P.W. Langhoff

Stieltjes imaging of atomic and molecular photoabsorption profiles

Abstract The effective transition frequencies and oscillator strenghts obtained from principal representations of spectral moments, or from variational procedures using appropriate square-integrable basis functions, are employed in the Stieltjes imaging of atomic and molecular photoabsorption profiles.

Experimental and theoretical studies of the valence-shell photoion1zation cross sections of acetylene: strong autoionization in the (1πu−1) X2Πu partial channel

Experimental studies of photoionizaiion cross sections in acetylene reveal the presence of a strong resonance in the (1π_u^(−1) X^2Π_u partial channel that is attributed on the basis of corresponding theoretical studies to autoionization of the valence (2σ_u^(−1)1π_g) b^1Π_u and (2σ_u^(−1)4σ_g)^1Σ_u^+ states.

Casimir-Polder separation theorem for second-order Coulomb energies

Abstract The second-order spectral sums over two-electron Coulomb matrix elements which arise in connectin with long-range intermolecular potentials, related exchange perturbation theories, and perturbative treatments of correlation energies are separated in the form of Casimir-Polder integrals over appropriate response functions. The latter are obtained from the solution of first-order perturbation equations which describe the interactions of the separate systems with an electromagnetic plane wave, employing variational and related computational techniques.

Green's function calculations of ground-state correlation energies

Abstract A computationally convenient and numerically stable procedure is reported for the direct calculation of ground-state correlation energies employing one-particle Greens functions.

Sum-rule moment bounds on long-range van der waals potentials

Abstract Quantum-mechanical sum rules and results from the theory of moments are employed in the construction of rigorous bounds on the long range van der Waals interactions of atoms and molecules.

Stieltjes orbitals for molecular photoionization continua

Abstract Theoretical studies in the static-exchange approximation are reported of Stieltjes orbitals that provide optimal Hilbert-space representations of molecular photoionization continua.

On the angular distributions of electrons photoejected from fixed-in-space and randomly oriented molecules

Abstract New theoretical expressions are devised from a dynamical perspective for molecular photoionization cross sections differential in electron ejection angles which facilitate comparisons between theory and experiment and provide a convenient basis for ab initio calculations. The cross sections obtained for fixed-in-space molecules, including the lowest-order (nondipole) effects of retardation, are given in terms of invariant molecular body-frame transition moments and related normalized angular-distribution amplitudes which can be calculated employing interaction-prepared states without reference to specific scattering boundary conditions. Corresponding expressions for molecular dipole and nondipole anisotropy factors appropriate for randomly oriented molecules are obtained in closed forms involving expectation values of harmonic polynomials over the fixed-in-space body-frame angular-distribution amplitudes. The expressions are seen to be in the spirit of corresponding results for atomic photoionization anisotropy factors, to which they reduce in appropriate limits. Interpretations of recently measured angular distributions of photoelectrons from the K-shell of molecular nitrogen illustrate how the development relates measurements on randomly oriented molecules to those performed on fixed-in-space molecules. The theoretical formalism provides results in excellent accord with measurements of the molecular nitrogen K-shell dipole anisotropy factor, and accounts for the origins of large nondipole effects observed at relatively low photon energies (ℏω≤500 eV) in the measured angular distributions.

Tchebycheff-derivative approximations to photoabsorption cross sections in atoms and ions

Abstract Spectral moments and Tchebycheffs inequalities are employed in the construction of continuous, convergent approximations to photoabsorption and ionization cross sections in atoms and ions.

On Rayleigh-Schrödinger and green's-function calculations of ionization potentials and electron affinities

Abstract Connections between first-quantized Rayleigh-Schrodinger perturbation-theory and one-particle Greens-function calculations of the ionization potentials and electron affinities of atoms and molecules are reported.

Moment theory bounds on long-range casimir—polder interactions

Abstract Quantum-mechanical sum rules and results from the theory of moments are employed in the construction of rigorous bounds on the long range Casimir—Polder interactions of atoms and molecules.