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We propose a new variational principle for scattering theory which extends the Schwinger variational principle beyond the static-exchange approximation and to inelastic scattering. Application of this formulation to the scattering of electrons by hydrogen atoms at energies below k' = 0.64 demonstrates the rapid convergence of the phase shift with respect to the number of basis functions for both the open-and closed-channel orbitals. Furthermore, we show that the convergence of the phase shift with respect to the number of expansion functions (exact states or pseudostatesj is also fast. In our theory, the resulting phase shifts can be more accurate than those of the close-coupling method even if the same expansion basis is used. The phase shifts in our ls-2s-2p calculation are comparable to those of 1s-2s-2p-3p-3d calculation of Matese and Oberoi [Phys. Rev. A 4 569 (1971j],which are very close to the exact values. Several aspects of the convergence characteristics are also discussed.

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Dissociation pressures of some gas hydrates have been evaluated using the Lennard-Jones 12-6 28-7 and Kihara potentials in the Lennard-Jones-Devonshire cell model. The Lennard-Jones 28-7 pot~ntial 'gives the least satisfactory results. The Lennard-Jones 12-6 potential works satisfactorily for the monatomic gases and ClL but poorly for the rodlike molecules C2H6, C02, N2, 02, C2fu. This failure may be due to (i) d~~.tortio~s of t~e hydrate la~tice, (ii) neglect of.m?lecul~r shape and size in determining the cavity potential (m) bamer to mternal rotatIOn of the molecule m Its cavity. A crude model for the lattice shows that it is not distorted. The Kihara potential predicts better dissociation pressures for the hydrates of the rodlike moleules. U,nlike the previously used Lennard-Jones 12-6 potential, it depends on the size and shape of the mteractmg molecules. The absence of lattice distortions, improved dissociation pressures through the use of the Kihara potential and the restriction of the motion of the solute molecule to around the center of a cavity makes a large barrier to rotation unlikely. A small barrier may still be present.

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Measured and calculated differential cross sections for elastic ͑rotationally unresolved͒ electron scattering from two primary alcohols, methanol ͑CH 3 OH͒ and ethanol ͑C 2 H 5 OH͒, are reported. The measurements are obtained using the relative flow method with helium as the standard gas and a thin aperture as the collimating target gas source. The relative flow method is applied without the restriction imposed by the relative flow pressure conditions on helium and the unknown gas. The experimental data were taken at incident electron energies of 1, 2, 5, 10, 15, 20, 30, 50, and 100 eV and for scattering angles of 5°-130°. There are no previous reports of experimental electron scattering differential cross sections for CH 3 OH and C 2 H 5 OH in the literature. The calculated differential cross sections are obtained using two different implementations of the Schwinger multichannel method, one that takes all electrons into account and is adapted for parallel computers, and another that uses pseudopotentials and considers only the valence electrons. Comparison between theory and experiment shows that theory is able to describe low-energy electron scattering from these polyatomic targets quite well.

We report the results of theoretical studies of the time-resolved femtosecond photoelectron spectroscopy of quantum wavepackets through the conical intersection between the first two 2 AЈ states of NO 2 . The Hamiltonian explicitly includes the pump-pulse interaction, the nonadiabatic coupling due to the conical intersection between the neutral states, and the probe interaction between the neutral states and discretized photoelectron continua. Geometry-and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Photoelectron angular distributions are seen to provide a clearer picture of the ionization channels and underlying wavepacket dynamics around the conical intersection than energy-resolved spectra. Time-resolved photoelectron velocity map images are also presented.