H Ehrhardt

Rotational excitation of N2, CO and H2O by low-energy electron collisions

Absolute total and differential rotational and rotational-vibrational cross sections have been measured for N2 (weak quadrupole), CO (weak dipole), and H2O (strong dipole). In the beam experiments, the range of collision energies of the electrons was chosen between 0.5 and 6 eV, and the range of scattering angles from 15 to 105 degrees was covered. The overall energy inhomogeneity of the electron spectrometer was between 10 and 18 meV FWHM, depending on the experiment. Direct and resonant rotational excitation has been measured. In the case of N2 the rotational branches with Delta J=0, +or-2, +or-4 have been fitted to energy-loss spectra and for each branch a cross section is given. The authors have proceeded similarly in the case of CO (for dipole excitation Delta J=0, +or-1, for excitation via the 2 Pi resonance Delta J=0, +or-1, ..., +or-4) and in the case of H2O ( Delta J=0, +or-1). For N2 and CO the results are compared with calculated angular dependences. Finally, the cross sections are listed according to the different types of interaction potentials and interaction mechanisms.

Rotational excitation of diatomic molecules at intermediate energies: absolute differential state-to-state transition cross sections for electron scattering from N2, Cl2, CO and HCl

Absolute differential cross sections for the electron-impact rotational excitation of N2 Cl2, CO and HCl have been measured in a crossed-beam experiment. Results on rotational state-to-state transitions are obtained by a deconvolution procedure from the energy-loss peaks at primary energies between 10 eV and 200 eV and in the angular range from 10 degrees to 160 degrees . Details are given of an apparatus which allows absolute measurements with high-energy resolution and excellent statistics. At intermediate impact energies and scattering angles, unexpected large angular-momentum transfer was found. The results are very different for various energies and for different molecules. Four excitation mechanisms can clearly be distinguished. The experimental endings are compared with theoretical results using the semiclassical spectator model and a close-coupling calculation with an effective interaction potential.

Elastic electron scattering and vibrational excitation of CO2 in the threshold energy region

In a crossed-beam experiment e-+CO2 elastic scattering and the excitation of the three fundamental vibrational modes have been investigated in the energy region below the 2 Pi u shape resonance. Angular and energy dependences of the differential cross sections have been determined in absolute units below 1.05 eV collision energy. For elastic scattering the large increase of the cross section towards zero energy is consistent with the existence of a virtual state. The angular dependences of the elastic differential cross sections below 1 eV indicate that direct scattering is important even at very low collision energies. The excitation of the two infrared-active modes (010) and (001) below 1 eV is mainly determined by the direct excitation via the dipole potential, whereas the excitation of the symmetric-stretch mode (100) is dominated by s-wave scattering. A comparison with recent theoretical investigations shows that angular and energy dependences for this mode near threshold are strongly influenced by the virtual state of the e-+CO2 potential.

Direct and resonant vibrational excitation of C2H2 by electron impact from 0 to 3.6 eV

Energy and angular dependences of the elastic scattering and of the excitation of the nu 2, nu 3 and nu 5 vibrational modes have been measured in the angular range from 5 degrees to 108 degrees and for impact energies of the electrons from threshold to 3.6 eV. Absolute differential and integrated cross sections are presented. The study of the vibrations nu 1 and nu 4 is more limited because these Raman active modes are not excited strongly in this energy region. A previous theoretical treatment allows the determination of the amplitudes for direct excitation and for excitation via the 2 Pi g shape resonance and also the relative phases between these two amplitudes as a function of the impact energy. Branching ratios are presented for the decay of the resonance into the different vibrational channels of the molecule.

Threshold structures in the cross sections of low-energy electron scattering of methane

The energy and angular dependences of the elastic and inelastic scattering of electrons from methane have been measured in the range of collision energies from 0.1 to 1.8 eV. Of special interest are the shapes of the excitation functions of the four possible vibrational modes of the molecule, because the elastic cross section shows a pronounced Ramsauer-Townsend minimum and this is compatible with the existence of a short-lived compound state of the electron-molecule system being detectable very close to the thresholds of the inelastic channels. The measurements show structures right at the thresholds, which have half widths of 0.1-0.3 eV and seem to be predominantly in the s and f-waves of the scattered electrons. In addition, a cusp structure has been found in the nu 2, nu 4 channel. The transmission functions of both the electron gun and the electron optics of the detector have been carefully controlled and therefore the peak shapes and half widths of the threshold structures should be more accurate than the results of Rohr (1980) who first reported the existence of the threshold peaks in methane at a 60 degrees scattering angle.

Elastic electron scattering from CH4 for collision energies between 0.2 and 5 eV

Angular dependences of the elastic scattering of electrons from CH4 for impact energies between 0.2 and 5 eV in the angular range between 15 degrees and 138 degrees are presented. Absolute differential cross sections have been obtained. With the help of a phaseshift analysis, integrated cross sections are calculated. The Ramsauer minimum is found to be at a collision energy close to 0.45 eV. The sum of the integrated vibrationally elastic cross sections and the vibrational excitation cross sections yields the total cross sections. These data are compared with the recent data for sigma T of Ferch et al. (1985).

Low-energy electron impact spectroscopy of OCS and CS2

Energy and angular dependences of differential cross sections for elastic scattering and vibrational excitation of the fundamental vibrations and some strongly excited overtones for OCS and CS2 are presented. The impact energy ranges from 0.3 to 5 eV and scattering angles are measured between 12.5 and 138 degrees . Integral elastic, vibrationally inelastic and total cross sections are tabulated. The total cross sections are compared with theoretical results of Lynch et al. (1979). With few exceptions good agreement is found. The differential cross sections for elastic and inelastic scattering are compared with first Born approximations for dipole, quadrupole and polarisation potential interactions. A body-frame phaseshift analysis has been applied to the experimental data in the energy range below 1.2 eV.

Double differential cross sections for electron impact ionisation of helium

Double differential cross sections for electron impact ionisation of helium have been measured in a crossed-beam experiment for primary energies E0 of 100, 200, 300, 400, 500 and 600 eV. Angular dependences of the cross sections are presented for the scattered (primary) as well as for the ejected (secondary) electrons. The energies EB of the ejected electrons were chosen to be 2, 4, 10, 20 and 40 eV. The cross sections have been put on an absolute scale by extrapolating the generalised oscillator strength for the primary electrons to the optical limit. The authors also used the fact that the integrated cross section of a scattered electron of a given energy loss EO-EA corresponds to the integrated cross section of a secondary electron with kinetic energy EB=EO-EA-IP where IP is the first ionisation potential of He, when double excitations and ionisation are ignored. Cross sections for double excitations and ionisation of He by electron impact are negligible (<2%) compared with single excitation and ionisation cross sections.

Near-threshold vibrational excitation and elastic electron scattering from N2

Angular dependences of the elastic scattering and the vibrational excitation of N2 have been measured for impact energies between 0.1 and 1.5 eV in the angular range from 15 to 135 degrees . Absolute differential and integrated cross sections are presented. The elastic scattering can be described reasonably well by the modified effective range theory for 0.1 eV. The integral vibrational excitation cross sections are in good agreement with swarm results.

Rotational branch analysis of the excitation of the fundamental vibrational modes of CO2 by slow electron collisions

At 2 eV, the simultaneous rotational-vibrational cross sections for the fundamental modes are found to be well described by the Born formula with just long-range interactions. However, this result is not obtained for pure vibrational excitation (Q branch) in the Raman-active Fermi diads. At 3.8 eV, the infrared-active nu 2 and nu 3 cross sections agree with a previous theory incorporating resonant and direct scattering coherently. Measurements of the Raman-active nu 1 mode indicate that the theories need to account for the Fermi resonance.