Measurements are reported of the absolute total cross sections for scattering of electrons between ∼ 35 meV and 2 eV by C 6 H 6 , C 6 H 5 D and C 6 D 6 . Data are also recorded for scattering in the presence of an axial magnetic field over an energy range which extends from between 5 and 10 meV to 2.5 eV. Absolute values of the total scattering cross sections agree well with those of other groups down to 500 meV below which no other data are available. No differences in scattering cross sections are found within experimental error between C 6 H 6 and its deuterated derivatives.The 2 E 2u shape resonance around 1.1 eV in C 6 D 6 is found at an energy ∼ 25 meV above that for C 6 H 6 . This shift in energy is ascribed to the different zero-point energies of C 6 H 6 and C 6 D 6 , with a contribution from Jahn-Teller distortion in the temporary negative ion. Data at low electron collision energies show that the scattering cross section rises rapidly below 100 meV to values of 170-180 Å2 at the lowest energies ¶. A combination of data obtained in the absence and presence of the axial magnetic field leads to the conclusion that C 6 H 6 attaches electrons at very low energy, in agreement with earlier high-pressure attachment studies. The lifetime of the (benzene) − moiety, however, appears to be of the order of microseconds-very much greater than had previously been suggested.

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,

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