J W McConkey

Circular polarisation study of He 21P excitation using electron-photon coincidence techniques

A VUV polarisation analyser is employed in an electron-polarised-photon coincidence experiment to determine circular/linear polarisation correlations from the differential electron impact excitation of the 21P state of helium. Circular polarisation correlations are presented for electron scattering angles in the range of 20-90 degrees and incident electron energies of 50, 60 and 80 eV. The sign of the orientation of the excited state was unambiguously determined in each case, thus resolving uncertainties which excited prior to this work.

Dipole and non-dipole excitation in collisions of low energy electrons with diatomic molecules

The authors have measured the rotational envelopes of selected vibrational bands of H2, D2, N2 and CO following excitation of cooled ( approximately 25K) gas beams. The observed broadening in N2 and CO at low impact energies is attributed to a breakdown of the electric dipole rotational selection rule mod Delta J mod =1. No broadening is observed in H2 or D2.

Production and measurement of circular polarization in the VUV

A comprehensive analysis of the production and detection of circularly polarized VUV light using reflection optics is presented. Experimental results using gold reflectors are given. The necessary measurements to determine all Stokes parameters of an arbitrary VUV light beam are discussed.

Production of ground state OH following electron impact on H2O

Laser-induced fluorescence (LIF) measurements of OH(X 2Π) production following electron impact on water have been carried out over an energy range from threshold to 300 eV. Data have been made absolute using existing measurements of OH(X) production via dissociative attachment. The cross section reaches a maximum value of 2.1×10−16 cm2 at an energy of 75 eV. Dipole-allowed processes are shown to dominate the production of OH(X) particularly via the A(1B1) repulsive state, though significant population of higher rotational levels is demonstrated, indicating the relevance of other channels as in photodissociation.

On O(1S) and CO(a 3 Pi ) production from electron impact dissociation of CO2

A novel method is employed to selectively detect O(1S) and CO(a 3 Pi ) fragments following the dissociation of CO2 by electron impact. O(1S) atoms impinging on solid Xe produce a prominent emission feature at 725 nm, and less intense features at 375 and 550 nm. CO(a 3 Pi ) molecules give a single emission feature spanning 250 to 350 nm. Time-of-flight spectra are presented for O(1S) and CO(a 3 Pi ) from electron impact dissociation of CO2. The results for O(1S) production indicate contributions from pre-dissociation of excited states of CO2. Relative cross sections for the production of both species have been obtained. The cross section for O(1S) production has been made absolute by comparison with production of the same species from N2O. It has a threshold at 11.0+or-0.5 eV and reaches a maximum of 1.69*10-17 cm2 at 50 eV.

Dissociative excitation of SO2 by controlled electron impact

The fragmentation of SO, following dissociative electron impact excitation has been studied under single collision conditions for incident electron energies up to 500 eV. The emission spectrum in the far V.U.V. spectral range (45&l lOOA) shows many features arising from excited neutral oxygen and ionized oxygen and sulphur fragments. Absolute emission cross sections have been measured for the most intense lines and the maximum values were found to range from l-12 x lo-l9 cm2 with an uncertainty of approx. +_35%. Dissociation mechanisms are discussed and in some cases the dissociation path could be uniquely identified. The striking differences between the v.u.v. emission spectrum produced by single step dissociation of SO2 and the spectra emitted by the plasma torus around Jupiter are discussed.

Selective detection of O(1S0) following electron impact dissociation of O2 and N2O using a XeO* conversion technique

We have observed the metastable species O(1S0) following the dissociation of molecules by pulsed electron impact. A crossed beam apparatus was used to obtain time‐of‐flight spectra of the fragments. We have employed a novel detector which consists of a layer of freshly deposited Xe. O(1S0) atoms impinging on the surface quickly form XeO* excimers which rapidly decay (<1 μs) producing easily detected photons. Our present results indicate that this method is sensitive (with high quantum efficiency) solely to the 1S state of oxygen. Other metastables (such as 5S and 1D) and ground state oxygen atoms have not been detected. Low resolution optical spectra of the XeO emission reveal bands at 375, 550, and 725 nm. Time‐of‐flight and fragment kinetic energy spectra for both target gases at various electron impact energies are presented together with excitation functions from threshold to 1000 eV. These have been made absolute using a Bethe–Born calibration technique for N2O. O2 data are calibrated relative to N2O...

Rotationally resolved electron-impact ionization of N2 in a supersonic jet

Abstract Laser-induced fluorescence (LIF) is combined with a variety of electron excitation techniques to study the details of the electron-impact ionization process in N 2 . The “temperatures” of both the neutral N 2 and the resultant N 2 + have been measured and significant warming is demonstrated to occur in the ionization process. This warming is found to be relatively independent of incident electron energy above 80 eV in contrast to the findings of many previous studies. Reasons for these discrepancies are discussed. A simple model is proposed which accounts for the observed findings. Within the context of this model we find that dipole transition probabilities describe the ionization process down to relatively low impact energies (≈ 80 eV). Below this energy it is necessary to invoke multiple interactions to explain the observed effects.

Metastable fragment production following electron impact on CO2

An apparatus is described which is suitable for studying dissociative excitation of molecules into metastable fragments and results are presented for electron impact on CO2. Both Rydberg and non-Rydberg contributions have been observed in the resultant fragments. Among the processes which are probably being observed are those which yield the following fragments, CO(X1 Sigma g+)+O(HR), CO(a3 Pi )+O(5S), CO(a3 Pi )+O(HR) and C(3P)+O(5S)+O(HR). The results are compared with those of other workers.

Isotope effects in the electron impact break-up of water

Total fragmentation of and has been studied as a function of incident energy over the range from threshold to 325 eV. A small subset of possible dissociation channels has been selected by making use of a novel solid xenon matrix detector which is selectively sensitive to metastable atoms. O-fragment kinetic energies and appearance potentials have been measured and significant isotopic effects are observed. Within the errors of the measurements the cross section for production from is the same as that from . The cross sections reach a maximum value of at 100 eV incident electron energy.