L. E. Machado

Photoexcitation and ionization in acetylene

Theoretical studies are reported of photoexcitation and ionization processes in the acetylene molecule. The calculations performed employ Franck—Condon and static-exchange approximations, and implicitly invoke vibrational and rotational closure. Detailed comparisons with recent spectral assignments, electron-impact excitation studies, and total and partial-channel cross-sectional measurements indicate that the theoretical results provide a useful first approximation to the complete dipole excitation/ionization spectrum of C_2H_2. The calculated discrete spectra are seen to include contributions from virtual 4σ_g(n/σ^*) and 1π_g(π^*) orbitals, which appear an valence interlopers in the 1π_u→ nsσ_g, 2σ_u → nsσ_g, 1σ_u → nsσ_g and 2σ_u → ndπ_g, 1σ_u → ndπ_g Rydberg series, respectively, in good accordance with spectral measurements. By contrast, the 3σ_u(σ^*) virtual valence orbital gives rise to prominent diabatic resonance features above threshold in the 3σ_g → kσ_u, 2σ_g → kσ_u, and 1σ_g → kσu cross-sections, results that are in good quantitative accordance with corresponding experimental studies. Prominent features in the measured (1π_u^(−1))X^2Π_u partial cross-section, which is in good accordance with the present values, are attributed to autoionization 2σ_u → 1π_g(π^*) and 4σ_g(n/σ^*) intravalence transitions.

Elastic electron scattering by water molecules

Elastic differential and momentum transfer cross sections are reported for electron scattering by H2O at impact energies ranging from 4 to 50 eV. The iterative Schwinger variational method in the fixed-nuclei, static-exchange approximation is used to calculate the low partial wave scattering amplitudes and the higher partial wave contributions are included via closure using the Born approximation for a point-dipole. Comparison of our calculated cross sections with recent experimental and other theoretical results is encouraging.

Cross sections and photoelectron asymmetry parameters for photoionization of H2O

The iterative Schwinger variational method is used to obtain cross sections and photoelectron asymmetry parameters for photoionization of the three outermost valence orbitals (1b1, 3a1, and 1b2) of H2O for photon energies from near threshold to 50 eV. A comparison of these calculated results with available experimental data is encouraging.

Studies of the photoionization cross sections of CH4

We present cross sections and asymmetry parameters for photoionization of the 1t_2 orbital of CH_4 using static‐exchange continuum orbitals of CH^+_4 to represent the photoelectron wave function. The calculations are done in the fixed‐nuclei approximation at a single internuclear geometry. To approximate the near‐threshold behavior of these cross sections, we assumed that the photoelectron spectrum is a composite of three electronic bands associated with the Jahn–Teller components of the distorted ion. The resulting cross sections reproduce the sharp rise seen at threshold in the experimental data and are in good agreement with experiment at higher energy. The agreement between the calculated and measured photoelectron asymmetry parameters is, however, less satisfactory.

Photoionization cross sections and asymmetry parameters for silane

We present the results of applications of the iterative Schwinger variational method to obtain photoionization cross sections and phoelectron angular distributions for ionization out of the four outermost valence orbitals 1b_(3u), 1b_(3g), 3a_g and 1b_(2u) of ethylene for photon energies ranging from near threshold to 30 eV. The observed resonance-like maxima in the cross sections for ionization of the 1b_(3g) and 3a_g orbitals are reproduced in our calculations. The disagreement between our cross sections and the experimental data for the 1b_(3u) orbital below 17 e V is attributed to autoionization, which is not accounted for in our calculations.

A comparative experimental-theoretical study on elastic electron scattering by methane

Absolute differential cross sections for elastic electron scattering by methane have been measured at six incident electron energies between 5 and 100 eV and over scattering angles between 10° and 180°, using a crossed-beam electron spectrometer combined with a magnetic angle-changing device to extend the measurements to backward angles (125°–180°). Differential, integral and momentum-transfer cross sections are also calculated and reported for these energies. A complex optical potential was used to represent the electron–molecule interaction dynamics. The iterative Schwinger variational method combined with the distorted-wave approximation was used to solve the scattering equations. The comparison between our calculated and measured results, as well as with other experimental and theoretical data available in the literature, is encouraging.

Electronic excitation of H2O by electron impact

Recently we made calculations of differential and integral cross sections for the X1A1 to 3A1(3a1 to 3sa1) transition in H2O in the energy range of 12-30 eV, where the distorted-wave approximation was applied for the first time to study the electronic excitation of a nonlinear polyatomic target by electron impact. In the present work we make an extension of this calculation for that transition to the energy range 40-150 eV. Calculations of cross sections for the X1A1 to 3A1(3a1 to 3pa1) transition in the energy range 14-150 eV are also reported. The present study is the first theoretical investigation of electron-impact excitation of the channel 3a1 to 3pa1 in this nonlinear molecule.

A distorted-wave study of electronic excitation to some low-lying states of CO by electron impact

The distorted-wave approximation is applied to study electron-impact excitation leading to the , , and states of CO in the 20 - 100 eV range. Our calculated DCS and ICS are compared with available theoretical and experimental data. In general our cross sections are in quite good agreement with previous two- and five-state Schwinger multichannel results. Also, good qualitative agreement with the experimental data is obtained. However, in general our theory overestimates the cross sections. In comparison with a previous DWA study, our work reveals the strong influence of the description of the target wavefunctions on the calculated cross sections.

Elastic electron scattering at acetylene at low and intermediate energies

The authors report calculated differential and integral cross sections for e-C2H2 collisions in the 10-200 eV energy range. These cross sections were derived from fixed-nuclei scattering amplitudes in such a way that the low angular momentum components are obtained using the Schwinger iterative method and the large ones are treated by a Born closure. Using this combined theory, it can be shown that the elastic differential cross sections obtained at the static-exchange level are nearly exact, even at higher energies. The inclusion of a semi-empirical polarization potential is discussed as well.

Low energy elastic scattering of electrons by hydrogen sulphide molecules

Elastic differential, integral and momentum transfer cross-sections for electron-H2S collisions are reported at impact energies ranging from 2 to 50 eV. The Schwinger variational iterative method in a fixed-nuclei static-exchange approximation is used to calculate the low partial-wave scattering amplitudes. Higher partial-wave contributions were taken into account through the Born approximation with a point-dipole potential. Comparison of our calculated cross-sections with recent experimental and theoretical results is encouraging.