.

Notes: In the classification of finite-dimensional modules of Lie superalgebras, Kac distinguished between typical and atypical modules. Kac introduced an induced module, the so-called Kac module V¯(Λ) with highest weight Λ, which was shown to be simple if Λ is a typical highest weight. If Λ is an atypical highest weight, the Kac module is indecomposable and the simple module V(Λ) can be identified with a quotient module of V¯(Λ). In the present paper the problem of determining the composition factors of the Kac modules for the Lie superalgebra sl(m/n) is considered. An algorithm is given to determine all these composition factors, and conversely, an algorithm is given to determine all the Kac modules containing a given simple module as a composition factor. The two algorithms are presented in the form of conjectures, and illustrated by means of detailed examples. Strong evidence in support of the conjectures is provided. The combinatorial way in which the two algorithms are intertwined is both surprising and interesting, and is a convincing argument in favor of the solution to the composition factor problem presented here.

Notes: Kac distinguished between typical and atypical finite-dimensional irreducible representations of the Lie superalgebras sl(m/n) and provided an explicit character formula appropriate to all the typical representations. Here, the range of validity of some character formulas for atypical representations that have been proposed are discussed. Several of them are of the Kac–Weyl type, but then it is proved that all formulas of this type fail to correctly give the character of one particular atypical representation of sl(3/4). Having ruled out, therefore, all such formulas, a completely new extension of the Kac–Weyl character formula is proposed. The validity of this formula in the case of all covariant tensor irreducible representations is proved, and some evidence in support of the conjecture that it covers all irreducible representations of sl(m/n) is presented.

Notes: A computer simulation study of quantum effects in methane, butane, and octane is presented. Each molecular system is examined at three state points in the liquid region using novel extended system, multiple time step, constant pressure, path integral molecular dynamics methodology. In addition, the results of classical calculations are reported to provide a useful reference. Liquid butane is used as a test case on which to compare the predictions of two empirical force fields, CHARMM22 and AMBER95. Comparisons are made to experiment. Briefly, the models predict that quantum effects lead to an increase in molar volume of approximately 2 cm3/mole (i.e., relative to a classical calculation). However, a slight unphysical hydrogen–deuterium isotope effect is, also, observed. This may be caused by an incorrect parametrization of the anisotropy of the potential or by a reduction in the magnitude of the intermolecular induced dipole-induced dipole dispersion coefficient with increasing isotope mass that has not been parametrized in the force fields. In addition, the results show an interesting zero-point energy effect. The intramolecular regions of the radial distribution function exhibit less structure at lower temperatures than at higher temperatures. This is the inverse of the prediction of the model in the classical limit. The quantum effect occurs because the bulk density decreases faster than the intramolecular degrees of freedom lose zero-point energy as temperature increases in the highly harmonic intramolecular potential model employed in the calculations. Nonetheless, the phenomena is not likely to be an artifact and careful experiments could observe it. Finally, the efficiency of the path molecular dynamics methods employed in the studies are demonstrated on both serial and parallel computers. © 2000 American Institute of Physics.

Notes: The product-state-resolved dynamics of the reaction H+CO2→OH(2Π;ν,N,Ω,f)+CO have been explored in the gas phase at 298 K and center-of-mass collision energies of 2.5 and 1.8 eV (respectively, 241 and 174 kJ mol−1), using photon initiation coupled with Doppler-resolved laser-induced fluorescence detection. A broad range of quantum-state-resolved differential cross sections (DCSs) and correlated product kinetic energy distributions have been measured to explore their sensitivity to spin–orbit, Λ-doublet, rotational and vibrational state selection in the scattered OH. The new measurements reveal a rich dynamical picture. The channels leading to OH(Ω,N∼1) are remarkably sensitive to the choice of spin–orbit state: Those accessing the lower state, Ω=3/2, display near-symmetric forward–backward DCSs consistent with the intermediacy of a short-lived, rotating HOCO (X˜ 2A′) collision complex, but those accessing the excited spin–orbit state, Ω=1/2, are strongly focused backwards at the higher collision energy, indicating an alternative, near-direct microscopic pathway proceeding via an excited potential energy surface. The new results offer a new way of reconciling the conflicting results of earlier ultrafast kinetic studies. At the higher collision energy, the state-resolved DCSs for the channels leading to OH(Ω,N∼5–11) shift from forward–backward symmetric toward sideways–forward scattering, a behavior which resembles that found for the analogous reaction of fast H atoms with N2O. The correlated product kinetic energy distributions also bear a similarity to the H/N2O reaction; on average, 40% of the available energy is concentrated in rotation and/or vibration in the scattered CO, somewhat less than predicted by a phase space theory calculation. At the lower collision energy the discrepancy is much greater, and the fraction of internal excitation in the CO falls closer to 30%. All the results are consistent with a dynamical model involving short-lived collision complexes with mean lifetimes comparable with or somewhat shorter than their mean rotational periods. The analysis suggests a potential new stereodynamical strategy, "freeze-frame imaging," through which the "chemical shape" of the target CO2 molecule might be viewed via the measurement of product DCSs in the low temperature environment of a supersonic molecular beam. © 2000 American Institute of Physics.