Map Lima

Elastic scattering of low-energy electrons by CF3Cl, CF3Br and CF3I

We report integral, differential and momentum transfer cross sections for elastic scattering of low-energy electrons by CF3X (X = Cl, Br, I) molecules. We use the Schwinger multichannel method with pseudopotentials (Bettega et al1993 Phys. Rev. A 47 1111) at the static exchange approximation. Our calculations cover the energy range between 5 and 30 eV. We compare our results with available theoretical and experimental results for CF3Cl and CF3I, and in general find good agreement. In particular, our results show the shape resonances belonging to the A1 and E representations of the C3v group that have been reported by previous work.

Calculation of elastic scattering cross sections of low‐energy electrons by PbH4 and SnH4

We report elastic integral, differential, and momentum transfer cross sections from 10 to 30 eV for electron scattering by SnH4 and PbH4, obtained using the Schwinger Multichannel Method with Pseudopotentials [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)]. With these molecules we close the series of XH4 molecules, with X=C, Si, Ge, Sn, Pb. We compare the present results with those obtained previously for CH4, SiH4, and GeH4. We find similarities in the cross sections for SiH4, GeH4, SnH4, and PbH4 and a distinctive behavior of CH4. We discuss the role of the center atom size in the scattering process. To our knowledge this is the first ab initio calculation of the SnH4 and PbH4 electron scattering cross sections.

Electron collisions with isomers of C4H8 and C4H10

We report integral, differential and momentum transfer cross sections for elastic scattering of low-energy electrons by several isomers of the C4H8 molecules, such as isobutene, trans-2-butene, cis-2-butene, skew-1-butene and syn-1-butene, and by two isomers of C4H10, the isobutane and the butane molecules. To calculate the cross sections, we use the Schwinger multichannel method with pseudopotentials (Bettega et al 1993 Phys. Rev. A 47 1111) applied at the static exchange level of approximation for incident energies from 10 to 50 eV. Although the C4H8 isomers have different geometric structures, our results show that the integral cross sections for each of these isomers present a broad shape resonance around 10 eV; our results also show that for all C4H8 molecular targets, integral cross sections are very similar in shape and magnitude. Similarities are also found in the momentum transfer and in the differential cross sections of these isomers. The same pattern is found in the cross sections of the C4H10 isomers. Through comparison of the integral and momentum transfer cross sections of 1,3-butadiene (C4H6), trans-2-butene (C4H8) and butane (C4H10), all belonging to the C2h group and having similar structures, we discuss the role of the hydrogen atoms in the scattering process by these molecules. Further we show that the integral elastic cross sections of all simple hydrocarbons present strong similarities after a scaling. We present a simple geometric model for this scaling that works quite well for a whole family of CnHm molecules (with combinations of n = 1, 2, 3, 4 and m = 2, 4, 6, 8, 10).

Experimental and theoretical elastic cross sections for electron collisions with the C3H6 isomers

In the present work we report cross sections for electron collisions with the isomers propene (C3H6) and cyclopropane (c-C3H6). Electron-scattering differential cross sections (DCS) are reported for measurements carried out for energies 1.5-100 eV and the angular range of 20 degrees-120 degrees. Elastic integral cross sections (ECS), DCS, and momentum-transfer cross sections (MTCS) are reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range of 2.0-40 eV and angular range of 0 degrees-180 degrees. The resemblance of the pi* shape resonance in the cross sections, observed at 1.5-2.0 eV for propene, to those in C2H4 and C2F4 clearly points to the effect of the double bond in the molecular structures for these molecules. Below 60 eV, we observed clear differences in peak positions and magnitudes between the DCS, ECS, and MTCS for C3H6 and c-C3H6, which we view as the isomer effect.

Electron and positron scattering from 1,1-C2H2F2.

1,1-difluoroethylene (1,1-C2H2F2) molecules have been studied for the first time experimentally and theoretically by electron and positron impact. 0.4-1000 eV electron and 0.2-1000 eV positron impact total cross sections (TCSs) were measured using a retarding potential time-of-flight apparatus. In order to probe the resonances observed in the electron TCSs, a crossed-beam method was used to investigate vibrational excitation cross sections over the energy range of 1.3-49 eV and scattering angles 90 degrees and 120 degrees for the two loss energies 0.115 and 0.381 eV corresponding to the dominant C-H (nu2 and nu9) stretching and the combined C-F (nu3) stretching and CH2 (nu11) rocking vibrations, respectively. Electron impact elastic integral cross sections are also reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range from 0.5 to 50 eV in the static-exchange approximation and from 0.5 to 20 eV in the static-exchange plus polarization approximation. Resonance peaks observed centered at about 2.3, 6.5, and 16 eV in the TCSs have been shown to be mainly due to the vibrational and elastic channels, and assigned to the B2, B1, and A1 symmetries, respectively. The pi* resonance peak at 1.8 eV in C2H4 is observed shifted to 2.3 eV in 1,1-C2H2F2 and to 2.5 eV in C2F4; a phenomenon attributed to the decreasing C=C bond length from C2H4 to C2F4. For positron impact a conspicuous peak is observed below the positronium formation threshold at about 1 eV, and other less pronounced ones centered at about 5 and 20 eV.

Electron collisions with CS2

We present elastic integral and differential cross sections for electron collisions with CS2 molecules for energies up to 10 eV. To compute the cross sections we employed the Schwinger multichannel method implemented with pseudopotentials at the static-exchange plus polarization approximation. In particular, our integral cross section shows a sharp increase near zero energy, which is an indication of the existence of an s-wave virtual state, followed by a Ramsauer–Townsend minimum around 0.7 eV. Our results agree well with available experimental and theoretical results.

Elastic scattering of low-energy electrons by carbon disulphide

We report results from an ab initio calculation of low-energy electron scattering by CS2 molecules using the Schwinger multichannel method with pseudopotentials. We calculated elastic integral, differential and momentum transfer cross sections in an energy range from 5 eV up to 50 eV and compared our results with available theoretical results and experimental data. Through the symmetry decomposition of our integral cross section and eigenphase sum analysis, we found cross section peaks that may be interpreted as broad shape resonances in the cases of the Sg, Pg, Pu, and Du symmetries. Among these possible resonances, the Pg, Pu, and Du are being reported for the first time.

Elastic and rotationally inelastic cross sections for low-energy electron scattering by SO2 molecules

We show rotationally summed and rotationally inelastic differential, integral and momentum transfer cross sections for electron scattering by SO2 in the 3-30 eV impact energy range in the static exchange approximation. Our results were obtained with the Schwinger multichannel method with pseudopotentials, including a first Born approximation to describe the influence of the molecular permanent dipole moment on the scattering cross sections. The rotational excitation cross sections were obtained through the adiabatic-nuclei-rotation approximation. Our results show good agreement with available experimental and theoretical data. The rotationally inelastic cross sections were found to be very large.

Scattering of low-energy electrons by isomers of C4H10

We report elastic integral, differential and momentum transfer cross sections for low-energy electron collisions with the two isomers of C4H10, butane and isobutane. We employ the Schwinger multichannel method with pseudopotentials at the static-exchange-polarization approximation, and cover energies from 1 eV to 20 eV. We investigate the influence of polarization on the isomer effect comparing the cross sections of these isomers and concluded that polarization has a small influence in that effect. We also compare our computed elastic cross sections with available total cross sections. Total ionization cross sections by electron impact are computed using the binary-encounter-Bethe (BEB) model and the results compared with available experimental data. The ionization cross sections for butane and isobutane are similar, showing that they also have a small isomer effect in the ionization process.

Electron collisions with the hydrides PH3, AsH3 and SbH3

We report integral cross sections for the elastic scattering of low-energy electrons by the hydrides PH3, AsH3 and SbH3. To calculate the cross sections, we employed the Schwinger multichannel method with pseudopotentials (Bettega et al 1993 Phys. Rev. A 47 1111) at the static-exchange plus polarization level of approximation for incident energies from 0.5 to 8 eV. For PH3, we compare our integral cross section with the recent total cross section measured by Szmytkowski et al (2004 J. Phys. B: At. Mol. Opt. Phys. 37 1833) and find good agreement in shape. In particular, our calculated elastic integral cross section for PH3 also shows the two structures reported by the experiment. We have found that the three hydrides present a Ramsauer–Townsend minimum at around 1 eV.