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Dissociative excitation of methanol (CH3OH) by electron impact leading to the production of O(1S) and CO(a3Π) has been studied in the energy range from threshold to 100 eV using a unique detector which is selectively sensitive to low energy metastable particles. Time of flight (TOF) techniques are used to separate atomic and molecular fragment species. Energy considerations have allowed certain excitation channels producing O(1S) to be identified.

Pulsed electron impact ionization, time-of-flight (TOF) spectroscopy and a novel detector are used to study the production of metastable O+ in e-O2 collisions in the electron energy range from threshold to 450 eV. Measured fragment ion kinetic energies are in the range 2–20 eV. The ions are identified as being in the 2P state within the ground 2p3 configuration. Peak metastable ion production occurs at an electron energy of ∼130 eV in agreement with the energy for maximum production of all O+ ions but the fall off in ionization efficiency toward higher incident electron energies is faster in the metastable case.

A unique detector which is selectively sensitive to low energy metastable atoms, has been used to study the production of ground state N (2P) atoms following collisions of low energy (0-200 eV) electrons with molecular nitrogen. TOF techniques have revealed the existence of at least two distinct mechanisms yielding this dissociation product. Released kinetic energies in the dissociation have allowed positioning of the parent molecular states in the Franck-Condon region. This has allowed probable parent states, such as B' 3Σu¯, b' 1Σu+ and C' 3Πu, to be identified making use of recent theoretical calculations. Both direct and predissociation processes are shown to be involved.

We have studied the excitation of metastable O(1D) following dissociative excitation of CO2 in the electron impact energy range from threshold to 400 eV. A solid Ne matrix at ∼20 K forms the heart of the detector. This is sensitive to the metastable species through the formation of excited excimers (NeO*), The resultant excimer radiation is readily detected, providing a means of measuring the production of the metastables. Using a pulsed electron beam and time-of-flight techniques, we have measured the O(1D) kinetic energy spectrum and its relative production cross sections as a function of electron impact energy. Threshold energy data are used to gain information about the excitation channels involved. In addition, an emission excitation function for the red photons, emitted in coincidence with the exciting electron pulse, has been measured in the 0–400 eV energy range.

A novel detector is described in which solid nitrogen at 17 K is used as the most significant element. Metastable particles impinge on this element and immediately transfer their internal energy to the solid nitrogen producing photons, via excimer formation or otherwise, whose wavelength depends on the metastable being detected and the energy transfer process. The performance of the instrument for the detection of atomic oxygen and molecular nitrogen metastables is discussed.

A novel detector is described in which solid nitrogen at 17 K is used as the most significant element. Metastable particles impinge on this element and immediately transfer their internal energy to the solid nitrogen producing photons, via excimer formation or otherwise, whose wavelength depends on the metastable being detected and the energy transfer process. The performance of the instrument for the detection of atomic oxygen and molecular nitrogen metastables is discussed.

Metastable N-2 molecules produced by electron impact on N2 are detected using a unique solid nitrogen matrix detector. The time-of-flight system is shown to be selectively sensitive to a(1)Pi(g) and (1)Sigma(+)(g) or (1)Gamma(g) metastable species. The latter species had been identified theoretically previously but was detected experimentally for the first time in the present investigation. Their identification and excitation as a function of electron energy from threshold to 300 eV are presented. Comparison is made with the data obtained by other techniques. Published by AIP Publishing.

Dissociative excitation of thymine following electron impact was studied in the energy range up to 430 eV. Emissions in the vacuum ultra-violet spectral region below 150 nm were studied and found to be dominated by the hydrogen Lyman series. Emission cross section data reveal that Lyman-α excitation displays a broad maximum at an electron impact energy of 160 eV. The probability of extracting other excited atoms from the parent molecule is found to be insignificant. Possible excitation and dissociation mechanisms in the parent molecule are discussed.

The dissociation of a wide range of oxygen-containing molecules following impact with electrons of carefully controlled energy is critically reviewed. Molecules considered range from diatomics, like O_2 and CO, to large molecules of biological and technological interest. Dissociation mechanisms are discussed and, where possible, quantitative data for the various possible processes, ionization, attachment, dissociation, excitation, emission etc., are presented. Gaps and discrepancies in our current data base are highlighted. Both graphical and tabular data are presented.

A new magneto-optical trap designed specifically for the measurement of absolute electron impact cross sections is described. Experimental results for electron impact total cross sections involving 6 2 S ground state Cs are presented over the energy range 5–200 eV. Significant disagreement is obtained between our experimental data and convergent close coupling (CCC) or Breit–Pauli R-matrix calculations at the lower energies. Above 100 eV our data overlap earlier measurements from this laboratory made with a completely different apparatus and agree well with the CCC results. A comparison is made with other measurements using more conventional techniques.

In 1990, two significant reviews of electron–atomic-oxygen collision processes were published. Since that time, a large volume of both experimental and theoretical research into these processes has occurred. These data are reviewed and recommendations regarding existing data sets and future research in this area are made. Attention is given to the challenges associated with handling atomic oxygen in terms of both experiment and theory. PACS Nos.: 34.80.Bm, 34.80.Dp

Differential and integral cross sections (ICSs) for excitation in atomic oxygen have been measured at electron-impact energies of 15, 17.5, 20, 22.5 and 27.5 eV. Differential measurements were conducted with a conventional electron energy-loss spectrometer and a microwave discharge source of atomic oxygen. Relative differential cross sections (DCSs) were determined between 15 and 30 eV impact energy. With the help of theoretical predictions of the shape of the DCS at large angles, measured results were extrapolated to 180° scattering angle and relative ICSs were deduced. The relative excitation function was normalized to the 30 eV impact energy ICS given by Kanik et al (2001 J. Phys. B: At. Mol. Opt. Phys. 34 2647). Normalization of the ICS values allowed the measured DCSs to be put on the absolute scale. Theoretical calculations of the DCSs were carried out using the R-matrix with the pseudostates approach. A total of 22 spectroscopic bound and autoionizing states, together with 19 pseudostates, were included in the close-coupling expansion. The pseudostates were chosen to simulate continuum target states. Theoretical results, along with other available experimental data, have been compared with the current experimental results.

Using a time-of-flight technique, we have studied the production of neutral metastable fragments in collisions of electrons with neutral xenon clusters. The data reveal that at least six different processes are occurring, yielding fragments with most probable kinetic energies between 0.08 and 1.6 eV. Additional processes give rise to excited fragments that decay by vacuum ultraviolet photon emission, with a variety of lifetimes. Some excitation routes involve exciton excitation, followed by localization into a dimer state, and subsequent decay into the metastable atomic channel. Others involve localization into a bound dimer state, followed by infrared photon decay to a repulsive curve, with consequent production of energetic metastable fragments.

Experimental and theoretical results for electron impact total cross-sections in Cs are presented over the energy range 0.3-400 eV. Good agreement is obtained between Convergent Close Coupling calculations and experimental measurements carried out using a magneto optical trap. A long standing question regarding the energy variation of the total cross-section for ground state targets is resolved. For energies above 10 eV, total cross-sections involving the excited 6 2P state are about 1.5 times those for the ground state at the same energy.

A detailed study has been made of the fragmentation of neutral krypton clusters following electron impact. The vacuum ultraviolet fluorescence and metastable fragments were monitored using a time-of-flight technique. Fragment kinetic energy distributions were obtained, showing that the fastest fragments were ejected with energies near 1 eV. Slower fragments with a peak kinetic energy near 0.5 eV were observed to dominate the fragmentation at higher incident electron energies. At least six processes were observed to contribute to cluster break-up with some of these involving exciton production. At least three further processes involved vacuum ultraviolet photon production.

A special matrix isolation detector, sensitive to S(1S0), has been used to study the dissociation of OCS into this fragment over an electron energy range from threshold to 400 eV. The maximum cross section occurs at an impact energy of 55 eV and has been measured to be 3.3×10-17 cm2. Time of flight spectroscopy has been used to monitor fragment kinetic energies. Optically allowed processes are observed to dominate the dissociation. Striking similarities between electron and 157 nm photon dissociation have been noted.

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.

The polarization of Balmer-α radiation excited in collisions of electrons with atomic hydrogen is presented for an electron energy range from threshold to 1000 eV. Measurements are in good agreement with calculations carried out using either convergent-close-coupling or R-matrix with pseudo-states approaches. Cascade is demonstrated to have a significant effect. Balmer-α excitation function data are also presented. A previous measurement of the polarization of Balmer-α following dissociative excitation of H2 by electrons is confirmed and extended.

The interaction of electrons of carefully controlled energy with N2O molecules and clusters has been studied in a pulsed crossed-beam arrangement over the energy range from threshold to 200 eV. A variety of time-resolved techniques have been applied to isolate and study many of the dissociative excitation channels which occur. The data obtained are relatively insensitive to the degree of clustering in the target beam.

Dissociative excitation of SO2 by electron impact to produce excited oxygen fragments, in particular O (1S), has been studied in the energy range from threshold to 400 eV. Three processes leading to O (1S) production have been identified and the positions of the relevant repulsive states of SO2 in the Franck–Condon region have been determined. The multiplicities and symmetry characteristics of two of these states have also been established. The absolute cross section for production of O (1S) by all channels is measured to be 2.2 x 10-18 cm2 at 150 eV. The relative cross section for production of O (3p 3P and 3p 5P) from threshold to 400 eV has also been measured. The existence of a hitherto unidentified triplet state of SO2 that emits in the near infra red has been established. PACS No.: 34.80Gs

The dissociation of molecular hydrogen ions with energies up to 100 keV in H targets has been studied. An Si barrier detector is used to distinguish between the production of one or more fast fragment ions or neutrals and coincidence techniques involving the slow ion or photon produced in the collision allow the different dissociation channels to be quantified. The dissociative excitation and ionization channels are found to be about an order of magnitude weaker in H than those induced by collisions with electrons. Comparison with Born calculations shows that the vibrational states of H2+ play an important role in the dissociation process.

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.

A variety of techniques have been applied to study the interaction of electrons with beams of CO2 molecules which are heavily clustered. Fragmentation of the clusters is monitored by detecting excited fragments directly or via the vacuum ultra-violet photons which they emit as they decay. The evidence obtained points to the conclusion that the excited molecules are essentially decoupled from the clusters during fragmentation. The shape of the excitation functions are indicative of the dominance of optically allowed processes in the initial interaction.PACS Nos.: 36.40+d and 34.80.GS

Results of semirelativistic $R$-matrix calculations for electron-impact excitation of the ${(np}^{6}{)}^{1}{S}_{0}\ensuremath{\rightarrow}{\mathrm{np}}^{5}[n+1]s$ $(J=1)$ transitions in the noble gases Ne $(n=2),$ $\mathrm{Ar}(n=3),$ Kr $(n=4),$ and Xe $(n=5)$ are compared with experimental values for light polarizations and angle-integrated magnetic sublevel cross sections. The calculations focus on the near-threshold regime, where negative-ion resonances have a significant effect on the results. In light of the complexity of the structures, the agreement between theory and experiment is very encouraging. Nevertheless, further improvements in the theoretical treatment of both the target structure and the collision dynamics seem necessary.

A detailed study has been made of electron impact induced fluorescence from Ar clusters. Use of suitable filters in the vacuum ultraviolet spectral region together with time-resolved techniques has enabled the dimer emission from the clusters to be investigated in detail. Excitation function measurements have revealed a number of processes leading to exciton production. Two-center collisions within the cluster appear to be significant. A close parallel to what is observed with solid Ar targets is noted.

Making use of a novel solid xenon matrix detector which is selectively sensitive to $\mathrm{O}(^{1}S_{0})$ metastable atoms, a new dissociation channel, via a $^{1}A_{1}$ repulsive state, has been uniquely identified following electron impact on ${\mathrm{D}}_{2}\mathrm{O}$ molecules. Careful measurements of the O fragment kinetic energies and appearance potentials have allowed partial reconstruction of the potential energy surface involved. The excitation function for production of $\mathrm{O}(^{1}S_{0})$ has a shape which is characteristic of an optically allowed transition in the parent molecule and has a maximum value of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}{\mathrm{cm}}^{2}$ at 100 eV incident electron energy.

The optical excitation function of prompt Lyman-\ensuremath{\alpha} radiation, produced by electron impact on atomic hydrogen, has been measured over the extended energy range from threshold to 1.8 keV. Measurements were obtained in a crossed-beams experiment using both magnetically confined and electrostatically focused electrons in collision with atomic hydrogen produced by an intense discharge source. A vacuum-ultraviolet monochromator system was used to measure the emitted Lyman-\ensuremath{\alpha} radiation. The absolute H(1s-2p) electron impact excitation cross section was obtained from the experimental optical excitation function by normalizing to the accepted optical oscillator strength, with corrections for polarization and cascade. Our data are significantly different from the earlier experimental results of R. L. Long et al., J. Res. Natl. Bur. Stand. Sect. A 72A, 521 (1968) and J. F. Williams, J. Phys. B 9, 1519 (1976); 14, 1197 (1981), which are limited to energies below 200 eV. Statistical and known systematic uncertainties in our data range from \ifmmode\pm\else\textpm\fi{}4% near threshold to \ifmmode\pm\else\textpm\fi{}2% at 1.8 keV. Multistate coupling affecting the shape of the excitation function up to 1 keV impact energy is apparent in both the present experimental data and present theoretical results obtained with convergent close-coupling (CCC) theory. This shape function effect leads to an uncertainty in absolute cross sections at the 10% level in the analysis of the experimental data. The derived optimized absolute cross sections are within 7% of the CCC calculations over the 14 eV--1.8 keV range. The present CCC calculations converge on the Bethe-Fano profile for H(1s-2p) excitation at high energy. For this reason agreement with the CCC values to within 3% is achieved in a nonoptimal normalization of the experimental data to the Bethe-Fano profile. The fundamental H(1s-2p) electron impact cross section is thereby determined to an unprecedented accuracy over the 14 eV -- 1.8 keV energy range.

The vacuum ultraviolet and ultraviolet emission (115 nm

The linear polarization of resonance vacuum-ultraviolet radiation emitted by electron-impact excited Ne atoms has been measured in the energy range from threshold to 21 eV. An electron resolution of 80 meV has enabled the effects of negative-ion resonances on the polarization function to be examined in some detail. Predictions of the effects of negative-ion resonances on the observed polarizations have been made using a generalized Baranger-Gerjuoy theory. Excitation via electron exchange is shown to be the dominant process very close to threshold. {copyright} {ital 1996 The American Physical Society.}

The linear polarization of resonance vacuum-ultraviolet vuv radiation emitted from collisionally excited helium atoms has been measured in the threshold region below 25 eV. The apparatus consisted of a hemispherically analyzed electron beam interacting with a gas jet; the normally emitted radiation was analyzed using reflection optics. The threshold polarization was measured to be unity, in agreement with arguments based on the conservation of angular momentum. The measured vuv excitation functions and corresponding polarization curve were compared to 11-state R-matrix calculations of the excitation of the 2${\mathrm{}}^{1}$P state and excellent agreement was observed up to the n=3 threshold. Negative-ion resonances and cascade had pronounced effects on the observed sublevel excitation functions and on the polarization. \textcopyright{} 1996 The American Physical Society.

The electron--polarized-photon coincidence method is used to determine linear and circular polarization correlations in vacuum ultraviolet (VUV) for the differential electron-impact excitation of neon and argon resonance transitions at impact energies of 25 and 30 eV at small scattering angles up to 40\ifmmode^\circ\else\textdegree\fi{}. The circular polarization correlation is found to be positive in the case of Ne at 25 eV and supports the prediction of the present $B$-spline $R$-matrix theory concerning the violation of a long-established propensity rule regarding angular momentum transfer in electron-impact excitation of $S\ensuremath{\rightarrow}P$ transitions. Comparisons with the results from the present relativistic distorted-wave approximation and an earlier semirelativistic distorted-wave Born model are also made. For the case of Ar, at 25 and 30 eV, the circular polarization measurements remain in agreement with theory, but provide limited evidence as to whether or not the circular polarization at small scattering angles is also positive. For the linear polarizations, much better agreement with theory is obtained than in earlier measurements carried out by S. H. Zheng and K. Becker [Z. Phys. D 23, 137 (1992); J. Phys. B 26, 517 (1993)].

We have studied the production of neutral metastable fragments in electron collisions with neutral argon clusters. The fragments are detected using a time‐of‐flight technique. The time‐of‐flight spectra show that the metastable fragments appear in two velocity ranges. Kinetic energy distributions are obtained, showing that the faster fragments are ejected with energies from 0.2 to 1.5 eV and that the slower fragments have energies less than 0.2 eV. It is argued that the fragmentation of the clusters involves the excitation and decay of excitons in the clusters.The faster fragments are produced by n=2 excitons, which localize on an excimer or an excited trimer within the cluster and upon dissociation cause the ejection of a metastable atom. The slower fragments are produced by n=1 excitons, which tend to localize on the periphery of the cluster, leading to the ejection of a metastable atom due to weak repulsive forces with neighboring atoms. Four different production mechanisms for neutral metastable fragments are observed.

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.

Production of metastable O(1D2) atoms following controlled electron impact on N2O and CO2 targets has been studied using a neon rare gas matrix detector operating at a temperature of

It is demonstrated that both channel electron multipliers and channel plates may display pronounced polarization sensitivity when VUV radiation is being detected. Photon angle of incidence is demonstrated to be an important parameter for channel plates.

Absolute emission cross sections, for the VUV spectral features at 130.4, 102.7, 98.9 and 87.8 nm, have been measured for electron impact on atomic oxygen in the energy range from threshold to 100 eV. An RF discharge source containing NO was used to prepare a high density atomic oxygen beam. Significant differences were found with earlier measurements on these transitions. The data are compared with available theoretical results and with level excitation cross sections obtained using inelastic electron scattering techniques.

Dissociation of N2O into metastable O(1S) fragments following electron impact has been studied in the energy range from threshold to 500 eV. A novel detector, selectively sensitive to O(1S), has been used together with pulsed excitation and time-of-flight techniques. The data suggest that O(1S) production is dominated by a single dipole allowed transition in the parent molecule.

Electron-impact, energy-loss spectra of anthracene were obtained in the 12 to 100 eV impact energy range at angles ranging from 5' to 40'. From these spectra energy level information, relative electron-impact excitation probabilities and relative optical f values have been derived. Cross sections for 250-525 nm band emission have been measured over an electron-impact energy range from threshold to 60 eV. Many processes, both direct and indirect, are observed to contribute to the excitation. The results are compared with those obtained using naphthalene targets.

Electron-photon coincidence measurements are reported of the excitation of the 4s(3/2)o1 state of Ar at incident energies of 20 and 30 eV and electron scattering angles up to 80'. Isolation of the 4s(3/2) degrees 1 state was accomplished in the photon channel using a LiF filter. Significant non-zero values of the spin-flip parameter, rho 00, are reported at both energies as predicted by theory but the level of agreement with available perturbative theories becomes poorer (as might be expected) as the incident electron energy is reduced.

The electron-photon coincidence technique has been employed in experimental investigations aimed at elucidating the finer details of the excitation of the first excited states of the heavy noble gases by electron impact since the mid 1970s. The agreement between measured data and the predictions of first-order perturbative theories such as the distorted wave Born approximation (DWBA) and the first-order many-body theory (FOMBT) ranges from good in some cases to rather poor in other cases. This article will review the current status of electron-photon coincidence experiments with the heavy noble gases, critically assess the level of agreement and disagreement between experiment and theory, discuss apparent trends in the data, make recommendations for the direction of future experiments and suggest ways to ensure that measured coherence parameters are free from instrumental and physical effects to the maximum extent possible.

Absolute, apparent cross sections for 295-410 nm band emission from naphthalene have been measured over an electron-impact energy range from threshold to 300 eV. Numerous processes, including electron exchange, are observed to contribute to the excitation in the near threshold region. At higher energies direct excitation through optically allowed channels is preferred. The electron-impact-induced fluorescence was found to be unpolarized (

Time-of-flight detection of neutral metastable fragments has been applied to the study of electron-impact dissociation of CF4 molecules. The time-of-flight distributions indicate that a number of distinct processes are involved in the production of metastable fragments. Excitation functions for these processes have been measured up to 400 eV. The results presented are compared with other studies of fragmentation of CF4 and indicate that fragmentation into an excited CF3+ ion and an excited F atom is a prominent process.

Electron impact excitation of the resonance levels of Ne, Ar, Kr and Xe has been studied for electron scattering angles up to 50' and impact energies between 30 and 80 eV. The P1 and P4 Stokes parameters have been measured in each case so that the influence of spin in the excitation process could be studied through evaluation of rho 00, the relative spin-flip cross section. After careful account was taken of various depolarizing effects due particularly to the finite volume of the interaction region and, in the cases of Kr and Xe, to nuclear spin, very good agreement has been found with theoretical predictions thus resolving a previously reported discrepancy. No evidence has been found for spin-flip under the experimental parameters used in this study, even for the heaviest target studied.

[1] Vacuum ultraviolet emissions resulting from electron impact on O2 have been studied at 100, 200, and 300 eV. The ratio of the 83.4 nm and 130.4 nm emissions were used in combination with the recently revised recommendation of the O I (130.4 nm) cross section to determine the O II (83.4 nm) cross sections. The present results are compared with existing measurements and resolve the controversy that has existed regarding the magnitude and shape of the 83.4 nm emission cross section. We recommend an O II (83.4 nm) emission cross section of (1.50 ± 0.17) × 10−18 cm2 at 100 eV.

Electron-impact excitation of the [3/2${]}_{1}^{0}$ 6s state of xenon has been studied at 30 eV and over a range of scattering angles up to 50\ifmmode^\circ\else\textdegree\fi{}. Linear and circular polarization correlation measurements show very good agreement with distorted-wave Born-approximation calculations. For the experimental parameters under consideration, the excitation is demonstrated to be completely coherent. Hyperfine depolarization of the excited-state charge cloud is found to be an important effect.