Cross sections for short pulse single and double ionization of helium

In a previous publication, procedures were proposed for unambiguously extracting amplitudes for single and double ionization from a time-dependent wavepacket by effectively propagating for an infinite time following a radiation pulse. Here we demonstrate the accuracy and utility of those methods for describing two-photon single and one-photon double ionization of helium. In particular it is shown how narrow features corresponding to autoionizing states are easily resolved with these methods.Comment: 9 pages, 9 figure

Decoding sequential vs non-sequential two-photon double ionization of helium using nuclear recoil

Above 54.4 eV, two-photon double ionization of helium is dominated by a sequential absorption process, producing characteristic behavior in the single and triple differential cross sections. We show that the signature of this process is visible in the nuclear recoil cross section, integrated over all energy sharings of the ejected electrons, even below the threshold for the sequential process. Since nuclear recoil momentum imaging does not require coincident photoelectron measurement, the predicted images present a viable target for future experiments with new short-pulse VUV and soft X-ray sources.Comment: 4 pages, 3 figure

A relationship between the many-body theory of inelastic scattering and the distorted wave approximation

It is shown that the first-order results of the recent many-body theory of inelastic scattering (see abstr. A25430 of 1971) can be derived by a direct application of the distorted-wave and random phase approximations to the usual expression for the inelastic scattering amplitude. The result is derived both in the second quantized formalism and by the standard application of the distorted-wave approximation coupled with the random phase approximation (RPA). The RPA (or time-dependent Hartree-Fock theory) provides the transition density between the initial and inelastically excited states. Possible generalizations of the procedures are discussed

Photoabsorption cross sections of two-electron atoms by the coordinate rotation method: Application to H– and several states of He

The coordinate rotation method, recently extended by us to treat photoabsorption processes, is used to obtain photoabsorption cross sections for several two-electron atoms. The calculations are performed using standard configuration–interaction methods; the need for atomic continuum wavefunctions is completely avoided in this approach. We have computed the photodetachment cross section of H– and photoionization cross sections for He in its ground and 2 1S states. In all cases, the computed cross sections agree well with results obtained by numerical integration and with available experimental data

A simple method for evaluating low-energy electron-molecule scattering cross sections using discrete basis functions

We present a simple, approximate method for calculating low-energy electron-molecule scattering cross sections using only the results of a basis set diagonalization of the molecular Hamiltonian. The method is based on the approximate conservation of orbital angular momentum in collisions between slow electrons and molecules lacking a permanent dipole moment (low l spoiling). Results are presented for e--H2, and e--N2, in the static-exchange approximation

Dissociative electron attachment to the H2O molecule. I. Complex-valued potential-energy surfaces for the 2B1, 2A1, and 2B2 metastable states of the water anion

We present the results of calculations defining global, three-dimensional representations of the complex-valued potential-energy surfaces of the doublet B1, doublet A1, and doublet B2 metastable states of the water anion that underlie the physical process of dissociative electron attachment to water. The real part of the resonance energies is obtained from configuration-interaction calculations performed in a restricted Hilbert space, while the imaginary part of the energies (the widths) is derived from complex Kohn scattering calculations. A diabatization is performed on the 2A1 and 2B2 surfaces, due to the presence of a conical intersection between them. We discuss the implications that the shapes of the constructed potential-energy surfaces will have upon the nuclear dynamics of dissociative electron attachment to H2O. This work originally appeared as Phys Rev A 75, 012710 (2007). Typesetting errors in the published version have been corrected here.Comment: Corrected version of PRA 75, 012710 (2007

Cross sections for the elastic scattering of low-energy electrons by molecular fluorine: an approximate theoretical treatment using discrete basis functions

Phaseshifts and total cross sections for the elastic scattering of low-energy (0-13.6 eV) electrons by molecular fluorine are presented. The phaseshifts are obtained by an approximate technique based on the weak asymptotic coupling of orbital angular momenta and are calculated solely from the results of a discrete basis set diagonalization of the molecular Hamiltonian. Correlation and polarization effects are not treated. The elastic cross section is dominated by a Sigma u+ shape resonance at about 2.2 eV in the static-exchange model

Photoabsorption in formaldehyde: Intensities and assignments in the discrete and continuous spectral intervals

Theoretical investigations of total and partial‐channel photoabsorption cross sections in molecular formaldehyde are reported employing the Stieltjes–Tchebycheff (S–T) technique and separated‐channel static‐exchange (IVO) calculations. Vertical one‐electron dipole spectra for the 2b_2(n), 1b_1(π), 5a_1(σ), 1b_2, and 4a_1 canonical molecular orbitals are obtained using Hartree–Fock frozen‐core functions and large basis sets of compact and diffuse normalizable Gaussians to describe the photoexcited and ejected electrons. The calculated discrete excitation spectra provide reliable zeroth‐order approximations to both valence and Rydberg transitions, and, in particular, the 2b_2(n) →nsa_1, npa_1, npb_2, and nda_2 IVO spectra are in excellent accord with recent experimental assignments and available intensity measurements. Convergent (S–T) photoionization cross sections in the static‐exchange (IVO) approximation are obtained for the 15 individual partial channels associated with ionization of the five occupied molecular orbitals considered. Resonance features in many of the individual‐channel photoionization cross sections are attributed to contributions from valencelike a_1σ^∗ (CO), a_1σ^∗ (CH), and b_2σ^∗ (CH)/π_y^∗ (CO) molecular orbitals that appear in the photoionization continua, rather than in the corresponding one‐electron discrete spectral intervals. The vertical electronic cross sections for ^1A_1→^1B_1, ^1B_2, and ^1A_1 excitations are in generally good accord with previously reported CI (S–T) predictions of continuum orbital assignments and intensities, although some discrepancies due to basis‐set differences are present in the ^1B_1 and ^1B_2 components, and larger discrepancies apparently due to channel coupling are present in the ^1A_1→^1A_1 cross section. Partial‐channel vertical electronic cross sections for the production of the five lowest parent‐ion electronic states are found to be in general agreement with the results of very recent synchrotron‐radiation photoelectron branching‐ratio measurements in the 20 to 30 eV excitation energy interval. Most important in this connection is the tentative verification of the predicted orderings in intensities of the partial‐ channel cross sections, providing support for the presence of a strong ka_1σ^∗ (CO) resonance in the (5a_1^(−1))^2A_1 channel. Finally, the total vertical electronic cross sections for absorption and ionization are in general accord with photoabsorption measurements, photoionization–mass–spectrometric studies, and the previously reported CI (S–T) calculations. Although further refined calculations including vibrational degrees of freedom and autoionization line shapes are required for a more precise quantitative comparison between theory and experiment, the present study should provide a reliable zeroth‐order account of discrete and continuum electronic dipole excitations in molecular formaldehyde