Rotational excitation of H2O by cold electrons.

Experimental data are presented for the scattering of electrons by H2O between 17 and 250 meV impact energy. These results are used in conjunction with a generally applicable method, based on a quantum defect theory approach to electron-polar molecule collisions, to derive the first set of data for state-to-state rotationally inelastic scattering cross sections based on experimental values.

Virtual state scattering with cold electrons: para-xylene and para-difluorobenzene.

The scattering of electrons with kinetic energies down to a few meV by para-xylene and para-difluorobenzene has been observed experimentally with an electron beam energy resolution of 0.95 to 1.5 meV (full width half maximum). At low electron energies the collisions can be considered as cold scattering events because the de Broglie wavelength of the electron is considerably larger than the target dimensions. The scattering cross sections measured rise rapidly at low energy due to virtual state scattering. The nature of this scattering process is discussed using s- and p-wave phase shifts derived from the experimental data. Scattering lengths are derived of, respectively, -9.5+/-0.5 and -8.0+/-0.5 a.u. for para-xylene and para-difluorobenzene. The virtual state effect is interpreted in terms of nuclear diabatic and partially adiabatic models, involving the electronic and vibronic symmetries of the unoccupied orbitals in the target species. The concept of direct and indirect virtual state scattering is introduced, through which the present species, in common with carbon dioxide and benzene, scatter through an indirect virtual state process, whereas other species, such as perfluorobenzene, scatter through a direct process.

The determination of absolute anion formation cross sections from electron beam scattering data.

Using recent low energy electron scattering data for CCl4 and SF6, and accompanying theory illustrating the coupling of attachment and elastic scattering, absolute cross sections are derived for electron attachment to CCl4 and SF6 between impact energies, respectively, of 8-52 meV and 7-42 meV. Values of attachment cross sections are compared with those obtained by laser and threshold photoionization techniques, which include normalization to rate coefficient data. Excellent agreement with the latest CCl4 data is obtained, with less precise agreement for SF6, but still lying within experimental uncertainties.

Reaction and scattering in cold electron collisions.

Experimental data are presented for the scattering of electrons by CCl4 between 8 and 200 meV impact energy. These results are used in conjunction with data for the reactive process, yielding Cl-, to study the low energy behavior of a system which simultaneously displays both reactive and elastic scattering channels. Phase shifts are derived and illustrate how channel competition develops as the energy falls. This behavior and the involvement of vibronic effects at impact energies above approximately 30 meV pose a challenge to theory.

Giant resonances in cold electron scattering by CS(2).

Experimental data are presented for the scattering of cold electrons by CS2, for both integral and backward scattering, between a few meV and a few hundred meV impact energy. Giant resonances with cross sections in excess of 50 A(2) are observed below 100 meV, associated with the transient formation of CS(-)(2) at 15 meV and with the bend and symmetric stretch of CS(2) at thresholds of 49 and 82 meV, respectively. The resonance at 49 meV is 2 orders of magnitude greater in cross section than a dipole impulsive model predicts. These structures are superimposed on a sharp rise in the scattering cross section at low energy, which may be attributed to virtual state scattering.

The quantum world of cold electron collisions.

Collisions between electrons and molecules at thermal energy play a key role in the chemistry and physics of plasmas, ranging from those in space to the chemically active plasmas used for thin film and microcircuit fabrication. However, until recently, data for electron-molecule interactions below a few hundred millielectronvolts impact energy (a few thousand kelvin) were very sparse. Our experiments, using a high-resolution synchrotron photoionization source of electrons, have opened up this low-energy field. We present recent results which reveal some of the striking quantum scattering events which take place as long-wavelength, low-energy electrons encounter molecules, choosing H(2), N(2), benzene, halobenzenes, methanol, ozone, and chlorine dioxide as examples.