Low-energy scattering has been studied in , , , , , and in a transmission experiment using a synchrotron radiation photoionization source. Backward scattering cross-sections have been determined over the energy range 10-175 meV. The variation of these cross-sections with electron impact energy has been analysed for non-polar and weakly polar species using modified effective range theory, yielding scattering lengths and low-energy limiting cross-sections. Rotationally inelastic scattering cross-sections have been calculated for and using the first Born point-dipole approximation. Results suggest that rotationally inelastic events contribute strongly to low energy scattering in but only weakly in , reflecting the larger dipole moment in .

In a new experimental system synchrotron radiation from the SRS (Daresbury, UK) has been used to photoionise Ar at threshold and to produce an electron beam with an energy resolution of about 3.5 meV (FWHM). The authors have observed the variation of the total scattering cross section of O2 in a supersonic beam over the energy range 0.1-1.3 eV. Rotational effects are strongly apparent and the results suggest that there are important contributions to the scattering from the higher partial waves l=4 and 6 as well as l=2. This apparatus shows the potential to open up the field of electron-molecule scattering at rovibrational resolution to a large variety of molecules.

Electron beam transmission experiments have been performed in the energy range IO meV to 175 meV, with a magnetically collimated electron beam formed in a synchrotron radiation photoionization source using SuperACO, LURE The apparatus has been Calibrated with He and absolute backward scattering cross d o n s have been measured for the target gases Ht, N, and 0,. A relationship, involving sand p partial waves, has been established between the backward scattering cross sections (uB) and the momentum transfer cross sections (uM). This has been used to check the accuracy of experimental data and the consistency of values of uBr uM and total scattering cross sections. Experimental data and theory for Ha are in good agreement, whereas for N, experimental values of uB and uM conflict below 80 meV and agreement with theories is mixed. For 0,. discrepancies are greater than for N2 both in experimental data and between theory and experiment, which may differ hy up to a factor of 5 for the total scattering cross section at the lowest energies. 1. Inirnduclion Electron collisions with H2, NI, O2 and He at very low energy are of considerable interest and have been the subject of intensive theoretical and experimental study in recent years. The most recent theoretical work, at low energy, is that of Nesbet et a f (1986). Gibson and Morrison (1984), and Schneider and Collins (1983) for H2, Gillan et a1 (1988) and Morrison et a/ (1987) for N2, Noble and Burke (1992) for 0 2 , and Saha (1993) and Plenkiewin et a1 (1990) for He. The most recent experimental investigations are those of Subramanian and Kumar (1989) and Brunger el a1 (1991) for Hz, Sohn et al(1986) for N2, Ziesel et a/ (1993) for O2 (hereafter referred to as paper I), and Buckman and Lohmann (1986) for He. In the present work, beam-gas cell scattering experiments have been performed to measure absolute values of the backward scattering cross sections for Ha, Nz and O2 as a function of electron energy in the energy range of IO meV to 175 meV. Low energy electron scattering cross sections are very sensitiveto the formulation of long range polarization and exchange and these data provide a stringent test for present and future scattering calculations in this energy regime. Data for H2, N2 and Oz also have important applications in modelling astrophysical plasmas,

In a crossed-beam experiment the authors have studied elastic and vibrationally inelastic scattering of electrons by SF6 over the energy range 50 meV to 1 eV at a fixed angle of 90'. A synchrotron photoionization source was used to form a primary beam of electrons of energy resolution FWHM of about 5 meV. Scattered electrons were analysed with an energy resolution of 11.5 meV allowing us to study the excitation of close lying vibrational modes independently. The authors present excitation functions for v1, v3, 2v1 and v1+v3. Excitation of v3 takes place through a direct dipole mechanism whereas the authors suggest that v1 involves a long-lived negative ion resonance.

We have performed high resolution electron beam transmission experiments in 0, over the energy range 12 meV to I eV. We estimate an approximate trend in widths of the d = 4 , 5 and 6 resonance states of the 0; intermediate and again the approximate trend in the relative contributions ofthe I =2,4 and 6 partial waves. Widths lie in the meV range as found previously for U'= 6 by Field et 01 and higher partial waves contribute less at lower energies. Analysis ofthe non-resonant elastic scattering data below 100 meV shows that scattering takes place through an interaction dominated by s-waves.

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An experimental system for the study of electron-molecule scattering at high resolution is described. Synchrotron radiation from the synchrotron radiation source (SRS) at the SERC Daresbury laboratory is used to photoionize Ar at threshold (78.65 nm). The resulting photoelectrons are formed into a beam of energy spread 3.5 meV (FWHM) and crossed with a Campargue-type supersonic molecular beam. The primary electron beam energy can be varied over the range 0.04-20.0 eV. An electrostatic hemispherical monochromator with a narrow acceptance cone (+or-6 degrees ) together with a position-sensitive detector provides energy analysis of the scattered electrons. In addition a second detector comprising a cascaded pair of microchannel plates and a high-voltage anode can record without energy analysis those electrons scattered over a much wider angular range (90+or-30 degrees ). The performance of the apparatus is illustrated with data on O2, N2, SF6, NO and CO2.

Lowenergy electron scattering by a, C z a and CzH6 has been studied using two difkent synchrotmn radiation phmionization spectrometers. The variation in h total scattering cmss section with energy was measured in a transmission experiment at Super-ACO, LURB, Orsay, France and electron scattering specea were recorded with a crossed-beam a p e s at the Synchrotron Radiation Source (SRS), Dansbury Laboratory, UK h k m t was focused on the energy range below 2 eV where previously unrecorded threshold vibrational resonances were observed. Toe energy dependencies of the observed total and elastic sMltering cross senions below 1 eV were analysed by the modified effective-range theory (m).Excitation functions for vibrationally inelastic processes in C€b and CzH, were measured and shown to deprut radically from the predictions of the first-order Born approximation.