Energy and angular dependences of the elastic and inelastic scattering of electrons from H 2 and CO have been measured in the energy range from 0. 5 eV to 10 eV and for scattering angles ranging from 5 to 110°. The elastic cross sections are composed of potential and resonance scattering and therefore are difficult to interpret. The excitation of molecular vibrations, i.e., scattering into inelastic channels, contains predominant contributions from short-lived negative ion compound states. The qualitative agreement of the measured angular dependence with that predicted from pure resonant scattering considerations is shown to be able to fix certain symmetry quantum numbers of the molecular state. Moreover, characteristic and similar cr-, 7r-, A-. .. type angular dependences throughout the whole energy range of the molecular resonance (independent of the final vibrational state) are presented as a means of verifying the presence of a resonance, even when the short lifetime masks the usually characteristic resonant peaks. The half-width of the CO~~27r state is about 0.4 eV, that for H 2 2 2 M between 2 and 4 eV. Absolute total cross sections for the different inelastic channels are given. The resonances of N 2 , CO, and H 2 are compared*with the predictions of the single-particle-shape resonance model and their physical properties are discussed using the different potential energy terms at large distances between the additional electron and the molecule. Throughout, stress is laid on the importance of choosing the proper experiment (i.e., exit channel) when one wishes to study a resonance. Predictably, in using angular dependence, certain channels hide and others exhibit the presence of the resonance.The existence of short-lived negative ion states, formed in the scattering of low-energy electrons with molecules, has been established in several papers experimentally as well as theoretically. For this purpose measurements have been made of the: i, energy dependence of the attenuation of an electron beam after passing through a gas-filled scattering chamber, 1 " 3 ii, the energy dependence of the elastic and inelastic scattering cross section in the forward direction or into different scattering angles, 4 "" 8 and iii, the energy dependence of the total cross section for the dissociative attachment process. 9 ' 10 In certain energy ranges resonances are seen (methods i and ii), i.e. , considerable changes of the cross sections, which indicate compound states, if the variation of the wavelength of the colliding electrons in this energy range is small compared with the dimensions of the scatterer. This excludes diffraction effects as a reason for the cross section changes. Near the resonance energy, the cross section may increase and/or decrease, depending on the values of two scattering phases. The first is the phase of the outgoing electron wave of the direct collision process with the molecule M M+e-~Mj* + e, and the second, the value of the phase of the one (spherical for atoms, spheroidal for diatomics) partial wav...

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Differential cross section measurements for the elastic scattering of electrons from CH 3 Cl at energies from 0.5 to 9.5 eV are reported for scattering angles of 30° and 100°. The angular scattering dependence is determined at selected energies over this range. At energies below 1.0 eV, the cross sections are in excellent agreement with calculations using the Born dipole approximation. At large angles and higher energies, the scattering is dominated by a 2 A 1 temporary negative ion state near 3.5 eV. Energy loss data at 3.5, 5.0, and 8.5 eV are reported and the relative contributions of various vibrational modes determined. Differential cross sections for vibrational excitation of the 3 a 1 C-Cl and 4 (e) CH stretching modes have been measured. The latter reveals a broad shape resonance of 2 E symmetry peaking near 5.5 eV. Angular distributions for excitation of these same modes are also reported. Using fittings to the vibrational excitation functions, the resonance parameters have been extracted and used in a mixed semiempirical ab initio calculation to compute the relative strengths of the vibrational energy loss peaks. These compare favorably with the results of the experiment. Elastic cross sections integrated over angle are reported for low energies. They are substantially larger than the results from recent measurements of the total cross section.

For pt.I see ibid., vol.9, no.14, p.2521 (1976). Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of H2O by electron impact for collision energies from threshold to 10 eV. Differential and integral cross sections for the excitation of the (100, 001) and 010 modes are given in absolute units. Two distinct resonance regions are observed which are superimposed on a background of direct excitation: strong and sharp threshold resonances and a broad resonance region centred around 6-8 eV. The broad enhancement of the cross sections around 6 eV is attributed to a very short-lived 2A1 state of H2O-. The threshold resonances are interpreted as quasi-bound states in the dipole field of H2O. They are expected to be of general importance in electron-polar-molecule interactions.

Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of HF and HCl by electron impact at very low collision energies. The excitation functions for nu =1 and nu =2 have been measured from threshold to some eV above. In each case, an unusually strong and sharp threshold peak is observed with a broad resonance region following thereafter. The absolute cross sections reach values of the order of 10-15 cm2 for the threshold peaks in the nu =1 channel. The angular distributions are isotropic in the whole resonance region. The threshold resonances are interpreted as quasi-bound 2 Sigma + states in the dipole field possibly correlated with H*+X-. The broad resonance region is attributed to a very short-lived resonance state which may be the HX- 2 Sigma + state correlated with ground-state fragments H+X-.